Modern homes are shifting towards increasing levels of airtightness in order to secure heat-saving benefits. With increasing levels of airtightness comes a need for a ventilation strategy. For Passivhaus, this means an MVHR system.
Heat from the home is lost in two ways:
- through the fabric elements as it passes through our walls, windows, doors, floors and roofs
- through holes, draughts, open windows and doors through air movement
UK Building Regulations on home airtightness are terrible
The government building regulations on airtightness are pretty bad. They let our country’s house-builders build homes with an A4 piece of paper-sized hole in the building fabric. That’s the equivalent of leaving a window wide open constantly, all year round – even if it’s minus 20 DegC outside. Try and keep your home warm when all of the heat can escape out of an open window.
It’s almost impossible, and it’s very expensive to try, as people that buy new builds in England will know.
Building Regulations allow new homes a maximum of 10 air-changes per hour at 50 Pascals of pressure (the equivalent pressure of a light wind blowing against the house).
This means that every six minutes all of the air in your home is replaced with new, cold air from outside. It’s terrible, and that’s the standard of house-building in this country in 2023.
If you would like to learn more about airtight homes and the relationship with MVHR systems I’ve written about that here.
Aren’t airtight homes stuffy and smelly?
Not if you install some form of mechanical ventilation (with or without heat recovery).
If you have an airtight home up to Passivhaus standards, a Mechanical Ventilation Heat Recovery system (MVHR) is vital.
An MVHR is not a new piece of technology. You already use one everyday in your car. When you turn on the heater in a car it brings in fresh, cold air from outside and passes it over the engine, exchanging heat, through a filter and into the car as warm, fresh air.
Our nose is also a kind of biological heat exchanger in winter. Cold dry air is breathed in and the nose warms it safely via the warm air breathed out.
A home MVHR system does the same thing. It extracts warm air from the rooms that create heat and moisture from occupants (kitchens, bathrooms and media rooms) and it passes in little pipes through cold, filtered outdoor air to warm that air up. That warm, clean air goes into the supply rooms (living rooms, bedrooms and dining rooms).
This means that 90 per cent of the heat created in your home stays in your home. That’s one of the ways that Passivhaus saves up to 90 per cent on household heating bills.
This video shows the energy saving in action in a residential property we retrofitted one of our MVHR systems into:
One block of flats in London hasn’t turned their heating on in winter for two years because the MVHR is so efficient.
So what can go wrong with an MVHR system?
Local councils are on a drive to promote Passivhaus amongst its private-public housing projects, partly for the environment, but also because it saves the council money in heating bills in the long run (which they often pay for).
However, badly performing Passivhaus multi-development projects that aren’t designed/installed correctly have been showing up over the last few years.
Mould, damp and increases in asthma problems are being reported, which sounds like humidity problems; ie, the flats are struggling to remove the moist air created by its occupants. Both UK Building Regs and Passivhaus demand minimum levels of ventilation in rooms like bathrooms and kitchens because the danger of lingering moisture is so serious to human health. MVHRs have to be designed so that they operate efficiently in clearing out moist air, but also aren’t too noisy.
To do this, MVHR designers like ourselves calculate total pressure drops in the system as air moves through ducts around the building. Too many twists and turns in the ductwork and the air will struggle to move through into the rooms where it needs to be.
The five most common issues with MVHR systems are:
- Poor initial MVHR design (no proper airflow rate calculations, so the fan isn’t optimised to push and pull air through the property). Our MVHR design team completes the ventilation calculations bespoke to each project every time.
- Cheap ducting that is easily crushed or torn, or a cheap MVHR unit – stay away from the very cheap ones, they do nothing. Our company has banned flexi-duct from our projects, forever.
- On site deviations from the MVHR design (the system has been designed properly but the installers have changed duct runs, or not sealed the ducts properly so air can escape, or they’ve accidentally or negligently crushed areas of ductwork so that air escapes before it can get to the rooms). At HSL we quality check every system at commissioning to ensure the design has been followed and works.
- The MVHR hasn’t been commissioned properly (so each room has not been individually balanced to ensure proper airflow rates to each room). We won’t sign off a system until we’ve confirmed it’s working as designed.
- Filters haven’t been changed (In smoggy, dirty London the filters will have to be changed every 6 to 12 months or they’ll get clogged and the system won’t work effectively). We remind our clients to change filters, and offer the cheapest pricing for new filters that we believe is available.
It’s a great shame that public schemes have had issues with the MVHR system, because when the MVHR system goes right it provides clean, warm fresh air to the home quietly and effectively – and with 90 per cent plus efficiency it is definitely a future technology more homes need.
My aim is to help raise the standards for MVHR systems in UK homes, so if you want to discuss your build project incorporating MVHR, or you’d like a sizing and specification proposal for your own home for a ventilation system that is optimised for health and comfort, please contact me via the Contact Page or submit your details here.
I have recently moved into a shared ownership property where I have had a co come round to look at mvhr system and rates are very low and the last tenant had not ever changed or cleaned the filter. It has a flat roof and there seems to be no access to the loft and the housing association are stating I would have to have someone create access to look at the problem! How can it be legal to install this in not create access to it?
Hi Sharon, we spoke via email at Patrick [at] heatspaceandlight.com, but in short, this needs to be looked into as it doesn’t sound like a compliant set-up. Access for servicing must be made available for all ventilation systems, as we would for a gas boiler or heat pump.
We have the same problem, no one would come to service it our HRV2 Q Plus because there was no safe access. It begs the question how was this passed at building inspection of it is now not safe for anyone to service the unit!
Hi Annie,
When we design MVHR system for retrofits and new builds we always ensure adequate access for servicing and maintenance of the MVHR system.
When we commission and sign off our MVHR systems for Building Control we also double-check the system is straightforward to access for the homeowner. This is because MVHR systems have filters which need to be checked or changed every year.
So yes, it sounds like whoever supplied your system didn’t follow the rules in Approved Document Part F.
Where exactly is your MVHR unit located?
Feel free to contact the HSL team directly at the top of the page.
Best wishes,
Patrick
I am concerned that the ventilation system can stop working— unnoticeably , because they are so quiet– for several reasons, e.g. yo go away for 3 weeks and by the time you have come home there is mold everywhere
Because the homes are so airtight , humidity can not escape.
The other issue is that it is a money pit , as it is a 24 hour lifetime electricity user .
The third problem is that if the heat exchanger does not work, you will not know then you are pumping out warm air , needing to reheat the cold air that has been sucked from the outside
Hi Amrita,
I can answer all your concerns and I hope then you will feel better about MVHRs going forward.
Firstly, if the MVHR breaks or there is a power cut whilst you are away from the home, there will not be mould everywhere. This is because it is us as occupants who create water vapour in our homes by washing, cooking and drying clothes (20 litres of water vapour a day for a typical four-person family). A vacant property does not generate any water vapour, so no mould will form whilst you’re away.
Although the MVHR is near-silent, you will be able to tell when it’s turned off because the air will start to feel stuffy, and you’ll notice way before any mould occurs. The MVHRs I personally work with have very few warranty claims – I can count five out of 500 in the last four years, and they were fixed easily and quickly by the manufacturer. For the most part MVHRs are very simple, stable technologies.
The energy to run the two high performance MVHR fans is the equivalent of running two LED light bulbs – so it’s not very expensive to run long term in terms of energy usage either – 10 to 20p a day.
Finally, the heat exchanger is passive – there are no working parts that could break, it’s simply running the warm exhaust air from the home through the heat exchanger and warming the fresh, incoming filtered air. Compare this to all other forms of ventilation, which bring in cold air from outside through trickle vents.
I hope the above helps. Please let me know if you have any other concerns and I’ll be happy to respond.
Best wishes,
Patrick
Hi Patrick,
Apologies for commenting on an article from some time ago. I’m hoping you can give me some advice. I recently had an mvhr system activated in a new build, but the thing is we feel that it’s gotten more stuffy since then, not less. I know this should not be the case, would you have any idea what has gone wrong?
Thanks,
Martin.
Hi Martin,
If the MVHR isn’t working or hasn’t been installed properly it may not be supplying and/or extracting enough air to clear the rooms of excess humidity, CO2 and other indoor pollutants – that would cause stuffy air. This issue is exacerbated in a new build because the home will have been built very airtight, so you don’t have any airflow coming in through window trickle vents or draughts under doors.
Stuffy air could be happening because
1) it’s been installed incorrectly (crushed ducting, undersized fan unit, blocked ducts, missing valves);
2) commissioned incorrectly (fan speeds too low, out of balance) or;
3) designed incorrectly (wrong flow-rates, wrong unit specification, poor quality ductwork).
I will send you a private email so we can discuss more and diagnose the problem together. It may need one of our MVHR technicians to visit the property and assess the MVHR system, measure flow-rates and conduct a detailed check of the unit itself.
Best wishes,
Patrick
Is it possible that an MVHR system which is either not working properly, not been cleaned or has been turned off could cause water in a glass to change taste?
Dear Haden,
No, an MVHR system is not linked in any way to the water supply or system. An MVHR is a closed system of ductwork for carrying air only.
I hope this helps.
Best wishes,
Patrick
Hey Patrick,
Just came across this article, I hope you can help me. I’ve a heat recovery in my home, however one of the air ceiling vents is on a sloped ceiling upstairs. If the power goes off, which has happened a couple times in the last 3 years, condensation then travels down the piping and out the vent, staining my ceiling. Also, if the power goes out, a couple other vents also stain the ceiling because of condensation gathering. The piping is plastic from the main unit, but the casing just above the ceiling vent is metal. Is there anything I can do to help maybe remedy this, or is it just part and parcel of the system?
