The enthalpy heat exchanger extends conventional heat recovery by adding moisture transfer. Using hygroscopic membranes or coated honeycombs, water vapour diffuses directly from the more humid to the drier air stream without producing condensate. In summer, moisture is removed from incoming air via cool exhaust air; in winter, moisture from exhaust air is transferred to dry outdoor air, significantly reducing humidification demand. Enthalpy exchangers are designed as cross-flow plate units or rotating enthalpy wheels. Efficiency for both heat and moisture transfer typically ranges between 65 and 85 percent depending on operating conditions.