What puts people off about MVHR systems? Cleaning ducts, airborne viruses and maintenance explained

In my role as the lead MVHR technician at a Passivhaus / low energy design building consultancy we have received lots of questions and queries about MVHR systems. I’ve collected together my responses to some of the most pressing concerns that people have with a home ventilation duct system – from airborne spores to cleaning, maintenance, kitchen extraction and financial payback.

Residential MVHR systems – and the ducting they require – are a new concept for UK households, and it is entirely reasonable to be sceptical or concerned about them if you are building or renovating to very energy efficient standards. If you have any concerns that you’d like me to answer please write to me in the comments – I answer every comment I receive.

Please don’t be put off MVHR by these horror stories – we can design, install and commission MVHRs which will supply healthy, filtered, good quality air. If you’re researching this subject because you are considering an MVHR system for your project and would like to receive a free pricing proposal for a fully safe, healthy and compliant MVHR system, please complete the form here with your details.

“There are many stories of poor installation and consequent problems. How can this be avoided?”

Our company has a network of trusted installers that we recommend. The Heat, Space and Light MVHR team conducts a 36-check Quality Assurance (QA) service on our own projects if installed by others. We are also not against having a client’s builder/plumber/electrician install the ducting themselves, and we make sure they are supported with guidance documents.

Poor installation usually stems from a lack of proper design, using cheap and easily-damaged materials (eg, flexiduct) and no QA process. We create a full 2D and 3D visual document of the detailed MVHR ductwork layout, we use high-quality semi-rigid and steel ducting and we check everything installed is as per the design.

“How do you avoid ducting getting dirty over time, and how do you maintain it?”

The semi-rigid ducting we use is smooth internally, which will limit dust sticking to the walls. The ducting we design in a radial format is a single length from valve to manifold, so it can be cleaned with a pipe cleaner.

We put filters on each extract valve (where dust from the room will naturally accumulate) which can be taken out easily and cleaned/replaced to keep dust out of the ducts. We also put a grease filter on the kitchen valve to keep grease and dust out of the ducts. Finally, the unit has F7 (ISO ePM1 >50%) filters on the fresh air intake which keeps pollen, carbon dust and other very small particles from entering the system/fans from outside, and these can be replaced and cleaned also.

“There are family members who remember well the issues of sick building syndrome. How can this be avoided?”

Sick Building Syndrome has many causes, but the main problems which cause health issues are excess humidity, excess VOCs (chemicals from glues, cleaners, etc) and excess CO2. These are problems caused by no or low ventilation in buildings.

Proper continuous ventilation prevents build-up of any of these problems, and an MVHR ensures fresh filtered and tempered air is consistently supplied, whilst extracting waste humid air that contains CO2 and VOCs.

Using Low VOC paints and cleaners, and avoiding candles and air fresheners, is also a good strategy – no matter what type of ventilation system you have.

Another cause of Sick Building Syndrome is standing water in the ductwork, causing rust, mould spores or similar nasty viruses. The pollutants are then carried around the building to infect inhabitants.

Rusted condensate drain in poorly-installed MVHR system

Rusted condensate drain in poorly-installed MVHR system – not one of our systems

In an MVHR system, however, the air is always moving – the unit is never turned off completely (instead, it has a Holiday Mode speed which turns it down very low when the occupants are away).

In the above horrific image the MVHR was installed in a basement that had flooded, rusting the base of the unit.

There are no dead legs in a radial MVHR system, and generally the system will be completely dry – cold air brought into a warm home will be naturally dried out. The condensate drain is a dry trap. All in all, then, there is little chance of mould forming, and the risk of legionnaire’s such as in water-based air conditioning systems (that offices have) is entirely minimised.

“How do you clean the ducts – it seems a really tough challenge?”

With a radial system it is one continuous duct, so a long pipe cleaner can do it easily. With a branch system (lots of bends) it’s more difficult but still not impossible. This video shows how straightforward it is to connect and disconnect ducts from the manifolds:

“Who carries out maintenance (outside of filter changes)?”

The manufacturers have dedicated technical teams, or generally the original installer will carry on the maintenance, but some won’t offer it. MVHRs are incredibly simple machines (basically it’s just two high performance fans, a few sensors and a touchscreen) so there’s not a lot to go wrong. What’s more, the higher quality MVHR units are built modular, so if a fan does fail it’s straightforward to replace.

“Do different rooms need different temperatures?”

This depends on the client preference (eg, cooler bedrooms are nicer to sleep in). MVHRs are whole house ventilation systems, so they will generally create an even temperature range across the home. However, localised heating and cooling can still be used (eg, heater in a bedroom, or an AC unit in a loft area). There are also now MVHR systems with integrated cooling options.

“Surely bedrooms need lower temperature than living rooms – so how can you achieve that with MVHR?”

MVHRs supply to bedrooms/living rooms and extract from bathrooms and kitchens. As an example, right now it’s really hot in London during the day, but temperatures at night are at 13 DegC. The MVHR will be supplying this cold outdoor air directly to the bedrooms throughout the night (automatic summer bypass), which is cooler air, so those rooms will be cooler.

