Five most common problems with Mechanical Heat Recovery (MVHR) Units

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:

  1. through the fabric elements as it passes through our walls, windows, doors, floors and roofs
  2. 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 2021.

Aren’t airtight homes stuffy and smelly?

Not if you install some form of mechanical ventilation.

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.

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 air from outside to warm that air up. That warm, clean air goes into the supply rooms (living rooms, bedrooms and dining rooms).

MVHR schematic showing how heat and air flow within the heat exchanger core to warm the home



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.

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 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:

  1. Poor initial MVHR design (no proper airflow rate calculations, so the fan isn’t optimised to push and pull air through the property)
  2. Cheap ducting that is easily crushed or torn, or a cheap MVHR unit – stay away from the very cheap ones, they do nothing.
  3. 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)
  4. The MVHR hasn’t been commissioned properly (so each room has not been individually balanced to ensure proper airflow rates to each room)
  5. 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)

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, that’s the future of home heating.

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.



  1. 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], 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.

  2. 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,


  3. 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?


    • 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,


      • 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,


  4. 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

    • 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,


  5. 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,


  6. 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,

    • 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,


  7. 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,


  8. 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,


  9. 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 [@] 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,


  10. 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,


  11. 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,


  12. 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,


  13. 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,


  14. 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,

    • 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,


  15. 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,


  16. 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,


  17. 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?


    • 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,


      • 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

        • 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,


          • 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,


  18. 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?



    • 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,


  19. 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,


  20. 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,


  21. 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,


  22. 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

    • 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,


  23. 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,


  24. 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,


    • 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,


      • 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,


  25. 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,


  26. 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,

  27. 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

    • 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,


  28. 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,

    • 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,


  29. 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,

    • 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,


  30. 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,


  31. 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:

      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,


  32. 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,


  33. 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,


  34. 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,


  35. 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,


  36. 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,


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