The heating seasonal performance factor (HSPF) of an air-source heat pump is degraded by the defrost cycle. Electric resistance heat was added to the refrigerant through the use of a 750-watt immersion heater inserted at the base of the suction-line accumulator and by heating the accumulator shell via a 900-watt electric band heater attached to the outside of the accumulator. The electric resistance heat and its hot metal surface was used as an active enhancement technique to increase the rate of refrigerant boiling during the initial few minutes of the reverse-cycle defrost. The unit was fully instrumented and equipped with two possible expansion devices: a short-tube orifice and a thermal expansion valve (TXV), for cooling / defrost mode operation and a TXV for the heating mode.

Tests were run in psychrometric rooms that maintained outdoor conditions at 1.7°C dry bulb and 80% relative humidity and indoor conditions at 21.1°C dry bulb. Overall performance characteristics of particular interest were integrated COP, cycle time, defrost cycle time, along with other instantaneous comparisons of power, refrigerant flow rate, temperatures and pressures. Tests with a 1.8 mm diameter orifice defrost mode expansion device and the immersion heater enhancement showed the best system improvement. This test had a 3.1% increase in integrated cyclic COP and a 11% reduction of the defrost cycle time. The reduced cycle time came primarily as a result of a 1.0 minute reduction (31%) of the outdoor coil post-melt (draining) time.

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