The influence of ambient temperature and relative humidity on the heating mode performance of a high-efficiency air-to-air heat pump

A high-efficiency air-to-air split system residential heat pump of nominal three-ton capacity was instrumented and tested in the heating mode under laboratory conditions. Performance of the system was measured during steady-state, dehumidification, and frosting defrosting conditions with major emphasis placed on dynamic frosting operation of the system. The study encompassed an evaluation of system and component performance for ambient temperature levels of 8.3°C, 4.4°C, 1.7°C, -].1°C, and 8.3°C (47°F, 40°F, 35°F, 20°F, and l7oF respectively); and discrete humidity levels ranging from 50% to 90%.

For non-frosting, dehumidification conditions the performance of the heat pump improved somewhat with increasing relative humidities greater than 60%. At 4.4°C (40°F), supposed frosting conditions, the performance of the system initially improved for relative humidities greater than 70% as a result of higher mass transfer rates of moisture to the outdoor heat exchanger. At ambients less than 4.4°C (40°F), the rate of performance degradation due to frost formation increases with increasing relative humidity due to the increased rate of frost accumulation on the outdoor heat exchanger. The frosting zone was seen to occur between 4.4°C and -1.1°C (40 to 30°F) for a relative humidity of 70%. At higher humidity levels the zone ranged from 4.4°C to -8.3°C (40°F to 17°F).

The influence of volumetric airflow rate, under frosting and non-frosting conditions of the outdoor heat exchanger, needs to be investigated for the possible elimination of frosting at 4.4°C (40°F) and possible improvement in system performance. A defrost sensor based on the temperature difference from ambient air to evaporator tube wall would better sense the proper initiation of a defrost cycle and could possibly reduce the losses due to severe defrost operations.