Thanks again
James
Hi James,
So I understand you correctly, the duct slopes down towards the room valve? Is it in a supply room (bedroom/living room) or an extract room (bathroom/utility)?
It doesn’t sound right at all – there shouldn’t be that much condensation in the system. It needs more investigation – it could be that with the power out the warm, humid air normally picked up by the MVHR condenses in your roof space and trickles down onto the valve. How frequently and for how long is the power going out? I know you can’t help a power cut, but in principle, MVHRs should never really be turned off once they’ve been running a while – they use the power of a couple of LED bulbs to run the fans, so they’re not a signficant draw on energy.
It may help to insulate that area in the sloped ceiling void – in brief, condensation is about humid air reaching cold surfaces. I’m going to email you directly as it’s worth having a chat and seeing if we can have one of our technicians visit you to troubleshoot the issue.
Best wishes,
Patrick
Hi
I’m hoping someone can help me, I have a MVHR fitted in my house, I was cleaning the valve in a small bedroom and it was blowing cold air into the room. My wife has been saying upstairs is a lot colder than the rest of the house, I’ve got the temp set indoors at 23 and out doors is set at 18, am I doing something wrong, as it seems to be just sucking hot air out and replacing it with cold.
Hi John,
It sounds as if there’s a fault with your MVHR system, it should be blowing warm, tempered air back into the room at a gradual airflow rate. It could be a few issues:
1) The MVHR unit heat exchanger is not exchanging heat effectively
2) There is a hole in the system
3) The airflow rate from the bedroom valve is too high (system is not commissioned effectively)
If you’d like to discuss it in more detail, please contact us via the Contact Page at the top.
Best wishes,
Patrick
Hi Patrick, we have a new build project coming to the end of roof/soffit works. We are having a MVHR system in place. Can you tell me if we can do away with any soffit/roof ventilation if we are having this system please?
Kindest regards,
Mark
Hi Mark,
No, they are two separate aspects – an MVHR is for ventilating the home within the thermal envelope, and the soffits are for venting the roof space on the cold side to dry out the roof structure. It depends if your roof is open or closed in that respect as to whether you “need” roof ventilation, and I can’t tell you that without more detail. I’m sure your architect/builder/roofer will know how it should be.
Best wishes,
Patrick
Hi, can you advise on a problem I’m having with my Dantherm HCH5 Air recovery unit, it keeps switching off after a short time (5mins or so) after resetting it, it is flashing a red light on the control. I’ve replaced the filters to see if that would solve the issue but it’s still the same & then I’ve showered all the dirt out of the heat exchanger (this was Hollingsworth a fair amount of water when removed) which hasn’t made a difference.
I only moved into the property 6 weeks ago and it has been running fine until the last 2 weeks when it has been colder outside
Thanks in advance for any advice you may give or if you have or can recommend an engineer in Lancashire region
Hi Damien,
I would contact Dantherm directly and see if they have a technician or recommended supplier that they can send out, as it sounds like the motor has failed or something is tripping the motor out. When you say a fair amount of water was in the system, this could mean the condensate drain below the unit has blocked and sent water back up into the heat exchanger, potentially due to freezing. If so, water in the MVHR system could have fried something. To prevent this freezing, if the MVHR is in a cold room it should be insulated.
I’ll admit I’m not hugely knowledgeable about Dantherm MVHR units so I wish you the best of luck, and email me via the Contact Page if you have any more questions.
Best wishes,
Patrick
Patrick,
we have a MVHR system within our passive house. Our daughters bedroom and the 2 upstairs bedrooms are being supplied at 5 degrees lower than the rest of the house. The installer suggests that the system is working as specified but am I missing something? It was claimed that the house would be of the highest level of comfort but this is not the case at al. The heat is on far more than we were led to believe….. Could there be an easy fix???
Hi Martin,
How are you measuring the 5 Degree air temperature difference? Is it the room temperature at the thermostat, or the air coming directly out of the valve with a handheld thermometer? Do these rooms have heating, or do they differ in any way to other bedrooms, eg, do they have dormers or larger windows? Generally, the heating system will have a much larger impact on warmth than the MVHR, so it would be first port of call if a room is colder than others (eg, undersized or no radiators, or more external walls/ceiling than other rooms).
If it is an MVHR issue it could be that these bedroom valves are being over-ventilated, but I suspect it’s more to do with insulation and/or heating system issues.
A certified Passivhaus must have a space heating demand of less than 15 kWh per m2 per annum, so this would theoretically be your heating bill costs (your floor area multipled by 15). Is this not ringing true? This is only for space heating, so electrical demand is more, plus any gas/electrical standing charges.
I’ll also send you an email so we can chat directly.
Best wishes,
Patrick
Hi Patrick, and Happy New Year to you! I wonder if you could sense-test something for me please? We have an old (solid wall) house out in the country. We heat using wood and oil. We can keep the house warm but have an issue with damp/mould at the far ends of the house (think “L-shape”). I wanted to deal with the mould (and rising bills!) using MVHR; increate the ventilation in the upstairs rooms by drawing air into them then pushing it through the heat exchanger into the room below (we have a handy unused dual chimney connecting the two – and complete access to loft spaces). It’s an old house, albeit with double glazing, so not draft free and with a log burner and open fire to boot. I don’t need 90% recovery, I just need to extract wet air from the wet-generating rooms, and push (warmer,) drier air into the room which needs it – the rest should be OK as is. Am I barking mad or could this work please? Many thanks, John
Hi John,
Mould is always a case of two issues – overly humid indoor air and cold surface temperatures. Both are needed to create black mould (plus something for the mould to consume, but that’s a separate matter).
An MVHR deals with the first issue by extracting warm, humid waste air constantly at low level from the house and bringing tempered (warm) air back inside. This helps ensure indoor humidity levels remain around the 50% Relative Humidity level “sweet spot”, which is healthiest for humans and good for structural protection.
However, if there are very cold surfaces, even if indoor humidity is fairly low, mould can still occur if the surface temperatures are cold enough. In simple terms, the dewpoint (ie, water saturation) at 10 – 13 DegC is around 50% Relative Humidity. If it’s 0 – 5 DegC outside then those walls or windows could be near those temperatures – condensation would be inevitable (even if not visible as droplets, it could be occurring deeper within the wall).
This is why mould removal treatments don’t work incidentally. They treat the symptom not the cause.
Saying all that, I would look at insulating the external walls internally with either Diathonite or a Lime Plaster + Woodfibre Insulation finish. This will allow the walls to keep warmer on the inside skin, and the insulation will be vapour permeable to allow moisture to move through harmlessly. You don’t want to use too much insulation internally as it will make the outside of the wall too cold which may cause spalling, but enough to ensure that the walls can stay at a warmer temperature when it’s cold outside to prevent dewpoint.
We’ve applied lime plaster+woodfibre insulation at a conservation project successfully – please email me at patrick [@] heatspaceandlight.com if you’d like more details.
Secondly, once the walls are “warmer”, increasing the ventilation will help. What you describe is close to what an MVHR does – it draws humid air out of bathrooms and kitchens, and supplies drier, warmer air into bedrooms and living rooms, to allow for complete and balanced ventilation around the home. I realise you want to do that on a smaller scale, so extractor fans in bathrooms and trickle vents in living rooms would be the simplest way to do this – but with a negative comfort/energy efficiency impact as it would draw in cold air.
I hope this helps – happy to discuss in more detail over email (above).
Best wishes,
Patrick
Hi Patrick
We’ve recently installed a [redacted MVHR brand name] in a new build. It works really well and we are pleased with it. One technical issue baffles me and I haven’t been able to find an answer, maybe you can solve this one..
The MVHR is set up so the the extract fan is set at a higher speed than the inlet fan which must create a slight negative internal pressure (normal setting is 55% input fan 65% extract fan).
What is the logic for creating negative internal pressure? It means that if there was a leak in the building fabric then cold air would draft in. Wouldn’t it be better for warm air to draft out?
Thanks for your help on this one.
Rgs ….. Paul
Hi Paul,
Extraction of air and supply of air should match almost exactly, with a small 5% tolerance allowable, but no more than this. This criteria is set for the reasons you provide – that if the system was imbalanced it would either be pushing warm, moist air through the building fabric (very bad) or pulling cold air in through the fabric (not energy efficient, healthy or comfortable).
The above may not be happening with your MVHR system though. The reason the fans in a Vent Axia run independently is because they aren’t calibrated together, so one may push out 30l/s of air at a fan duty percentage, whereas the other might push out 20l/s, or 40l/s. This means they have to be set at different speeds. I think this is what you are describing. You should have a separate Commissioning Certificate which tells you the actual flow-rates in l/s across the supply and extract points – these should balance to prevent negative or positive internal pressure.
For our projects at HSL, the MVHR units we typically specify are “Constant Flow”. This means a flow-rate is programmed, eg, 30l/s, and the fans will regulate and correct the airflow constantly and instantly under any external influences. For example, as the filters block, the MVHR unit will counteract this and ensure the air flow-rates are met by subtly increasing fan speeds slightly.
Other units that do not have constant flow will provide lower and lower airflow until the filters are replaced, potentially jeopardising air quality indoors. If one filter blocks before the other, negative or positive internal pressure issues can occur.
Constant Flow also deals with wind hitting the external grilles to prevent any whistling noises, which would otherwise be annoying to hear.
