Most air-source heat pumps uti/ize the reverse cyc/e defrost to eliminate frost that forms on the outdoor heat exchanger during normal winter operations. During the defrost, the heat pump is switched from the heating to the defrost mode to provide heat to the outdoor heat exchanger to melt the frost. Once the frost is melted, the unit is switched back to the heating mode.

The objective of this research was to characterize the dynamic response of an air-source heat pump during the reverse cycle defrost. Relevant fiterature was reviewed, an experimental apparatus was constructed, and experiments performed. The test setup contained a nominal 3-ton capacity residential air-to-air heatpump that used a thermal expansion valve (TXV) for expansion during the defrost. Measurements made of refrigerant temperature and pressure throughout the system, refrigerant flow rates, air-side capacity, compressor/ outdoor fan power, and refrigerant level in the accumulator. A detailed description of the physical processes that occur during a defrost are provided.