Ventilation unit location is very important to both the efficiency and also the long term usability of the system. Ducts connecting to outside contain air at the outside temperature, so are effectively outside walls – we use insulated ducts but can’t possibly insulate them to the standard of the wall. The ducts therefore need to be as short as possible, basically the unit needs to be located adjoining an external wall, or failing that, within 2 m of one. It is possible to install the unit in a frost free location outside the thermal envelope, e.g. a garage, but the connecting ducts still need to be as short as possible and of course the external terminals have to be outside the garage.
Access is needed to change filters (every 3-6 months, depending on how dirty the outside air is), so you should be able to walk up to the unit and reach it without needing a ladder. Some units are too noisy to have in a living room or bedroom, so the ideal is a utility room or WC/cloakroom. Allow space for ducts and ancillary equipment when allocating space – look at previous installations for guidance.
Condensate drainage is needed from the ventilation unit – this should be to the internal soil pipe system via a trap and not just straight to outside, otherwise it could freeze and the unit would flood.
Excerpt from “How to build a Passivhaus: Rules of thumb”, chapter by Alan Clarke, Passivhaus Trust, April 2015.