An MVHR system is a relatively new piece of kit for the UK residential market, as our building stock has traditionally been built so leaky that we haven’t had to worry about ventilation before. We get lots of questions about how compatible an MVHR system is with other services in the home, so today, as one of Enhabit’s ventilation technicians, I’ll answer the most common questions here.
Let’s start with the most common question from clients. MVHR systems are fine to run with open windows in both summer and in winter – the MVHR system will already be supplying enough ventilation for healthy indoor air, so opening windows will be an additional help in cooling the home. The real benefit of an MVHR system is in winter, as the ventilation requirements for good quality indoor air are satisfied by the pre-warmed filtered air of the MVHR system alone, so you don’t need to open the windows. The MVHR ensures you have enough clean air to prevent condensation and humidity issues – and that is a significant health and comfort benefit in winter.
MVHR systems are ventilation systems, not heating or cooling systems (although heating and cooling of the supply air can be provided). They are entirely compatible with all heating systems, from gas condensing boilers and radiators, to underfloor heating, air-source heat pumps or even district heating (with the exception of old-fashioned fire-places – see below). The MVHR system is designed to work alongside the central heating system to reach your target internal air temperature – normally 19-21°C.
Air conditioning systems, despite the name, do not typically provide ventilation. Instead, they recirculate the air in the room, running it through a heating or cooling coil to heat or cool it. If you put an AC unit in a basement you would soon have ventilation problems, because it’s not changing the air. Air-conditioning system are entirely compatible with MVHR systems because, unlike the MVHR system, they’re not bringing any air in or out of the property.
In a home at high risk of summer overheating the combination of an MVHR system with an air conditioning system can be a solution. The only problem is that air-conditioning is very expensive and noisy to run – and air-conditioning is terrible for the environment.
MVHR systems come with their own touchscreen controls, but as they’re designed to run in the background it’s unlikely they’ll be used much. If you have a home automation system such as Loxone you can connect it to your MVHR system to control the “boost” or “away” function (ie, ramping up the ventilation to clear the air after cooking or partying, or turning down ventilation when you’re on holiday). Generally, the MVHR system is designed to work silently in the background, providing clean, warm air to the home without any need for busy controls.
MVHR systems don’t do away with cooker hoods, as they are still needed to remove the airborne grease and smells arising out of the cooking process.
The most compatible option with an MVHR system is a recirculating cooker hood with a carbon filter to remove any cooking smells. This is the simplest to install and there are hundreds of design options. They work essentially by running the kitchen air through a filter to remove grease and odours.
The second but less preferable option is a kitchen cooker hood with a dedicated exhaust duct to outside. Although still compatible with an MVHR system, when the cooker hood is extracting air from the kitchen it will create a slight depressurisation of the property, which could cause draughts around the house.
However, as long as the cooker hood isn’t running constantly it’s not particularly impactful to the MVHR system or your home comfort.
The MVHR system would still have its own extract valve in the kitchen, operating at low levels to recover background heat in the air 24 hours a day. This valve can be fitted with a removable grease filter, so that any grease not captured by the cooker hood is picked up before it hits the MVHR ducting.
As per Building Regulations, traditional open fireplaces that are rated above 5kW require a dedicated air supply in order to feed the fire. This is normally an air brick that runs directly to outside, located in the room and close to the fireplace. Those below 5kW dodge the Building Regulations requirements for a dedicated air supply, but there are other issues with even small fireplaces.
All fireplaces also obviously have open chimney flues to allow smoke and poisonous gases to be drawn out and into the atmosphere. If for some reason the room with the fireplace were to become de-pressurised, i.e., because the chimney was blocked and the fire had consumed all of the oxygen, the fire and smoke would stop going up the chimney and fill the room. Hence why it’s crucial that there is a dedicated air supply to the room from outside.
Although fireplaces are lovely, they’re not very modern. They smoke, release dangerous gases, need regular cleaning and to be made safe for children – and after all that, most of the heat escapes up the chimney anyway.
And what’s the point in spending a lot of money on insulation, airtightness and high quality building construction if you actively leave a hole in the wall for cold air to draught through in winter?
Traditional fireplaces also kick out far too much heat for the requirements of most modern low energy homes. We’ve heard of people in energy efficient homes opening their windows in winter when they’ve lit the fireplace, because it’s just too hot. This makes no sense.
But what are the solutions if you still want that focal point in your room?
Fireplaces can be beautiful centrepieces to a living room, and most of the UK housing stock features at least one old fireplace (whether used or not).
One solution therefore is to fit a room-sealed stove with a dedicated air supply to outside through a closed steel duct. This means there’s no air leakage to outside through an airbrick, as the air is drawn through a pipe within the fireplace itself. These stoves are also much more efficient, and can be gas, coal or wood-fed.
An enclosed glass compartment frames the fire and ensures that no smoke or ash escapes into the room. The only time you open the door is to put in more fuel and to clear out the old ash.
As an extra precaution Zehnder MVHR units have a fireplace mode, which keeps the property every so slightly pressurised to ensure the fireplace never spills back into the house.
Another option which we’re seeing more of our clients consider is to have an outdoor fire pit or fireplace in the garden. In the summer or winter seasons it’s much nicer to sit around the fire in the fresh outdoors and under the stars. The smoke keeps insects away and, being outdoors, you won’t need to worry about dealing with any ash or smoke in your home. You need to watch out for local regulations on their use.
The key is to ensure that you work with an experienced MVHR designer who knows how the system will interact with the other plant in your home. A well designed system should only require you to use the boost mode when smells or excessive humidity need to be purged and to regularly change the filter.