What is an MVHR system?
An MVHR supplies fresh air to all habitable rooms whilst also removing moist stale air, and reusing the heat in extracted air to pre-heat the fresh air entering the building.
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 as it is really hard to add an MVHR system into a low energy or passive house building design later on in the process. SWES will work closely with customers, builders and architects to overcome any possible issues at the design stage.
At what stage in the building process should the MVHR system be installed?
At joist erection if a new build as most of our systems use either spiral wound rigid or glue and screw ducting which is usually designed to pass along timber joist runs or through metal web ‘pozi’ joists. In any case it is essential that the ductwork passing through the ‘pozi’ joists are installed as they 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, can effect performance and requires extra materials. The electrical and MVHR unit are reasonably straight forward to install and can be planned in once building is watertight.
For existing dwellings, either plasterboard or floor boards will require lifting in order to install ductwork.
Is MVHR suitable for all types of property?
Although MVHR could be installed in all types of building, we would only recommend it for buildings with good airtightness levels. As a rule of thumb, this would be where the air permeability of the thermal envelope is at or below 3 m3/m2.h @ 50 Pa. If this level of airtightness is not achieved, then the heat recovery and consequent energy saving benefits of MVHR would be lost.
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?
The MVHR systems typically used have energy efficient motors which push the air through your system with a heat exchange. Most MVHRs use around 22 Watts. This is similar to having a low energy light bulb on all day in your home, and 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.
Are MVHR systems noisy?
The MVHR systems specified all 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 carry out?
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 every 3-6 months, and ventilation unit filters vary from 6 months up to 3 years as recommended by manufacturers.
Do I 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 an MVHR system 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 Passive house?
An MVHR system, with a recirculating cooker hood, performs as well as a standard extractor in a non-Passive house. 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 wired/wireless control panel and touch the boost button. It will stay on for a pre-determined time (usually around 30 minutes), and your system will then revert to normal operation. You may have an additional boost switch in your kitchen or bathroom which will have the same function.
Is it possible to have a woodburning stove with a MVHR?
It is possible to have a log burner as well as a 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 cold spots cold.
What do I do when I 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. If you forget then systems can be boosted after the period of high occupancy.
What happens to water vapour that is produced within 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 via a small 20-25mm pipe into nearest the nearest wastepipe run. 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) within the ductwork as appose to the heat exchange point.