“How can noise transmittance be avoided, and is it a problem for a properly fitted system?”

Proper duct sizing and installation, the use of drum acoustic attenuators and ensuring the specification of a properly sized unit from a good quality brand will ensure there are no noise issues. Noise issues should be designed out during the desk-based detailed ductwork stage, before a single duct is laid on site. 

We haven’t seen any acoustic issues in hundreds of projects over the past six years, except one where a drum sound attenuator was missing from the installation (which we fixed and the issue went away). If the design carefully considers air velocities, fan break-out noise and pressure, an MVHR should not have acoustic issues.

“In the kitchen it seems highly likely that the MVHR valve will get fouled given grease etc in the hoods. How do you overcome this?”

With our systems, the kitchen valve has a removeable grease filter to protect the ducts from airborne grease. It can be washed and/or replaced. The MVHR kitchen valve is located at least 600mm away from the cooker for this reason – we don’t want grease going inside it at all.

You still need a dedicated cooker hood extract separate to the MVHR system, to catch airborne grease and exhaust it straight outside. The cooker hoods we recommend are re-circulating and use a carbon filter.

“I am sure the MVHR unit works well in the first few years. But what about ten years from now?”

The units are a large investment and therefore you are going to be living with it for some time. Generally though, as long as the filters are replaced regularly (at least every year, and more often in high pollution areas) there is little else to an MVHR system – as mentioned, they are mainly two high performance fans, sensors and a control panel. They use the energy of an LED lightbulb to run, so they’re very low energy. An MVHR costs about 20p a day, and in an energy efficient home will recover more heat than it costs to run.

Yes, an MVHR should last a long time (a minimum of 10 – 20 years) with very little maintenance. In Germany where MVHR has been prevalent for longer there are units that have been running for 25 years. Some MVHR suppliers will say that ducts must be made of steel, but the plastic ducts are strong and stable, and should last the lifetime of the house.

Do you ever consider any of the other forms of ventilation at all?

Yes, we use MEV systems, which extract continuously from wet rooms, in retrofit homes where the airtightness isn’t very good – but these are not as comfortable or energy efficient as MVHR systems. This is “System 3” in Part F Building Regulations, with MVHR as “System 4”. Given the comfort, health and energy efficient aspects of the MVHR system, it’s the industry leader for low energy homes.

What do you think about the ground-tempered inlet pipes – good for keeping the intake air at reasonable temperatures in summer and winter?

Yes, they are, but maintenance is a huge issue with these pipes so we don’t typically recommend them. Mould and mildew can build up in the ducts underground, which we absolutely don’t want as they’re hard to access and clean.

“Does the MVHR system “pay back” over time?”

MVHR systems are for renovated or newly built homes that have good levels of airtightness as a result of works (eg, insulation, new windows and doors, proper weatherproofing), and thus require active ventilation (rather than trickle vents which we have in traditional homes). As such, ensuring good indoor air quality is the first priority for an MVHR system, not the financial payback.

However, because they recover heat that would normally be lost to outside in winter, your heating system will not work as hard in winter, as up to 96% of the heat will be recovered back into the supply air. This is their second role.

Finally, the high performance fans are very energy efficient – about the same energy usage over a year as an LED light bulb. Once you’re recovering more heat than you’re using running the light bulb, they are cost-positive.

Overall, MVHR systems are excellent at providing comfortable, even air temperatures (pre-warmed air in winter) and good, healthy air quality – these are their main paybacks.

“Does it help at all with cooling the house during the summer heat?”

Yes, the MVHR system will continuously extract out warm air from the home and input supply air, removing humidity also. Because this is a continual process 24hrs a day, when it’s cool at night the incoming air will cool the home. In summer the MVHR has an automatic summer bypass as well, which means when it’s very hot inside the air will be exhausted straight outside by the system.

“Does it filter the air for purification?”

Yes, all of our MVHR systems have F7 and G4 filters which will filter out pollen, carbon dust, insects, etc from the incoming air. A NOx filter to filter out pollutants related to diesel exhaust fumes can also be fitted to an MVHR unit.

If you’d like any help, guidance or pricing for your MVHR system, please contact me at patrick [@] heatspaceandlight.com or via the Free MVHR Pricing and Specification Tool here.

Patrick

13 Comments

  1. Hi, with your comment below, i was wondering in a new build with underfloor zoned heating, in winter if i have the living room warm from central heating or a stove with air from outside supply on it, will this warm air not make my bedrooms hot too?

    “Surely bedrooms need lower temperature than living rooms – so how can you achieve that with MVHR?”
    MVHRs supply to bedrooms/living rooms and extract from bathrooms and kitchens. As an example, right now it’s really hot in London during the day, but temperatures at night are at 13 DegC. The MVHR will be supplying this cold outdoor air directly to the bedrooms throughout the night (automatic summer bypass), which is cooler air, so those rooms will be cooler.

    • Hi Daniel,

      Yes, you’re right that during summer nights the incoming air to bedrooms is nice and cool – this is a benefit of living in the UK.

