Everything you need to know about mechanical ventilation with heat recovery
Whether you’re self-building or renovating, there are important factors to consider when specifying MVHR, from installation to maintenance.
What is MVHR and how does it work?
Mechanical ventilation with heat recovery (MVHR) is a system that circulates warm, fresh, filtered air around the home. Filters in the main unit remove particles so that the circulating air is free from pollutants. Usually installed in a loft, basement or plant room, the air-handling unit is the hub of the system. It is connected to a series of ducts that are attached to extraction and supply points.
Stale air is drawn out of each room at the extraction point, passed through a run of ducting to a heat exchanger and expelled outside. The extracted warmth is transferred via the heat exchanger to fresh air that flows through a separate run of ducting into the rooms.
Is it right for my project?
MVHR should be installed in a well-insulated, airtight home where warmth isn’t going to leak out via the building fabric. Installation is more straightforward in a self-build, where it can be designed into the plans from the start. However, it is possible to incorporate the system into a renovation. Homes with open fireplaces are not suitable as the air bricks that supply air to the fire make the spaces too draughty.
‘When you walk into a home with well- functioning MVHR, you feel the freshness and warmth in the air,’ says Patrick Chester, project manager and Passivhaus consultant at Enhabit. ‘The airflow is constant but low, so the system is near-silent and ensures there’s no build-up of excess humidity, carbon dioxide or toxins from gas appliances.’
Filters also remove potentially harmful particles from the incoming air. ‘The filters remove pollution and airborne allergens, such as pollen, which can be beneficial for allergy sufferers,’ says Larry Soper, technical training manager at Envirovent.
Which MVHR system is right for me?
There are several things to bear in mind when choosing MHVR. ‘The highest quality units are able to recover 96% of the heat, while the poorest recover 70% or less,’ explains Patrick.
Selecting a model that runs quietly is very important. This aspect of performance is measured in decibels (dBA), with lower numbers indicating a lower noise level. Most main units operate at about 40-45 dBA, similar to a fridge freezer, but this doesn’t matter so much as the unit is usually installed away from living spaces. Check the noise level of the air valves in each room too. They should be less than 25 dBA.
Take a close look at the controls and what they provide. ‘Are they easy to use? Is wireless functionality available? Are humidity and temperature monitored automatically?’ says Patrick. Check that there is a F7 pollen filter for the fresh-air intake, especially if anyone in your home suffers from asthma or allergies. Some units only have a G4 sponge filter, which keeps insects, hairs, large pollen spores and dust out. The F7 blocks smaller pollen spores, dust, bacteria and some smoke/oil fumes. If you want to zap everything, though, an electrostatic filter is even better.
What other elements are important?
The unit itself should be well insulated. ‘It’s no good having a model with a great heat exchanger if the recycled warmth gets lost through the unit’s skin,’ says Clarissa Youden, associate director at Total Home Environment. ‘Check that there is about 30mm thermal and acoustic insulation to the inside of the unit on the specification sheet.’
You should also consider an automatic summer bypass. In warmer months the heat from the outgoing air will not be recycled, but you’ll still get a fresh incoming flow. ‘With the UK’s erratic weather, having a modular bypass that automatically opens in specific increments when the temperature changes is a must,’ says Clarissa. ‘But manual ones can be inconvenient.’
There are many variables that will affect the final cost of a system, from the size of your home to the type of ducts used. ‘A good-quality MVHR, including the ducting, insulation and the controller, costs anywhere between £2,500 and £7,000,’ says Clarissa. Installation is likely to cost upwards of £2,500, depending on the complexity of the project.
Before placing an order, check the guarantee your manufacturer is offering in case anything goes wrong with the system. Guarantees for the whole set-up will typically cover you for between two and seven years. This depends on the supplier and whether you have opted into a maintenance scheme.