That’s why we recommend Constant Flow fans with our MVHR units – they are quieter, ensure better indoor air quality and maintain balance.
I hope this helps. If you have any other questions about MVHR, please contact me again.
Best wishes,
Patrick
Hi Patrick, just found this thread and was wondering if you might be able to help with some insight.
We moved in to a new build block of flats last June and last month decided to get the MVHR serviced as we weren’t sure if it had been done before.
They came to look at it and said that the bathroom and kitchen weren’t extracting and that it likely needs a new motor. He said it was strange and looks like it hasn’t ever worked.
We’ve been told that it’s out of warranty by a year and our model doesn’t allow part replacement it needs a whole new system.
Do you think there’s any recourse with the manufacturer to resolve instead of getting a new system installed?
System is a greenwood Airvac
Hi Charles,
It depends how new the new build is. If you think it’s poorly installed you may be able to contact the builder under their defects warranty. MVHRs in new build flats are notorious for being poorly specified and installed unfortunately – often by the lowest bidder.
I think that it’s unlikely the manufacturer would see it as a warranty issue as from the manufacturer’s perspective it doesn’t seem as if the unit has been installed or maintained properly. That would be a prerequisite of a warranty claim. But no harm in contacting them – and I’m sure they would be helpful in replacing it for you with a like-for-like.
I may know a very good MVHR installer who could switch the units for you – please contact me via the Contact Us page at the top of the webpage if you do help.
Best wishes,
Patrick
Hi
Recently I noticed water drops from the air vents in my bathroom. I changed the filter 6 months back. Could you please tell me why this happens and what needs to be done.
Hi Jas,
As per my reply on your other comment, we’d need more information but it could be water vapour condensing on your vents, possibly because the vent is within a cold ceiling/roof/wall void. The air in summer is much more humid than in winter, which may be why it’s happening the last six months. It could also be a blockage within the vent stopping the warm, humid air leave the bathroom.
Please send an email through the Contact Us page at the top and we’ll reply directly to discuss.
Best wishes,
Patrick
Hi
Recently I noticed water dripping from the air vents in my bathroom. I changed the filter 6 months back. Could you please tell me why this happens and what needs to be done. Thanks
Hi Jas,
We’d need more information but it could be water vapour condensing on your vents, possibly because the vent is within a cold ceiling/roof/wall void. The air in summer is much more humid than in winter, which may be why it’s happening the last six months. It could also be a blockage within the vent stopping the warm, humid air leave the bathroom.
Please send an email through the Contact Us page at the top and we’ll reply directly to discuss.
Best wishes,
Patrick
Hi Patrick, I just came across this post and was wondering if you could help me with a question.
I just moved in into a flat, it’s a new build ( about 2 years old) with a [redacted MVHR brand name] system. As soon as I came into the flat I noticed a strange smell coming from the room where the ventilation system is. The smells becomes stronger over night (when we close our windows) and it lingers over the entry and our bathroom, possibly even our bedroom. It’s clear that the filters have not been changed as the controls in the panel are infested with mould.
At this point, I am trying to get someone to come and asses the condition of the ventilation system and conduct the maintenance required, including changing the filters.
My concern is that I suffer from allergies and asthma, and I am not sure how bad the problem is or how long it would take for my property manager to send someone to fix this.
While I wait for this to be taken care of, I was wondering if it’s best to turn off the ventilation system completely or to keep it on?
We turned it off one night, but the smell seemed to have gotten worse the next morning, so I am not sure how to proceed here.
Thanks in advance,
Laura
Hi Laura,
In most scenarios it’s best to keep the MVHR unit running.
It sounds like your MVHR unit has been poorly maintained. Changing the filters is something a homeowner can do – it doesn’t require a professional. Turning the unit off safely and cleaning it should also be fairly straightforward, although be careful.
If the smell gets worse when the MVHR is off it seems the MVHR might be helping to clear the bad smell.
Does the MVHR have a condensate drain on its base for water to drain to a waste water pipe? Does this pipe have a visible trap? It could be the air is coming in through this pipe and that’s why it smells bad.
You can contact me directly and I may be able to ask someone local to you to arrange a maintenance visit. My Contact Page is at the top of the page.
Best wishes,
Patrick
Hi Patrick,
We recently purchased a house with a MHRU, which we had never had before. The seller replaced one unit, which had stopped working. We soon replaced the other unit, which was 12 years old and stopped working. The problem is the pipes on the floor below the unit (in the wall) suddenly start making a terrible loud noise, usually at night, which goes on for 30-45 minutes. Not every night, but tonight is the first time it has been making this noise two nights running! What could it be and how can we fix it?
Dear Mrs Gould,
What does the noise sound like? Please feel free to send an email via the Contact Us page and perhaps I can see a video of the noise.
I’m inclined to think it’s not the MVHR ductwork making that noise, and I’ll explain why.
An MVHR system carries air consistently at very low flow-rates, which means it wouldn’t typically make any noticeable noise (and certainly not terribly loud). An MVHR also works at low level 24 hours a day, and there’s no reason it would suddenly ramp up for 45 minutes each night. For this reason it’s unlikely to be the MVHR ductwork making the issue.
Is it possible your hot water pipes or system are heating up or re-heating the thermal cylinder at this time of the day? It’s worth checking what systems are automated to turn on at night or re-heat and see if any of those are making the noise. It could also be a tank cooling down, or solar thermal pipes contracting at the end of a hot day.
Please contact me to discuss in any case.
Best wishes,
Patrick
Hi Patrick we have recently purchased a new home in the Netherlands with a mechanical ventilation (blue Itho box) already included in the loft (2nd floor). It has vents in the bathroom (first floor) and kitchen (ground floor). We have a switch in the kitchen with three settings in the dial (1, 2 and 3) which should allow to make more powerful or turn it down. However I don’t think the system is working and I cannot see any lights or settings on the blue box itself in the loft. The house is very hot and humid everywhere in all rooms. What can I do to problem check what is happening?
Hi Andrew,
A very simple way to check if the MVHR is working is to turn the system on to boost at the switch and hold a piece of tissue paper up to the valves – it should stick to the bathroom/kitchen valves and blow out from the living / bedroom valves. The valves should be relatively open with lots of free area to blow air through.
It sounds like there may not be power to the unit – is there a switch nearby? MVHR units are installed with a local fused spur in the UK.
If the unit is working but not driving enough air, there will be a way to re-commission the MVHR unit to increase its fan speed. Also, check that ducts are not accidentally disconnected or blocked in some way.
Let me know how you get on.
Best wishes,
Patrick
Hi Patrick,
We recently moved into a property that was renovated in 2010, it has an MVHR unit, and we have just had an engineer come out to carry out a service, see what was wrong with it, and basically teach us how it should be working/how to use it.
He ran a series of tests and the outcome is that the extraction side works well, but the supply side has a vastly decreased flow rate that is practically non existent at the end of the ducts. Now from this we can determine that there is a leak (or more likely lots of little leaks) however the entire ducting is either behind boarded walls or ceilings.
Is there a (simple) way to determine where the leaks are on the ducting (smoke bomb, firbe optic camera), or are we resigned to having to rip out and replace entirely?
Thanks
Michael
Hi Michael,
It’s not easy but there is a way to be fairly logical about investigating and minimising how many ceilings/walls you have to open.
The lack of air at the furthest ducts could be due to air leakage in the ducts or poor design (managmenent of air resistance).
Regarding the first possible issue, I’m going to assume you have a trunk-and-branch layout for your ductwork – difference explained here.
Do you know what kind of ducting you have? Rectangular PVC, circular steel or semi-rigid radial ducting?
If it’s jointed ducting (ie, not radial, which we use) it could be a little air leakage from every single joint in the system, or it could be that there is a major air leak – if, for example, a duct has fallen out or a previous occupant has accidentally knocked a pipe out of place. If it’s the first, you’ll probably have to replace the ductwork at some point.
However, because the extract pipes seem fine I suspect it’s the latter issue.
I would measure the flow-rates from the furthest valve and mark the point at which air stops or significantly slows down. If it’s a piece of ducting which has slipped out, it’ll be ahead of that valve.
If you don’t want to open up, at this point you could use a duct camera in the valve to see if something is missing.
It could also be the supply fan motor is failing and needs replacing. Or it could be that the supply motor needs increasing its fan duty.
I hope the above gives you a good start. If you want to speak anymore, please feel free to contact me via the link at the top of the page.
Best wishes,
Patrick
Hi Patrick,
thanks for the response. It is a trunk and branch layout from my understanding of the plans that I have managed to find in the online planning portal.
As for the type of ducting from what I can see near the MVHR it is Rectangular PVC jointed ducting. And whilst flow rates in one branch direction are satisfactory, on the opposite branch there appears to be very little flow rate by the 2nd valve!
I’m toying with getting an LED duct camera so will try one off amazon and see if I can spot a major slippage from inside.
thanks again
Michael
Hi Michael,
It’s either a joint has fallen out or the supply fan motor is failing. Good luck in the search and feel free to keep in touch and send me details of what you discover via my contact page at the top.
Best wishes,
Patrick
Hi Patrick….
My heat recovery system in not working…it coming up alarm on the screen and now today it not working ,i
Hi Francie,
I need more information to help, please. What is the brand name of the MVHR unit and do you have any photos? Please feel free to send me an email via the Contact Us page at the top of the page.
Best wishes,
Patrick
Hi, I live in a converted victorian 1 bed flat and in a conservation area in Islington. Is it even possible, or what would you recommend for a MVHR system with limited space (~40m^2 in total). Can a single unit in the bedroom mounted on an external wall be sufficient for the whole flat?
