In order to keep cost to a minimum and efficiency to a maximum, it is necessary to locate the unit in a fairly central location in the house, but yet ensuring easy access to an external surface to take in and exhaust out air. There are usually two options for location, (a) the attic area and (b) a utility/plant area.
In the maritime climate of Ireland and the UK it was common to install the Heat Recovery Ventilation unit in the attic space even if it is an un-heated space. This may have been acceptable in the past when these areas were partly heated due to low levels of insulation and heat rising from the main body of the house. These days, houses are built to a more demanding energy standard and this means that the attic area should be as cold as outside and this can get down below -10oC regularly. Because of this, it is advisable to install the HRV unit inside the insulation and air-tight envelope of the house, unless the ventilation unit is very highly insulated as is the case with the ProAir Heat Recovery Ventilation units.
Even when installed inside, either in a warm attic or in a utility space, it is still quite advantageous to have a very highly insulated unit. This is because very cold air from outside, enters the unit and remains cold until it reaches at least half way through the plate exchanger. Likewise, air exiting from the exchanger will be almost as cold as the air entering, and this means that half the unit, plus the ducting leading to outside can be at sub-zero temperatures inside in the house. High levels of insulation on the unit and this ducting protects against potential heat-loss.
Alternatively, if installed in an unheated attic, the fresh and exhaust is not that important to be insulated, but the duct leading to the house should be highly insulated.
Many storey and a half and two storey houses have larger ground floor areas (and volumes) when compared to first floor areas. Locating the air handling unit on the ground floor will imply smaller movement of air throughout the house as a whole. This will therefore require less power making the system more efficient.
An important aspect to be considered when choosing a location is that of maintenance and the ease with which it can be carried out. It is more likely that filters will be checked etc. more regularly if the unit is installed in a location that is easily accessed.