MEV uses a constantly operating fan which extracts warm moist air from the warm moist rooms (ie. bathrooms, kitchen etc) via ductwork, with fresh air effectively being sucked in via trickle vents and gaps in the structure. MEV is fairly economic to install, especially as it eliminates the need for dedicated extractors in the bathrooms. However, for very airtight buildings additional openings in the structure need to be introduced and warm stale air is replaced with fresh but cold air, thus driving heat out of the building and reducing its efficiency. As an aside to this, an MEV option is available for our heat pump which actually uses the heat from the outgoing stale warm air to pre-heat the ‘brine’ before it goes into the heat pump, thus recovering some of that energy. This is a great idea and an option well worth considering for self builders opting for a heat pump, but we eliminated it on the basis that that our first floor is largely unheated and such a system might lead to the cooling of that area via the trickle vents in the Velux windows which would need to be open for this system to work properly.
MVHR combines MEV with an intake system which supplies the ‘dry’ rooms with fresh air, preheated via a heat exchanger which takes heat from the extracted air. This is the system we have chosen, a (claimed) 95% efficient unit made by Dutch company Renovent and supplied by Ubbink in the UK. The unit has three settings and is virtually silent in operation. At the lowest setting (normal operation) it uses just half the power of a 60W light bulb and should be adequate to ‘heat’ the three first floor bedrooms alongside the heat convected from the ground floor. Other advantages are good air quality by using fresh, filtered air from outside, and the ability to use the unit to provide cooling in summer by bringing in cool air at night into a solar heated building.