Cheers
Bhav
Hi Bhav,
An MVHR system can be very useful even for small properties. Even though the property may be small, the occupancy may still be two or three people – that could mean around 8 litres of water vapour generated by cooking, cleaning, washing, breathing, etc, and that moisture needs to be dealt with daily.
An MVHR system would exhaust the waste humid air and bring in fresh filtered air that’s been pre-warmed in winter. You’re right that the MVHR itself would be very small, often cupboard or ceiling mounted and features only a few small ducts to each main room.
Amongst all the different ventilation systems, MVHR is the gold standard for energy efficiency, comfort, health, moisture management and longevity, so it’s worth investigating. If you want to chat more please contact me at the top of the page with more information.
Best wishes,
Patrick
can an mvhr bring in smells from outside, i.e if the neighbours have a bonfire can it bring the smell of that fire into the rooms in the house?
Dear Leon,
Our MVHR systems have F7 (ISO ePM1) filters to remove carbon dust, pollen, insects and other nasties before they get into the home.
If a window was accidentally left open whilst the neighbour had a bonfire, it would take the MVHR only 2hrs to refresh the indoor air with fresh, filtered air entirely.
Best wishes,
Patrick
Hi there. We have a mvhr installed in our new build flat. There was quite an accumulation of condensation on windows so assumed the drain was blocked. When opening the unit, it appears the heat exchanger contained quite a large amount of water. the drain was not blocked. Is it common for the heat exchanger to be filled with condensation? Thanks in advance, Kai
Dear Kai,
This is not normal and should be investigated as soon as possible. The risk is that the moisture could touch the electrical elements (fan motors) of the MVHR unit and cause a shock.
I’ve emailed you directly to find out more information but it seems there is a blockage somewhere in the system which is preventing condensation from draining out.
Best wishes,
Patrick
WE moved into a house 2 years ago that has this system. Our neighbour is complaining as the susyetm is at the side of their property and is admittedly quite loud at times. Also they have a coal fire in the winter so it blows a smoke smell all around our house (petit justice I suppose). I would like to re-route the vents, is this possible without completely changing the balance of the system? At the moment it vents straight out the wall to the side. When we moved in the system was extremely noisy but I cleaned out all the vents and filters and the noise got a lot quieter, but still annoying. I don’t want to annoy our wonderful neighbours but believe it is a poor design. Thanks. Roger
Dear Roger,
I’ve emailed you directly as I’d like to understand more about the distances between the two properties and the lay of the land in terms of how wind hits the buildings.
In principle it’s not a very difficult job to move the intake/exhaust grilles, or a better option may be to simply add in a couple of our sound attenuators on the ducts to and from outside. This will limit the fan break-out noise significantly.
Best wishes,
Patrick
Hi Patrick
We installed an mvhr about a year ago (new build) but it’s only been operating since March. However after changing the filters the error message SENSOR ERROR has appeared & keeps recurring despite several resets (error takes a couple of minutes to come back). When it does the intake fan stops working. I’ve removed the sensor & cleaned it – it looked fine & the interior looked clean – but no change on reassembly.
Considering this is a highly rated unit I can only say how disappointed I am at this early failure; the only thing I’m wondering is if it’s possible the fact that the inlet filter was totally blocked could have caused the problem; I looked at the filters around June time & they were a bit grey, so I just vacuumed them & put them back. I can only think that the blockage occurred at harvest time when the combine was as close as about 15 m away, there’s no other sources of dust out here in the country.
I’m having difficulty buying a new sensor, being told different things by different people, though the official video (which I followed when checking the sensor) makes replacing it seem a straightforward process. I would appreciate any advice!
Kind Regards
Andy
Hi Andy,
We’ve spoken via email and your issue is now resolved. You mentioned you had purchased the MVHR from eBay – it could be it was being sold because it had this fault already, or it had been stored or left somewhere where it shouldn’t have been and the fault had developed as a result. In any case the manufacturer sent you a new sensor free of charge which you’ve installed yourself and the MVHR unit is now working well again. A great result.
As an aside, this is the benefit of technology companies building in a modular way that allows people to fix their MVHR systems in their own home. I hope more tech companies think about modular building of their products.
Best wishes,
Patrick
We recently moved into a new build and have had one issue after another with our MVHR (condensation, the fan turning off because the room was too cold even though it was 19 degrees, air moving in the wrong direction (into bathrooms/out of sitting room…) We’ve had the installers back over 3 times now and they seem to have run out of ideas on how to fix it. We believe there’s just a fundamental issue with how it was set up originally but the developers are currently refusing to get anyone else, other than the installers, over to look at it. Do you have any advice/suggestions on what we can try next? Thank you!
Dear Lucy,
I think you’ve correctly identified that the reason for the continuing failures is likely the installers themselves. And as your developer keeps using the same installers (who don’t seem competent at all by what you’ve said), they are unable to solve the very issues they have possibly created.
Speaking generally, new build properties tend to use the cheapest ductwork materials (flexible and PVC ducting – both of which we’ve banned from our projects), which are highly prone to failure due to tears and cracks in the (now hidden) ductwork. I’m not saying this is the case, but it could be contributing to the issues.
Condensation issues sounds like there’s little or no airflow getting to rooms. Reverse airflow sounds like they’ve mixed up the ductwork into and out of the unit. The fan turning off simply shouldn’t be possible – MVHRs are not triggered by the temperature of rooms.
I shall send you an email directly to request more information to try and diagnose the issue(s) ourselves, but ultimately I think best practice would be to ask your developers to use someone more competent to fix the system. In a worst case scenario that may mean removing walls and ceilings to replace faulty ductwork.
Best wishes,
Patrick
Hi Patrick I’m after some advice if possible after seeing the great advice you have been giving, my MVHR system seems to turn off if you turn on bathroom and cloakroom lights, the fan stops, then turn light off fan revs up again I’m guessing this isn’t correct? Any advice greatly appreciated
Hi Darren,
No, that’s not normal. I suspect it’s been wired incorrectly.
I think your electrician was intending for the opposite ( when the bathroom light turns on, the MVHR goes into boost) but this is also incorrect. An MVHR is designed to run at a very low steady speed over 24 hours at a ventilation rate designed to clear bathrooms and other wet rooms significantly better (and much more quietly) than intermittent extract fans. Boosting is not necessary except for very high spikes in humidity, at which point the MVHR systems we specify will boost automatically.
I would speak to your electrician and/or your home control person about setting up the system in a way more suited to the MVHR system.
Best wishes,
Patrick
Hi Patrick,
Moved into a new build with MHRV installed however just outside the warranty of 2 years a bearing went in one of fans and now makes a loud humming noise. Developer unwilling to fix it but the manufacturer says its impossible to fix as its a sealed unit and we a whole new unit. Can that be right. Seems very expensive rather than just fixing a fan/bearing.
Any help is appreciated.
Kind regards,
James
Hi James,
What unit is it? We always specify MVHR units which are designed modular, so that the fan motor or sensor can be easily replaced without replacing the unit. Although these units can be a little more expensive initially, they pay back in much lower disruption and maintenance costs in the long run.
Best wishes,
Patrick
I was wondering if you could assist me in trying to resolve a problem that presented itself yesterday. We had a power outage on Tuesday that lasted for 8 hours from 9.30am until 4.30pm. On Wednesday mid morning I noticed water on the table and floor below the MVHR – maybe around a litre or two.
I changed the filters: they were last changed in March and they did look rather grubby……… I made a mental note to change them more frequently. Water continued to drip from the front of the unit around .5l over one day.
Usually over the course of the day I can hear water gurgling down the drainpipe but at the same time as the power outage the temperature dropped by a few degrees and there is now no water gurgling down the pipe but as I mentioned before a small amount is dripping from the front of the unit. The humidity level is at an all time low – 50 and hasn’t risen since the temperature drop.
I’m wondering if perhaps some debris is blocking the drain pipe? How would I access this? Or maybe the large hexagonal filter/heat exchanger in the middle of the unit is wet/holding water and this is what is dripping out but then why isn’t it finding its way to the drainpipe instead of leaking from the front of the unit – mainly the Lhs corner but also dripping from the section of drainpipe immediately under the unit.
Could it just right itself in a few days? Should I continue to check for water inside the unit?
The Unit was installed by the previous owners. This is the first problem that we’ve encountered
Any advice would be greatly appreciated.
Dear Martha,
It could be a blockage of the condensate drain, or it could be that some very humid air was stopped up in the heat exchanger core temporarily during the power outage. The best practice would be to run the MVHR unit on boost for a few hours to thoroughly clear any moisture from the system.
If the MVHR unit is still dripping it could be the condensate drain is undersized, or that there is a blockage in the plumbing. During very cold weather the MVHR may dump more water vapour than usual, and this should all drain into the condensate drain. If that’s not happening then it could be an issue with the drain.
One way to check would be to remove the condensate drain and push a pipe cleaner or similar down it to check for a blockage.
50% Relative Humidity is perfect indoor humidity for health and comfort, by the way. Any level between 40% and 60% is comfortable and healthy for us to breathe, so it sounds as if your MVHR unit is otherwise doing a very good job.
I hope this help, but let me know if you need any more guidance.
Best wishes,
Patrick
Hi Patrick,
We live in NW London newly build block of flats. We been here for 6+ years. Yesterday I was watching TV in the evening and suddenly our [redacted MVHR brand] went dead. Strange noise was keep coming from the unit itself, but all lights where off. I immediately switch the unit off. This morning I once more removed and hoovered filters and did try to switch the unit back on. Result the same: no lights, but still strange noise, sounds like its trying to operate, but not happening. Is it anything you came across in the past? Any suggestions? Thank you in advance.
