MVHR FAQs

Solar space at Denby Dale Passivhaus

Answers to frequently asked questions

What is MVHR?

MVHR (mechanical ventilation with heat recovery) supplies fresh air to all habitable rooms. It also removes moist stale air, reusing the heat to pre-heat the fresh air entering the building.  For more information, see our detailed article ‘What is MVHR’

At what stage in the building process should the MVHR system be designed for my building?

As early as possible! Ideally just after the project has received planning permission. It is really hard to add an MVHR system into a low energy or Passivhaus building design later on in the process. Our designers will work closely with engineers and architects to overcome any possible issues at the design stage.

At what stage in the building process should the MVHR system be installed?

Most of our systems use spiral wound rigid ducting which is usually designed to pass through timber I joists or metal web ‘pozi’ joists. In this situation it is essential that the ductwork passing through the joists is installed as the joists are erected, otherwise it may be necessary to cut the duct into small segments, drop it between the joists then reconnect it, which is very labour intensive and requires extra parts. 

Is MVHR suitable for all types of property?

Although MVHR could be installed in all types of building, we generally would recommend it for buildings with good airtightness levels, for example where the air permeability of the thermal envelope is at or below 3 m3.hr/m2 @50Pa. For comfort, and to reduce the carbon emissions of homes, it is important that homes have good airtightness levels, as well as good ventilation levels.

Recent research by the Passivhaus Trust has shown that MVHR ventilation could be used in less airtight homes (eg even a fairly draughty home of 9 m3.hr/m2 @50Pa) as it would still have lower carbon emissions rate than natural ventilation. However, in very draughty homes (eg higher/ worse than 9 m3.hr/m2 @50Pa) the efficacy and efficiency of an MVHR will decrease.

Can I open the windows?

Yes you can and the MVHR system will continue to operate in the background. However the efficiency of the MVHR will usually be slightly reduced, affecting its ability to keep the building warm in winter and cool in summer. The exception to this is when windows are opened for summer purging (see below).

Does the system run 24/7 for 365 days of the year?

It is set for automatic use all year round so you do not have to adjust any settings. However you may want to over-ride the automatic system on some occasions (see below).

How much electricity does it use?

Green Building Store domestic MVHR systems typically have 2 very small motors which push the air through your system. An MVHR unit like Focus 200 uses 22 Watts. This is similar to having a low energy light bulb on in your home, costing around 10p per day.

What will happen if the electricity supply is cut off?

You may start to feel that the air is becoming a little stale, but do not worry you will not suffocate! In the unlikely event that it happens whilst you are asleep, then your bedroom might seem a bit stuffy in the morning. All of our MVHR units are set up to automatically restart when the power is restored.

Is it noisy?

Supply and extract ducts in MVHR system designWe design MVHR systems to have minimal noise impacts and they should run almost silently. It is important not to block any of the air vents as this will put the whole system out of balance.

What maintenance do I need to do?

Apart from keeping all the air vents clear and clean, the only maintenance is periodic filter changes. There are several filters which need to be changed to keep the air flowing properly. How frequently depends on where you live and how clean the air is. The filters keep your air clean, but are also needed to keep the system working properly. The kitchen extract filter may need changing/ cleaning more often than the others. Normally filters need to be changed every 3-6 months.

Do you need to clean inside the MVHR ducting?

We would advise that if an MVHR system is present in the property, a recirculating cooker hood is used rather than an extractor. This cooker hood will act as the primary filter, taking the air in through a charcoal filter removing smells and grease. The MVHR extract has a fleece filter to protect and leftover airborne grease from entering the ducting system. Providing that you keep on top of the filter changes including the kitchen extract filter then there should be no need for cleaning of the ductwork system. After a few years, it may be necessary to unscrew the extract valves in bathrooms and have a wipe in the duct behind them with a cloth, as wet dust and towel lint can collect here. As all of the ducts are within the thermal envelope the air does not condense until it hits the heat exchanger, by this time it has gone through the filter which will remove any airborne debris.

Will it heat my house?

MVHR will re-use the heat which already exists in your home produced by occupants and electrical equipment. Some systems can be designed to have an additional in line heater, to heat supply air, but this can only provide a small amount of heating.

Can I dry clothes?

Yes, clothes dry very well indoors and create some moisture which is beneficial. You also might like to grow plants.

What is summer purging?

If you experience unusually high outdoor temperatures and your home is too hot, it is recommended that you ‘summer purge’. (This is standard practice in hot climates).

Open windows during the night so that the cool air can enter and reduce indoor temperatures.
During the hot day close your windows and increase the fan speed on your control panel to increase ventilation.

How do I get rid of cooking smells in a Passivhaus?

An MVHR system, with a recirculating cooker hood, performs as well as a standard extractor in a non-Passivhaus. If all of the filters are maintained in the MVHR system AND the recirculating cooker hood, the smells will be minimised . A well-designed MVHR system with a high extract rate in the kitchen creates a negative pressure in the kitchen,  so air is being sucked under the doors into the kitchen and not the other way around,  therefore minimising the spread of smells.

Can I boost the fan whilst cooking?

Yes. Just go to your control panel and touch the boost button. It will stay on for a pre determined time (usually 30 minutes), and your system will then revert to normal operation. You may have an additional boost button in your kitchen or bathroom which will have the same function.

Is it possible to have a woodburning stove with MVHR?

It is possible to have a log burner as well as MVHR. However, there are a couple of things to consider:

  • The air supply to the log burner should be ducted directly to it, in order to avoid draughts and leakage in the fabric of the build.
  • If you have built a low-energy house, the room in which your log burner is situated may get too hot. A lot of people think that MVHR redistributes heat around a house, but it doesn’t, so any hotspots remain hot and coldspots cold.

What do I do when go on holiday?

You can reduce the ventilation while you are away. Simply go to your control panel and reduce the fan speed to the minimum. When you return home just return to the standard setting and your system with return to normal operation.

What should I do if there are more people in the house or if there is a party?

You can increase the ventilation for ‘high occupancy’. Simply go to your control panel and increase the fan speed. When people have left, return to the normal ventilation setting, which will keep the air feeling fresh.

What happens to water vapour in an MVHR system?

When water vapour is extracted from a kitchen or bathroom it is carried along the duct to the MVHR unit where it condenses and is drained away. This is why it is important that all of the ducting is contained within the thermal envelope of the building. If the duct was cooler, the air would cool upon entering the duct and it would lose its capacity to hold as much moisture, resulting in the air condensing (warm air holds more moisture than cold air).

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