AbstractThe performance of the heat exchangers of an unsophisticated air‐air heat pump are examined with particular emphasis on the evaporator frosting characteristics. The data relate to an operating domestic heat pump in Dunedin. Evaporator frosting reduces the seasonal COP by only a small amount (less than 1.7%) in this example but the defrost rate is shown to be very sensitive to small changes in the evaporator capacity. Thus frost formation generally has an important influence on the evaporator optimization calculation. The effectiveness of the expansion valve and of the defrost thermostat is also analysed, and particular opportunities for improving the system are identifie