Kind Regards,
Egis
Hello Patrick,
I wonder if you might be able to shed some light on this please – I recently had a filter change in my MVHR unit, about 2-3 weeks ago.
I was then away for a week, but since coming back I’ve noticed that the unit is quite noisy. It never used to emit any noise, other than a very soft hum which I only noticed at night when it is generally very quiet throughout. But the louder noise is starting to bother me, and I am wondering if something could be wrong? I haven’t noticed any issues with performance, and I was also wondering if it’s possible that the very cold weather we’ve had recently could be affecting it at all?
This is in a new build (2020), 1 bedroom flat. I’ve lived here since June 2020 and never had any problems with it before.
Thank you
Ania
Dear Ania,
It really depends on a number of things – please could you contact me with more information through the Contact Us page at the top?
If it’s a good quality unit and one of the fans has failed, it should be a straightforward fix.
Best wishes,
Patrick
Hi Patrick,
Our MVHR unit just switched itself off last night while we where watching a TV. Strange noise come out of storage area where MVHR unit fitted. When I went to see what’s happening, all lights on the unit was off. I switched off from the switch. After 20-30 minutes tried to switch back. Unit started, but none of the lights where working and strange noise keep coming out of the motors, sounds like someone pressing and releasing accelerate pedal in the car. We live in North West London and been in this one bedroom flat from 2016 October straight after its been build. I do hoover filters regularly, at least ones every two month. Any advice would be appreciated.
Thank you.
Kind Regards,
Egis
Dear Egis,
Please email me to discuss, as it could be a number of things. I can help with more information but it’s possible the fan motors have failed.
If the unit is one of the brands that we specify for our projects with modular parts then it is a very quick process to identify the fan motor issue and replace if needed.
The cheaper MVHR brands are not typically built modular however, and some suppliers will tell you to replace the whole unit – which is poor practice from both a cost and environment perspective.
Best wishes,
Patrick
Hi Patrick,
I was hoping you had some advice on an issue I am having with my MVHR unit.
The Extract Fan is particularly noisy when having Boost on, and I think I have identified the problem: the extract fan itself is not firmly/properly attached, and is hitting the side when it spins, creating a horrid noise (not much on normal mode, mostly on boost).
We are out of warranty, and the company quotes close to £800 for replacement. This seems steep, and the fan itself is currently working, just out of place (maybe an issue with the bearing).
What do you recommend I do? – Is there a way I can fix it myself so the fan is more stable? – Or should I decrease the boost setting for extraction?
Please let me know if you have any suggestions – any advice appreciated!
Thank you,
Abs
Dear Abs,
As with Ania, it really depends on a number of things – please could you contact me with more information through the Contact Us page at the top?
If it’s a good quality unit and one of the fans has failed, it should be a straightforward fix.
Best wishes,
Patrick
Hello Patrick, my Mvhr unit is pulling in chimney smoke and smells from a neighbors wood burner is this normal for these systems? My intake and out take are at the side of the house at 10/12 feet of the ground.
Dear Anthony,
No, that’s not normal. Either the wood-burning stove flue has been installed too close/low or there are prevailing winds which are driving the smoke downwards. Can you contact me directly for more information please?
Best wishes,
Patrick
Hi I have this system.Ihave a vent in ceiling in each room.How does it work. I know it’s sucks the air out by the vents in ceilingbut how does it return clean warmer air back.
Dear Wayne,
The MVHR system has extract valves in wet rooms, and supply valves in the living areas and bedrooms. The MVHR unit extracts warm, humid waste heat from the bathrooms and kitchen, runs it through the heat exchanger core and exhausts the humid air outside. At the same time the MVHR pulls fresh, filtered air from outside, runs it through the heat exchanger core and it transfers the heat over from the waste air.
This now-warm air is returned to the home to the supply points in living rooms, so it’s both more comfortable and saving on heating costs.
The MVHR units do this at more than 90% efficiency. Short video here: https://youtu.be/eyS1GW57dEM
If you want to test if it’s working, the simplest way is to hold tissue paper up to the valve and see if it blows/sucks.
Best wishes,
Patrick
We have a [MVHR BRAND NAME REDACTED] in our new converted barn ( 18 months old ) and it is great! However we are getting an intermittent error E106 ( NTC sensor ) which closes the system done altogether. We have no idea what this sensor does or where in the unit it is — can you shed any light on this problem for us?
Many thanks, David.
Hi David,
I am familiar with the type of unit you have, and I have been in touch with the supplier to find out what the error code means. I’ll be in touch.
Best wishes,
Patrick
Hi…I have a [MVHR BRAND NAME REDACTED] MVHR unit that seems not to be producing enough suction from the dwelling…I have checked the ductwork sizing and have run a camera through and all seems well…
Could you kindly advise as to how I can check the performance of the unit in its factory condition…I have access to any tools that may be required in order to test…
in my case I have graph (Max) 250 Pa and 45ls could you explain with 25Pa the performance is 20ls (practically no pressure). I would have thought that the ls would diminish as the pressure Pa rises…Many thanks
Hi Jeff,
The MVHR unit you have is a budget brand (which we don’t specify). It uses very basic potentiometers at the front of the unit to modify airflow speed (they look like flat-head screwdrivers tops).
The problem with this unit is that you get no visual feedback on what speed the unit is running at, so it’s very hard without airflow measuring kit to work out what speed the system is at. By turning the potentiometers you may hear the fans ramp up and you can check.
I think ultimately your MVHR unit will need servicing by a technician like ourselves with airflow measuring equipment, and as part of this service we would check for any problems, blockages or control issues.
Best wishes,
Patrick
Hi there, so glad to have found this page, hopefully you can help.
Bought a new build flat in Aug 2020 and it is fitted with a [MVHR BRAND NAME REDACTED]. I have replaced the filters annually but the last few months the machine was so loud it sounded like it was about to take off. It’s time for a filter change but they seem to be OOS everywhere but suddenly the machine has completely stopped now. It has power, and you can hear it run slightly when turning on but then it just cuts out. According to my HA it is out of warranty and there is no external parts that could be an issue but I don’t want to cover a hefty cost. Is there anything I can do or check for to see what the issue may be and how to fix it?
Hi Daniel,
The MVHR unit you have is unfortunately by a supplier who has a habit of discontinuing parts for their system, which is why we don’t specify that brand for our projects.
Being able to source replacement filters for the lifetime of the MVHR system is vital for it to be cost-effective and work properly.
It sounds as if the fan motor has failed, possibly due to a blockage that caused significant strain that caused it to burn out. It’s going to need a technician to come out and service it and take a look.
Best wishes,
Patrick
Hello Patrick, thank you for th article.
I have a flat in London with what I think is an MVHR system. It is located in the ceiling of the storage room (which also houses the electrical boiler system). The unit is not making a noise when the switch is switche on. I have replaced the fuse in the switch panel, I have tested that a current is running through the switch.
From what I can see, the unit has a hexagon filter and 2 fans. I’m not sure what is wrong with it. If you can recommend any companies that have experience with these type of units and can help repairing it, that would be great.
Many thanks, Fred.
Hi Fred,
There should be a brand name somewhere to identify the MVHR unit. If you know this please contact us via the page at the top and if it’s a unit we service we’ll be in touch to help.
Best wishes,
Patrick
Hi Patrick, I have a HRV2 Greenwood whole house airvac in new build about 7 years ago (so no warranty left). I had recent service from [REDACTED] group which couldn’t identify why sulphur/egg smell coming from all air vents when bathroom/boost is on. Any ideas on source of issue?
Hi Jag,
My first port of call would be to check the condensate drain. It’s used to drain away any moisture dumped from the air when very cold outdoor air meets very humid indoor air.
The condensate drain is normally linked to a waste water pipe using a dry siphon. Using a “dry” rather than a “wet” trap is key because if the trap dried out it could let air in from the waste water pipe.
For our projects we always specify a dry trap.
With that explained, I wonder if the sulfur smell could be coming from the trap? How does it look? One option may be to temporarily open the trap with the dry siphon but let it drain into a bucket and see if the smell stops (remember to seal the waste water pipe also).
Please let me know how you get on.
Best wishes,
Patrick
Hi Patrick
I have been quoted a couple of MVHR system designs for our 145m2 self-build house. One of the designs locates the supply valves in the bedrooms and living area only and the extract points in bathrooms, kitchen and utility. The other design indicates all of the above valves, plus one additional supply valve in the ground floor hallway and one extract point in the upper hall.
What would be the benefit of having these extra valves?
Thanks
Hi Alex,
It’s difficult to say without the plans but we typically consider hallways as air transfer zones, where air meanders from extract points in rooms to supply points in rooms.
Hallways can also be considered protected fire zones for egress routes depending on the height of the property, and in that scenario you wouldn’t want any ventilation points puncturing this very important fire enclosure.
I’ve sent you an email as we can also provide our own MVHR design, supply and commissioning sizing and specification proposal for your project.
Best wishes,
Patrick
Hi Patrick,
My MVHR unit started making a loud noise a few months back that you can hear though the top floor of the house. I had it serviced and had the filters changed to see if this got rid of it but it didn’t. The person who did the service claims that the noise is normal but the unit has never been noisy until this year.
