[AIVC, Air infiltration and ventilation glossary, 1992]:a device operating on a refrigeration cycle in which both evaporator and condenser are refrigerant/air heat exchangers. As a heating season heat recovery device, the evaporator transfers heat from the exhaust warm air to the refrigerant and the condenser transfers heat from the refrigerant to the incoming air.Arrangements are often made to allow the refrigerant flow to be reversed making the condenser the evaporator and vice versa – thus energy may be recovered in the cooling season.