In order to keep cost to a minimum and efficiency to a maximum, it is necessary to locate the unit in a 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) a utility/plant area (b) the attic area. Houses are built to a demanding energy standard and this means that the attic area is often as cold as outside and this can get down below -2˚C regularly. It is advisable to install the HRV unit inside the insulation and air-tight envelope of the house if possible, 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 can be uninsulated, but the fresh and exhaust ducting to the house should be highly insulated, a minimum of 200mm is recommended. 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 to make 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.