Thank you
Adam
Hi Adam,
That seems unusual, and it should be investigated. I’ll contact you via email but it sounds like something environmental may be making the noise and it’s travelling into the pipes.
I’ll send an email to discuss directly.
Best wishes,
Patrick
Hi Patrick, i’ve just moved into a new build apartment and the MVHR system seems to be pulling in dust through the vents. I’ve had to switch off the system but the developer is slow to respond to my concerns as I think it’s a health & safety issue (I have asthma). Any advice would be appreciated. Thanks
Hi Jules,
Dust in the valves is an issue that a poorly installed MVHR system, or one made with cheap flexi-ducting may cause. The issue is that there may be gaps or holes in the ducting, and the dust from the void space is getting pulled in and ultimately blown into the room. Cheaper MVHR units also have poorly-fitting filters which allow air to pass around them, stopping the filter from actually filtering the air.
That’s why we at HSL only ever use rigid or double-walled radial ducting in our projects, and we have o-rings and seals on every single joint in the ductwork system as standard. We have banned aluminimum flexi-duct from all of our projects for this reason forever.
I would persist with your developer that the MVHR ductwork in your apartment needs to be checked and ensure there are no gaps or holes in the ductwork. They can take out the room valves and look into the ductwork as a first port of call.
Best wishes,
Patrick
Hi Patrick,
I wondered if you could help with an issue I am having with my MVHR unit [an airflow DV72]. It seems not to be extracting, but it’s unclear why. We had a company to service the machine and we do think it may have become worse since then (they measured to extraction after taking the machine apart to clean the fan but not before, so it’s hard to prove). The company wanted to send someone back at further cost to try to locate the problem, which they said would need someone to look on the roof to find the external vent to see if it was blocked, and a camera to see if there was any blockage in the duct work. We are not overly keen on getting the same company back and I wondered if you had any advice – could they have done something when they opened the machine up? How might we remedy this? We do still seem to be getting air flowing in.
Thanks in advance (and what a great forum this is!).
Best wishes,
Edd
Hi Edd,
If they measured the extraction after taking it apart, what flow-rates did they record? Presumably they weren’t zero or they wouldn’t have left site. Did the extraction rate seem to drop after their visit?
A simple test would be to hold tissue paper against a bathroom or kitchen extract point – the tissue should hold easily with the MVHR’s airflow.
We’re always exceptionally careful when our HSL technicians open up an MVHR unit, and it’s possible they may have done something wrong, but surely they’d have noticed if the fan weren’t running before they left?
It could be the roof cowl is blocked by seeds or leaves – is there any way for you to check this safely?
Are the filters clean?
It could be a blockage within the pipe, in which case I’d check every extract valve and see which work and which don’t and then trace it back.
I hope this helps but let me know if you need any more details.
Best wishes,
Patrick
Hi Patrick,
Thanks so much for all your informed answers to queries above. I have found it very interesting reading up on everything.
We have a Vent Axia Sentinel Kinetic Plus B in a new build and have noticed that there appears to be something blackening the roof slates that surround our extract vent. Filters are changed annually and cleaned every six months. I was hoping you might be able to shed some light on what could be causing this phenomenon?
Keep up the great work.
Sincerest thanks and regards,
John
Hi John,
Thanks for your kind comments, I’m glad to help.
I’m not sure exactly what it could be but if I explain what I think is happening it may help you decode it.
The exhaust vent of your MVHR system is pumping out waste air from the home. Typically it’s a bit more humid than ambient air, and a bit warmer. So the black area could be soot or dust from the home that’s getting pushed out onto the slates. Or it could be humid air that’s wetting the slates, prompting moss or mould to form around that area, or allowing soot to stick and settle from nearby chimneys.
That would be my best guess as it is an unusual phenomenon, so please let me know if you manage to get up there and work it out.
Best wishes,
Patrick
Hi Patrick, Thanks so much for the answers to the questions above. I couldn’t quite find one that relates to our problem so would really appreciate your advice.
We have noticed some stains on the ceilings in two bedrooms. These appear as diffuse marks and in some of the areas seem to run in a line with an unaffected area (approximately 1cm white line) running through the stain. The affected rooms both have bathrooms on the other end that have extractor vents and one is an ensuite and the other room is separated by a corridor with the bathroom opposite.
At first we wondered if it could be a water leak from the flat upstairs or due to thermal tracking. We live on a busy main road and occasionally open the windows. The vents are clean and the MHVR filters have been changed. Do you think this could be due to insulation issues and is there any possibility that this could be a leak from the MHVR ducts or pipes ? Thank you.
Hi Nadia,
It’s really hard to say without looking myself, but the 1cm white line seems like a clue to pursue. If you look inside the MVHR supply valve, is there anything inside it that would interrupt the airflow (eg, a piece of the valve arm that secures the valve to the socket)? If the stain is caused by the airflow from the MVHR then the white line would align with the valve arm inside.
Otherwise it could be due to dirty air being pushed against the ceiling, or could it be something completely different – as you say, a leak or a cold spot due to missing insulation in the ceiling above.
I hope this at least helps prompt some more investigating and let me know the results.
Best wishes,
Patrick
Hi, I have MVHR in new-built (built in 2015). I can hear that it works and there is air flow from the ceiling valves, however the green light is off as well as all of the other lights. I remember that it is on when I just moved to the flat in 2019. I have tried switching it off the power and switching back on – however still no green light. Could you please advise on the possible issue?
Thank you,
Marija
Hi Marija,
Without knowing which brand it is, I can’t say whether there is an issue. If air is blowing through the green light may have been a temporary illumination when the unit was being commissioned / boosting / turning on for the first time.
It doesn’t sound like there’s an issue but please send me further details or even a photo of it via our Contact Us page and I’ll be sure to reply.
Best wishes,
Patrick
Hi Patrick,
I hope you could offer some advice. The humidity in our three bed flat is currently around 59% in the summer with indoor temperature around 26 degrees celsius. I feel that the extractors in the bathrooms are just not extracting as I would like but I’m not an expert so not quite sure how strong the airflow extraction should feel.
The MHVR system we have is the [redacted] which was installed in 2016 as part of a new build. It’s boxed in the boiler room so we don’t have access to any controls. Does this model have controls and is there a way to change the settings and extraction rate ?
We also recently changed the filters from what I think are G3 filters which were provided with the unit to M5 for both intake and extract as we live in an area with car emissions and building work outside. Is this choice of filter suitable or could it cause problems by affecting the air flow rate and humidity levels and cause problems due to increased workload on the MHVR unit ?
Thank you
Hi Ross,
That unit isn’t one we recommend, as unfortunately if one part of the system breaks the manufacturer will tell you to replace the whole unit. The MVHR units we specify are built modular to prevent this.
Yes, there should be controls – is there nothing on the unit or nearby to allow you to boost the system, for example?
Have you tried to hold tissue paper up to the bathroom valve? If it’s flowing well it should hold the tissue with ease.
Changing the filters for higher grade versions will affect its performance, yes, and it could be causing the fans to slow. That’s why our MVHR units have “constant flow” technology, so that as the fans saturate with grime the fans ramp up automatically to accommodate until the filters are refreshed.
If you want to improve airflow but not affect the system performance one tip would be to only put the M5 filter on the incoming air side, and leave the G3 filter on the room-side.
Let me know how you get on but it may be time for a service – a little earlier than scheduled but it may not have been set up correctly since installation.
Best wishes,
Patrick
Hi Patrick,
This is a great service you provide, thank you. We have a renovated airtight bingalow in Dublin Ireland (A2 rated), with a Hitachi Yutaki air-to-water heat pump and a [redacted] MVHR.
Last winter, our first with the new system, we noticed that the MVHR was blowing relatively cold air, sometimes as low as 15C, despite the extracts from wetrooms being approx 21C. We could see from the MVHR app that there was a significant loss of heat by the time the air got to the MVHR unit, located in a cold attic. After a long period of inaction on the part of the installer, it was discovered that much of the ducting from the wetrooms was very poorly insulated (it was out at the edge, very near the eaves and not easily seen. So the builder put in a lot of insulation and the issue is significantly improved. Last night, for example, the external air temp was 10c, the wet room extracts were 20.5, but the air at the intake on the MVHR was 17.8 and the air being circulated was also 17.8. As nights get colder, we fear the MVHR will blow cold again, with consequent discomfort and placing more demand on the Hitachi heat pump to maintain temperature. Any advice or thoughts?
Hi Andrew,
It’s really good that you had the ducts insulated, because being in a cold space it is vital that they are kept as warm as possible.
I think the difference now between your outgoing temperature from rooms and the air coming back in with heat recovery is down to the heat exchanger efficiency itself. I’ve redacted the brand name in your comment, but I’ll admit this MVHR brand has a lower efficiency at 70-80% (it’s always given as a percentage) than the MVHRs we typically specify which are 96% efficient.
Saying all that, I think 17.8 DegC is satisfactory, with your heating system only having to “top up” the warm air to 20 DegC.
Also, please keep in mind that as the outdoor temperature drops the MVHR heat exchanger becomes more efficient (not less) because the movement of heat is much faster when the temperature gradients are larger.
I hope this helps but please let me know if you have any more questions.
Best wishes,
Patrick
Hi Andrew
I’ve recently moved into a flat that has a [redacted] which was installed when the block was built in 2016. Before I bought it, the surveyor identified the unit as being very noisy!
I negotiated a small reduction because of this and bought the flat, which I really like.
My excellent handyman who I trust has found out that one of the 2 fans is faulty and making the noise. I’m wondering if it makes sense to just replace the noisy fan, which will be about 20% of the cost of a new replacement unit. Does that make sense to replace the fan in your opinion?