      In terms of heating a living room in winter, air is actually quite a poor carrier of heat. A stove next to an extract valve won’t necessarily carry that heat through the MVHR to the far end of the house and into the supply valves in the bedroom. It will recover the heat, but the actual heating emitters (eg, radiators or underfloor heating) will provide far more localised heat than the MVHR. “Air heating” is only really a thing for super-insulated Passivhaus homes, and even then it’s not hugely effective.

      If bedrooms don’t have any heating emitters (or they’re turned off), they will be cooler than the rest of the house.

      Hope that helps.

      Best wishes,

      Patrick

  2. I live in a maisonette in a block with mvhr fitted about five years ago. Maintenance done by landlord. Twice in five years I have had to ask about filters being cleaned as they don’t bother. I have lots of dust covering everything in my home on a daily basis. Surely this is harmful?

    • Hi Chris,

      Yes, dust is harmful, we don’t want to be breathing it in, particularly if it’s being drawn in through ceiling voids due to leaky ductwork. Filters also need to be replaced when they’re full, not when a maintenance programme decides, although at least the maintenance is (eventually) being done, as filters aren’t that expensive in the long run but will contribute to a longer life for the MVHR system and much improved indoor air quality.

      Best wishes,

      Patrick

  3. Hi Patrick, I have MVHR and, because the house wasn’t being used, it was left turned off for 6 months. Would it now potentially be dangerous to restart it?

    • Hi David,

      Yes, it’s risky. The issue is one of mould from any part of the system that was (possibly) left damp when it was turned off. You should be able to remove the heat exchanger element itself and give it a good clean with warm, soapy water. Let it dry and clean any other visible elements with antibacterial products. Replace the filters and run the system on its highest setting to blow out any accumulated dust. If after a few hours it smells damp or mildewy, it may be worth looking into a replacement heat exchanger, unfortunately.

  4. Thank you so much for your reply Patrick – that’s very helpful. I was thinking more of the ducting and hadn’t thought of the exchanger itself! I have checked the ducting and it seems largely clean and dry, except for one supply pipe where I found a little condensation (just slight dampness no pooling) – I think because it was a section not covered by the main insulation. Would you think that this small amount of dampness would be a problem (no mould or mildew I could find). David

    • Hi David,

      Without inspecting it myself I can’t say for sure, but if you feel comfortable and you’ve cleaned it all thoroughly then it’s your call.

      Best wishes and good luck,

      Patrick

  5. Thanks, Patrick. I appreciate your replies. I’ve decided to get a professional in to take a look/give a thorough clean. But, as I’m in SW France, I’m sadly
    probably not on your patch!

  6. Hi Patrick,

    I connected our MVHR to the app you can get for it (Zehnder) and was surprised to see the supply temp is just 14-15 degrees, which accounts for us struggling to get the living areas up to temp with the underfloor heating (downstairs) and rads/towel rails (upstairs).

    Our thermostats read between 18-20 with the heating on (which it mostly is), and the Zehnder app reports an extract temperature of only 15-16 degrees.

    The unit is based in the loft, which is cold, but the pipes seem fairly well insulated save for a few small areas.

    If the bathroom/kitchen temps are 18-20, why is the extract temp 15-16, leading to supply air temps of only 14-15?

    New filters in today, and insulation topped up. Flipping the MVHR to away mode led to both supply and extract temps of around 14 degrees….

    Any ideas what could be causing this?

    • Hi Nick,

      The MVHR extract temperature is normally pretty faithful, and I think therefore the supply is also – heat exchangers improve efficiency with larger differences between temperatures (ie, very cold and very hot transfers more efficiently than slightly cold and slightly warm), so the heat exchanger seems to be working well.  

      With the thermostat reading 18DegC, that makes me think that there is some heat loss in the loft via the pipes or the unit itself – although both may be very well insulated, when they are in an unconditioned space they are at the mercy of cold in winter and heat in summer.  

      Finally, I would recommend checking the unit flow-rates and that they are matched to the size of the home. If you want to let me know what the boost mode is, and your floor area, number of wet rooms and occupancy I can do a back-of-the-envelope calc. Too high a flow-rate and the MVHR may be over-ventilating. 

      Best wishes,

      Patrick

  7. Hi Patrick We have a Nuaire system which has G3 filters. If I upgrade the filters to G4 will I need to change the fan settings to maintain the same air flow? Thanks Richard

    • Hi Richard,

      The pressure drop from G3 to G4 is likely to be quite small, so I wouldn’t expect you’d need to increase the fan settings. If anything, it may need to be increased very slightly. However, if you detect an increase in fan noise or a decrease in system performance (stale, humid air) then please do consider re-commissioning/balancing the system.

      If the system is more than five or six years old it’s probably worth looking into getting it serviced around this time – we can help advise on what to look for and who to ask.

      We’d also only recommend MVHR systems which have fresh air filtration of F7 (ISO ePM1) rather than G3 (ISO Coarse), as F7 will filter out more of the particles which can be bad for our health (viruses, nanoparticles, vehicle exhaust gas), whereas G3 or G4 will only catch larger particles in the air (sand, hair, paint spray).

      Best wishes,

      Patrick

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