Learn everything you need to know about MVHR systems with our FAQ guide. Here we try to answer all the questions we’ve had regarding MVHR systems so that you can be up to date with the most relevant information.
Still have questions? Get in touch with our experts and they’ll be happy to answer.
We’ve separated this guide into different topics to help you get the most out of it:
- MVHR Basics
- MVHR Systems and Construction
- MVHR Systems and Electricity
- MVHR Systems and Manual Ventilation
- MVHR systems and Water Vapour
- MVHR Systems and Cooking
- Are MVHR Systems worth it?
MVHR Basics
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’.
How Does an MVHR System Work?
An MVHR (Mechanical Ventilation with Heat Recovery) system provides continuous fresh, filtered air to your home while recovering heat from the outgoing stale air. It works by extracting moist, polluted air from wet rooms, passing it through a heat exchanger to transfer its warmth to incoming fresh air, and then distributing this tempered air to living areas. This process, driven by a central unit with fans and filters, significantly improves indoor air quality, reduces energy consumption by retaining heat, and prevents issues like condensation and mould, making it an ideal ventilation solution for modern, airtight homes.
MVHR Systems and Construction
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 hard to add an MVHR system into a low energy or Passivhaus building design later 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 (e.g. 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 (e.g. higher/ worse than 9 m3.hr/m2 @50Pa) the efficacy and efficiency of an MVHR will decrease.
Maintaining your MVHR System
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.
Look at our in depth MVHR aftercare guide here.
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 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.
How to balance an MVHR System
This process involves meticulously adjusting the airflow rates at each supply and extract vent to match the specific design requirements for each room, ensuring that the total incoming and outgoing air volumes are equal to prevent positive or negative pressure within the dwelling. A qualified professional typically uses specialist equipment, such as an anemometer, to accurately measure airflow at each terminal and then makes fine adjustments to the individual room valves and the MVHR unit's fan speeds, ensuring adequate ventilation in "wet rooms" (kitchens, bathrooms) and a comfortable supply of fresh, tempered air in living spaces and bedrooms, thereby preventing issues like condensation, poor air quality, excessive noise, or wasted energy from an imbalanced system.
MVHR Systems and Electricity
Does an MVHR system use a lot of electricity?
21° domestic MVHR systems typically have 2 small motors which push the air through your system. An MVHR unit like Focus 200 uses 22 Watts. This is like having a low energy light bulb on in your home, costing around 10p per day.
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).
What will happen if the electricity supply is cut off?
You may start to feel that the air is becoming a little stale, but in the unlikely event that it happens whilst you are asleep, then your bedroom might seem a bit stuffy in the morning. All our MVHR units are set up to automatically restart when the power is restored.
MVHR Systems and Manual Ventilation
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).
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.
MVHR Systems and Water Vapour
Can I dry clothes indoors with an MVHR System?
Absolutely, an MVHR system is highly effective for drying clothes indoors, making it a preferable method compared to traditional window-opening or even some dehumidifier uses. As clothes release significant moisture, the MVHR's continuous extraction from "wet rooms" efficiently removes this humidity, preventing condensation and mould, while simultaneously recovering the heat from the outgoing damp air to pre-warm the incoming fresh air. This not only maintains excellent indoor air quality and comfort but also does so in an energy-efficient manner, by placing clothes in areas with extract vents and ensuring good airflow around them.
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 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).
MVHR Systems and Cooking
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 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.
Miscellaneous MVHR System FAQs
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 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 cold spots cold.
Does an MVHR system heat homes?
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.
Does an MVHR system cool a house?
The short answer is yes, but there will be some additional steps you'll need to take. We wrote a complete article around MVHR and cooling here.
Is an MVHR system noisy?
We 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 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.
Are MVHR Systems worth it?
MVHR systems are an incredibly valuable investment for any modern, energy-efficient home. Far from being a mere luxury, a well-designed and installed MVHR system is the cornerstone of a truly healthy, comfortable, and sustainable living environment. By continuously supplying fresh, filtered air while recovering up to 95% of heat from outgoing stale air, MVHR dramatically improves indoor air quality, reducing pollutants, allergens, and excess humidity that can lead to condensation and mould. This not only enhances your family's well-being but also significantly lowers energy bills by reducing the demand on your heating system, ensuring your home remains consistently warm in winter and cooler in summer.
For those seeking to create a genuinely low-energy, comfortable, and future-proof home, an MVHR system is worth every penny.