A mechanical ventilation heat recovery (MVHR) system has a ventilation unit with two fans: one draws outside air in and supplies it to bedrooms and living rooms; the other extracts air from kitchens and bathrooms to exhaust to outside. Inside the unit there is a heat exchanger which transfers the heat from the outgoing air to the incoming air without any recirculation of the air itself.
MVHR – The fans run all the time, but can be switched to different speeds – usually low, normal and boost – and these rates are adjusted at commissioning to suit the individual house. The unit includes filters which remove dirt (and insects) from the air supply. A system of ducts is needed to distribute the air from the ventilation unit to and from the rooms in the dwelling, and there are also ducts between the unit and outside to bring in fresh air and exhaust stale air.
Excerpt from “How to build a Passivhaus: Rules of thumb”, chapter by Alan Clarke, Passivhaus Trust, April 2015