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.

Patrick

32 Comments

  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] 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.

  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,

      Patrick

  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?

    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

  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
    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

  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,

      Patrick

  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,
    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

  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,

      Patrick

  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,

      Patrick

  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 [@] 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

  10. Hi Patrick

    We’ve recently installed a Vent Axia 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

  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,

      Patrick

  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,

      Patrick

  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,

      Patrick

  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 MVHR eco Nuaire 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

  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,

      Patrick

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