Thanks very much for your help (v impressed with your website).
Peter
Hi Peter,
Yes, you can replace the fan, but bear in mind that both fans have been on together for their entire lifetime so the other one may not be far gone also. Perhaps you can ask for a better deal for the pair?
However, it could also be that this fan is under more strain than the other fan due to a blockage somewhere in the system (eg, leaves blocking the external grille), and this is what has caused the issue. As part of replacement I would have the system checked over to make sure the new fan won’t also get fried. This would be a “re-commissioning” service using an airflow meter to measure the flow-rates at each valve and adjust as needed.
Best wishes,
Patrick
Hi Patrick
Thank you for a very informative article, and for your responses to the diverse range of questions.
My query is hypothetical, as we are currently building a passive house which will obviously have an MVHR system.
I understand the theory behind the system extracting damp stale air and replacing it with filtered fresh air. However, I read a comment somewhere about the ‘fresh’ air in rooms feeling ‘over dry’. In particular I am concerned that the bedrooms might feel uncomfortable for sleeping. Some of our bedrooms have windows which will be hard to open, so we will be wholly reliant on the MVHR system for fresh air.
My fear is waking up with a dry throat(!), because we will be breathing air overnight which is much drier than we are used to!
Is my fear unfounded, and if not, is there a way to introduce some limited, controlled moisture content to incoming air, to prevent this situation?
Thank you.
Niall
Hi Neil,
My website often attracts comments from people with MVHR problems asking for help, so the comments section tends to skew towards the negatives of MVHR – but there are many benefits to MVHR and people like us are trying to promote good standards of workmanship and more education about the higher performance MVHR systems in the daily work we do.
The good news is that a properly designed MVHR system such as we design at Heat, Space and Light Ltd is carefully optimised to remove excess waste humidity at a level that stays comfortable and healthy for the occupants all year round. So you won’t have overly dry or overly humid air.
Generally speaking, even if you did find that the air was overly dry with an MVHR system, the solution would be simply to lower the ventilation rate and/or increase humidity generation (ie, drying clothes, indoors, steamier showers, cooking more, etc). This would balance the humidity levels and return the indoor air to a 50% relative humidity that we as humans find most comfortable and healthy.
I hope this puts your mind at rest that you are doing the right thing by choosing a high performance MVHR system.
Best wishes,
Patrick
Hi, I have the hr155wm greenwood airvac, I’ve changed the 2x media filters and have reset it, but it’s making a whirring sound now. As in I can hear the purifier sounding much louder now that I’ve changed the filters. Do you know what might be wrong?
Also, the vent nearest to the kitchen seems clogged, the dust inside feels like a thick layer of Velcro. I can’t see the tin foil piping that I can see very clearly in the other 2 vents. Could the dust clog in the vent be the cause of the loud whirring sound coming from the purifier? It wasn’t this loud before I changed the filters!
There is also no green light showing as it did before I changed the filters and reset it.
Hi Mihir,
Did you turn off the MVHR unit when you changed the filters? This is important because if air is flowing then dust can get into the heat exchanger core. It’s simple to remove and clean the heat exchanger core – you can pull it out and wash it clean with warm, soapy water in a bath-tub.
It could be the kitchen vent is very clogged with dust, and this would also cause an issue. Is it possible to clear the dust and clean all ducts? For our systems we include a grease filter on the kitchen valve to prevent dust and grease ingress.
Please feel free to contact me back if you have any more questions.
Best wishes,
Patrick
Hi Patrick,
I’m hoping you can help me, we moved into a new build home 1 week ago and have a [redacted] unit installed.
The unit is installed in the attic of a 2 storey house, however we can hear the unit at night turning on and off repeatedly, turning on for 4 seconds and then off again immediately, this can go on for 1.5 hours, repeatedly turningon/off. It happened on night 1, and again 6 nights later. The repeated on/off is keeping us awake! Why is the unit doing this and can it be set to only do this on daytime run hours?
Thanks in advance
Niall
Hi Niall,
I’ve redacted the brand name in your message because all I have to say is that it is in my opinion a very basic, cheap MVHR system that’s prone to noise issues.
Without looking at the unit I don’t really know why it’s making such strange noises. It could be the fan motors are failing, it could be that it’s been set on some humidity / sensor trigger, or it could be an electrical fault.
I would recommend investigating with the building contractor / sales agent first of all and asking them for the commissioning certificate to review. It could be they need to replace the MVHR unit, which should be simple if it’s under warranty.
Do push them on it, and don’t be fobbed off with an excuse – MVHRs should be silent or near-silent when in operation.
Best wishes,
Patrick
Hello,
I was wondering if you have any advice, I have a [redacted brand] MVHR installed in our attic, connected to a control panel on the wall. There seems to be a problem with the sensors, as the humidity value has disappeared, as has one of the temperature recordings, and I suspect the second temperature recording is incorrect as it is reading 24-26 degrees Celsius and I’m sure that is supposed to be the temperature of the air from outside. I am sure the system cannot be running effectively. I am not sure where the sensors are to check them, or if I need to have the whole system checked. I am in a new build house and have tried to contact the electrician who installed the system but I am struggling to get someone to get back to me about it.
Hi Rosie,
It sounds as if the sensors that check the external / internal humidity and temperature have failed. They are usually inside the machine housing, between the top ducts and fan motors. I would try speaking directly with the manufacturer’s technical team, as for the MVHR brands that we use (not yours) it is very simple to change a sensor. It takes around ten minutes.
If you can’t get in touch with the manufacturer please feel free to contact me again and I’ll investigate myself.
Best wishes,
Patrick
Hi Patrick,
I have a quick question, I have a [redacted MVHR brand] for the last year in a new build, yesterday I noticed a trail of water down the gable from the extract vent outside and also condensation inside the unit when I took off the cover, tonight the unit has shut off and displayed error code 02 which is extract motor not running, any idea what is up with it? Condensate drain is connected and clear but unit sounded like it had water in the fan compartment when it was running, thanks, alan
Hi Alan,
Has it been raining heavily where you are? It could be that driving rain has flooded the MVHR unit if the ductwork doesn’t have a slight fall to the outside. If that has wet the fan motor it could have fried it, which may mean a new motor is needed.
It doesn’t sound like a condensate issue if your drain is clear.
I would turn the MVHR off, dry it out with the cover off the front and see whether it’s working again. If not, it’ll need a replacement motor. The MVHRs we specify (not this brand) are easy to replace but you may find this one is difficult, unfortunately.
I hope this helps, let me know if you have any follow-up questions.
Best wishes,
Patrick
Hi Patrick,
Are there any not so expensive devices for checking efficiency of heat exchangers along with flow rate. My problem is that during winter, i feel cold draughts when i am sitting around/ under the duckts. the house was rebuilt and is very air tight with no draughts from windows etc.
The unit seems to work fine as no mould or damp or stale air in house even if closed for a month.
I was expecting air to be warm but it feels cold on bare skin.
Any other things i should check.
thanks
Hi Nav,
That doesn’t sound right – a good MVHR exchanger such that we design and supply should return the air at nearly indoor ambient temperatures. If it’s 20 DegC in the home the air coming in via the MVHR should be around 19 DegC.
There’s an image on our FAQ page which shows how a good MVHR will take cold outdoor air and warm it up.
Does the MVHR unit have any data you can read on the main panel? Again, a good MVHR unit should display this so that you can check it (and also see the humidity levels).
Please feel free to contact me in more detail via the Contact Us page at the top and I’ll investigate. It may be that your MVHR unit needs servicing, which we can help with.
Best wishes,
Patrick
Hello Patrick,
We moved in to a new house in 2021 and had a heat recovery system installed by the builders and commissioned by a Joules engineer – he told us to let the settings at 1 all the year around which we did. From our 1st winter we noticed that the air coming in was noticeably much colder from all vents – this has significantly worsened in that Its almost at outside temperature – we have to turn it off at night. We turn it on again during the day esp when showers are taken etc…. Also, from the 2nd winter we noticed 2 small water stains after frosty night surrounding 2 extractor vents in our kitchen which are located on a downward sloping ceiling. This winter the latter problem has significantly worsened, we would appreciate any advice. The filters have been changed as per recommendations. Many thanks.
Hi Patrick,
If the air coming in to the MVHR unit is cold, and you’ve had condensation at the extract valves I think it’s because your ducts above the ceiling are in a cold space but uninsulated. Uninsulated ducting is causing transmission losses. In this scenario the air starts warm in the MVHR unit but then cools as the ducts run in a cold space before it gets into the rooms.
Our MVHR units bring in air almost at room temperature when working correctly.
Can you have a look in the loft and see if the ducts are exposed / uninsulated?
Please let me know if you want to discuss in more detail – my contact details are at the top of the page.
Best wishes,
Patrick
The
Hi Patrick, I was wondering if you could shed some light on an ongoing problem. We live in a block of apartments with MHVR system and ceiling air con. There is an odd smell in one of the bedrooms which I suspect is coming from the MHVR. It smells like smoke and I’m convinced the smell is somehow coming from the communal corridor (although bedroom is located far end from the front door). I can’t definitively smell it in the other rooms. The problem seems to have started when the new tenants moved in next door and smoke cigarettes inside their flat and it can be smelt in the corridor. If I use an odour neutraliser and open the communal windows the smell in the corridor and the smell in the affected bedroom disappears. The MHVR filters have been changed. Is this even plausible ? The system is a Silavent HRX with a Domus dry condensate trap. Many Thanks.
