Living with MVHR (Mechanical ventilation with heat recovery)

Who should read this guide?

If you are living in a home with MVHR (mechanical ventilation with heat recovery), this guide will give you some general information about how it works and what you should do to get the best from it. It isn't specific to any particular sort of MVHR system. It does explain some things you should know if you move into a new home that has it.

What does MVHR do?

picture of house with MVHR

MVHR extracts air from bathrooms and kitchens
and feeds fresh air into other rooms.

All homes need fresh air so normally we open windows but this allows heat to escape; in the winter time, this can be a large part of your heat loss. Also, opening the windows isn't always possible, for example because of traffic noise or burglars. Your MVHR will provide enough ventilation to keep the air fresh without you opening windows and at the same time the heat recovery part uses the warm air leaving your home to heat up the fresh air coming in. This means you can have ventilation without losing much heat (so reducing your energy bill) and without getting cold draughts.

MVHR units usually have air filters to keep the heat recovery surfaces clean. However, we like clean air too so the improved air quality is an added advantage. Many people with MVHR find this is as important to them as the heat saving. It also reduces dust.

An MVHR unit uses electricity to pump the air through the system. However, this should be much less than the energy saved by recovering the heat.

There are two main kinds of MVHR. You can have a whole-house system with a central unit and ducts carrying air to each room, or you can have single room through-the-wall units. The whole house system is generally more effective at heat recovery. You should be able to recover up to 90% of the heat that would have been lost. However, there will also be some system losses, especially if the unit is in a cold space or the ducting goes through a cold space and is not properly lagged.

In a whole house MVHR system the main unit will be tucked away somewhere such as in the attic or under the eaves or in a cupboard. The ducting from there leads to extract valves in bathrooms, toilets and the kitchen, and supply valves in all the other rooms. These valves should be balanced so that the total supply equals the total extract.

How do I know it is working?

When the unit is working you should be able to hear the air flow in each room, unless there is too much other noise. When it is on normal setting it should be only just audible, but on boost it is likely to be a bit more noisy. If you get up close to where the main unit is you will be able to hear the pump/fan running.

The other thing to check is that the rooms are not stuffy. If you find that some rooms are not getting as much ventilation as others, this could be because the balancing is a bit off. Alternatively, you may get poor ventilation in a room if the valve is too close to the door, so that the air circulates in only a small part of the room.

What do I need to do? Is there anything I shouldn't do?

MVHR does not usually have a lot of controls. For a whole house system there will be an on/off switch and usually some kind of boost switch or level selection for low, medium, high. You should normally leave it running on medium most of the time. You should turn it up to boost if you want extra ventilation for a few days, for example if you have wet plaster or paint to dry, or some new furniture that is a bit smelly.

Similarly, single room units usually have a normal and boost setting. If you have a unit in a shower room then you could put it on boost for a short while after using the shower.

Many MVHR systems have a summer bypass mode so that in the summer you can have ventilation without the heat recovery, when it is cooler outside than inside. When it is warmer outside then the heat recovery actually helps to keep the house cool, because the air coming in is cooled by the air going out. If you have a reasonable level of thermal inertia in your house then this gives you night time cooling - using the bypass overnight helps cool the house and then the heat recovery comes on during the day to retain that coolth when it is warmer outside. You may need to flick a switch to enable the bypass mode and then it automatically activates by temperature.

You can switch the MVHR off completely in the summer if you can get enough ventilation by opening the windows instead and don't need the cooling effect. However if you have MVHR you are presumably pretty airtight so you should only switch the MVHR off if you can open windows for ventilation in every room. Also, apparently there is a potential for mould growth within the heat exchanger unit if it is switched off for more than a few hours (see Yougen: which retrofit ventilation system and also this comment on the Green Building Store blog.). This is probably unlikely but if it did happen you might need to replace the unit which would be very expensive.

In the winter when you are running your heating system you should turn off the summer bypass if it does not happen automatically and then keep your windows closed. The MVHR system should give you enough fresh air on its own, so you don't need to waste heat by opening the windows. If you do open windows, then the MVHR will ensure that you have ventilation in all the rooms but it won't be so effective in saving heat.

Why is it making that noise?

A whole house MVHR system should sound like an air conditioning system, with just a quiet woosh of air in each room. If it is loud, then check the setting isn't on boost. If you are getting noise from the pump in the main unit then it probably has not been installed correctly. There should be rubber feet and flexible tubing to prevent the fan vibration noise from travelling down the ducts.

With single room units some fan noise is inevitable but it should still be reasoably quiet except when on boost.

What maintenance does it need?

For whole house MVHR it is very important to clean or change the filters regularly - perhaps every 3-6 months. Some people clean the filters with a vacuum cleaner (being gentle) or by rinsing them in clean running water.

The filter is important to protect the heat exchanger from dirt, because dust and grease will interfere with the flow of heat. If you live on a main road you will be amazed at how mucky the incoming air filter gets. If you are an allergy sufferer you can get pollen filters for your MVHR.

You may need to clean the heat exchanger directly, occasionally.

Single room MVHR units may not have a filter in which case you will need to clean the heat exchanger directly.

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