A heat recovery unit transfers heat (some units also moisture) from the exhaust air stream over to the supply air stream, thus reducing the heat loss due to ventilation, and reducing the need to condition the cold supply air. Conversely, in hot and humid outdoor conditions, a heat recovery unit can keep heat (some units also moisture) outside, thus reducing air conditioning costs.
Heat recovery may be used in balanced ventilation systems (i.e. fan powered supply and exhaust air flows). The building should be satisfactorily airtight - air leakages constitute an extra heat loss since they do not pass though the heat recovery unit. For dwellings, the infiltration rate should not exceed 10-20 % of the flow rate through the heat recovery unit.Heat recovery is equally appropriate for buildings with any space heating system. Correctly dimensioned and maintained heat recovery units with high efficiency will pay for themselves in a few years, in terms of reduced ventilation & space heating costs. This profitability is higher if the exhaust fan is located before the heat exchanger. It should be possible to reduce the heat recovery efficiency outside the heating season, to prevent over-heating indoors. Some heat exchangers can also recover moisture. It can be desirable to recover moisture this way in buildings with central humidification in winter, to reduce humidification costs. For AHUs with cooling (air conditioning) moisture recovery can be desirable in summer (when the outdoor air is hot and humid) to reduce the cooling energy needed for dehumidification. If the exhaust air has water-soluble odours/pollutants, one should nevertheless use a heat exchanger that does not recover moisture, i.e. totally separate air streams. Heat recovery units require regular inspection and maintenance, though anyone with normal technical aptitude can do this, on the condition that a proper operation & maintenance manual is available.