Hi Natalie,
The MVHR unit wouldn’t have any airflow connections with your communal corridor, so there wouldn’t be a direct pathway for smoke to travel from there into the MVHR.
But even if the MVHR was pulling in smoky air, the air from extract vents never mixes with the incoming supply air (only the heat moves across), so smells and smoke would be exhausted directly outside.
Because the smoke is only affecting your back bedroom I think more plausible may be that your ceiling void is shared across the communal corridor and that potentially smoke is getting in through gaps in the ceiling and into the bedroom through this route. This shouldn’t be the case but there may be a pathway.
Long term the best solution would be to stop your neighbours smoking in the hallway, as it’s both anti-social and dangerous if it caused a fire in what is your only escape route.
I hope this helps.
Best wishes,
Patrick
Hi Patrick,
I hope you can advise us. We purchased a property 2 years ago and refurbished it totally (creating a sizeable extension as well as refurbishing the old part) and have installed an MVhR system. At the time of the purchase we had a very bad moths infestation (old carpet). We treated the moths twice in the old property (via professional pesticide); 2 years later, having had the property fully gutted, we are ready to move in but are finding moths in the property (50 per day on average). The mvhr system has been installed last October but only activated 3 weeks ago, and the moths problem is now getting out of hand. We have hoovered, cleaned, and the mystery remains. The house is totally empty. All insulation is man made. We have no soft furnishing at all in the property so in principle no food for the larvae to thrive in). Having done lots of research, only the month larvaes feed and they only feed on natural fibres that contains kératine (eg cotton, silk, leather, hair, fur etc…).
Some rooms are worst than others (even though the moths appeared more on the old site of the building) and interestingly enough where the ducts vent air out. In the new part where I have many intake ducts, (kitchen, utility, wcs) we have very few moths. I have placed hormone traps (releasing hormones to trap the male ) in two of the worst areas one by an mvhr vent and the traps are covered in male moths.
When I look at my filters: the filters to the outside are ok, trapping spiders, bees etc…. But the filters to the inside have lots of moths in them. I have cleaned them daily and I trap 10-15 moths in them each time…. I am about to do a heat treatment to kill all the moths stages, adults and larvae (at a very high cost) but I fear it may not work if the mvhr pipes are harvesting the moths.
Now my question: could the moths live in my mvhr system and being blown out on a regular basis in the rooms? And if so, how do I solve the problem?
Thank you in advance!
Chris
Hi Chris,
This is a really strange case but I hope I can help.
There are two bits of information which are particularly relevant, both related to the filters.
First of all, the filters are trapping spiders and bees. Are the moths smaller than these insects? If not, then surely the filters should trap those too if they are coming from outside. Do you think the moths are coming from outside?
Secondly, the fact that there are many insects on the inside (room-side) filter means that the insects are coming from the home, not from outdoors (like the bees and spiders).
I guess there is a possibility that the moths are somehow inside the ducts or the MVHR itself, but there’s nothing made from keratine in there.
I’m speaking speculatively but I wonder if there is still something in the home that is feeding the moths?
If you do feel the MVHR is sucking them in, and they’re getting past the filters, then I would check for anything around the external grilles.
I’m curious as to the conclusion here so please do feel free to contact me to discuss.
Best wishes,
Patrick
Hi Patrick,
Thanks for your reply. I m not sure how to check the external grilles but we are finding 10-15 moths everyday in the filter. We know the moths are coming from the home but there is absolutely nothing of natural fibre to feed the larvae inside the home, and yet I am now finding very small moths as well as big ones….so the mystery remains…. Can we flush the mvhr system by any chance? Or is it safe to put insecticide in the system?
I am thinking of using thin mesh like tights on the vent in one specific area to see if the moths come from those vents…. Or set up a small camera to see?
But I am running low on ideas.
Thanks
Chris
Hi Chris,
I think putting a thin mesh over a valve will at least help isolate if it’s something inside the MVHR because moths would get caught in it on the duct-side if so. I wouldn’t recommend putting insecticides through it if possible, as this is the fresh air system for the home. There may be some natural remedies though (lemon, vinegar, alcohol) which won’t have any long term issues but as you can probably tell it’s not my specialty.
Please let me know if the above helps, and/or if you find the source.
Best wishes,
Patrick
I live near an airport, would the air inlets bring in noise from the outside? Is the noise completely dampened by the time it reaches the rooms in the house?
Hi there,
Yes, the MVHR systems we specify have additional sound attenuation to prevent fan noise from reaching rooms, so yes, it will dampen outdoor noise also as it’s on the same ductwork.
MVHRs also prevent aircraft noise because you can keep the windows shut when sleeping and still have healthy airflow via the MVHR system.
You can also generally build the home more airtight, which again will prevent noise ingress from gaps and cracks in the building fabric.
All in all, an MVHR system is a very useful ventilation strategy for homes with outdoor noise issues.
If you’d like for us to suggest an MVHR system for your property / project please contact me via our contact form at the top of the page.
Best wishes,
Patrick
Hi Patrick.
My heat recovery system keeps switching on the boost randomly and then stays on for hours on end. Do you know what it can be? The system filters are clean. Can you recommend a company that will fix faults near the Southampton area?
Thanks in advance
Hi Maria,
The MVHR system would automatically boost if it detects high humidity. We’re in the “slightly warm, very damp” season at the moment so it may be temporary whilst external humidity is quite high.
But it could also be because the standard airflow speed is too low, and the MVHR is having to boost to “catch up” to remove sufficient water vapour from the home.
We cover the Southampton area so it may be worth booking in a servicing visit with us if it has been four years since it was serviced.
Please contact us at the top of the page if you’d like us to service the MVHR system.
Best wishes,
Patrick
Hello Patrick.
We moved into our then newly built detached three bed house in June 2020. A Vireo HR185WM MVhR unit was fitted in the loft space as part of the houses construction and has performed well along with the well sealed and insulated house.
I have renewed the units filters annually and vacuumed the heat exchanger matrix and inspected the associated seals, although the matrix looked to be very clean on each inspection.
I have noticed that the initially almost silently running fans have developed an increase in noise level especially the extract fan. also with the matrix removed (during filter exchange times) the fans impellers can be seen to have a dust like deposit attached to their surface. The extract impeller also has a degree of play, small but not observed in the supply fan.
Any experience or advice you can provide would be very welcome.
Hi Dave,
It sounds like the fan motors / scroll housing need a clean. Zehnder / Greenwood recommends servicing the system every four years, which is something we can do.
As part of this I’d also check the intake and exhaust grilles are clear and clean.
Our servicing would also include a re-measuring and re-balancing of the room valves, as these could have been knocked which may affect airflow.
If you contact us directly we can help organise MVHR servicing.
Best wishes,
Patrick
Hello Patrick,
We moved into a new build house 3 yrs ago as second owners to the now 7 year old home.
In October we had the vents upstairs serviced (downstairs replaced) and were told while there was a quiet hum from one of the vents that they were ok and the engineer cleaned and serviced them, over the following few weeks the noise increased to a point where all the vents upstairs are making loud hums and need to be turned off at night due to the noise. So what I want to know really is if it’s just a coincidence that this has happened or could some part of the service have caused this.
Hi Nicole,
It’s really hard to say if the servicing affected it, as it’s an unusual scenario and it depends what they did for servicing.
Have you checked the filters – are they clean?
It’s not ideal to turn off the unit at all – have you contacted the company that serviced your MVHR system to ask?
Best wishes,
Patrick
Hi
I have moved into a new build flat a month ago, I have today checked the filters on the MVHR system and one of them is covered in what looks like black sticky oil/tar and needs replacing already, is there something wrong with the system or could this be pollution? I overlook a canal
Hi Kim,
It sounds like it could be grease, potentially from your kitchen. Is there a grease filter on your kitchen valve to prevent airborne grease ingress into the MVHR ductwork? There should be.
It could also be pollution from outside, in which case the filter is doing its job keeping it out of your home.
Hi I have recently moved into a house 10 years old which has a MVHR in every room. Main filter just been changed but the fans each room haven’t. . They seem to have a lot of fluff around the edges in some of the rooms. They vendor said don’t take these out as it will effect the settings but surely these need to be cleaned? If so how?
Hi Rob,
If the valves were commissioned correctly the balancing plate should be locked with a bolt at the correctly adjusted length behind the plate. This means you can dust them without affecting the calibration. You can also unscrew them from the ceiling by holding the outer edge to clean behind them if needed.
If the valves are loose then the system either wasn’t commissioned or wasn’t commissioned properly, and I would recommend a servicing to get it re-balanced correctly and locked. This is something we can do if you want to get in touch.
Hi Patrick.
I live in an apartment with a Nuaire MRXBOX95-AB MHVR. We have heard a rattling noise from the unit for some time so I took the front casing off and discovered one of the supply fans has come loose (the anti-vibration rubber mounts seem to have broken apart) so the fan still turns but brushes against the casing making a racket.
I spoke to Nuaire who advised the only thing they could do was replace the whole unit which is going to cost £1500+. I just wondered if you had any thoughts on if it would be possible to replace one of the supply fans, or even the anti-vibration mounts holding the fan in position?
Thanks!
Hi Joe,
I think unfortunately the MVHR can’t be fixed if the internal casing is broken.
This is why I personally only specify MVHR units which are built modular, so if a fan motor fails it’s very easy, quick and cheap to replace.
If you do replace the MVHR I would look for this type of modular MVHR.
Best wishes,
Patrick