Control system and method for defrosting the outdoor coil of a heat pump

Abstract

The current outdoor ambient temperature and outdoor coil temperature in a heat pump are sensed when the heat pump's outdoor coil is clean and frost-free, and from those current temperatures the split or difference that will later exist between the temperatures, when sufficient frost has built up on the outdoor coil to necessitate defrosting, may be determined. When that defrost temperature split, called the Defrost Value or DV, is reached, defrost is initiated and the frost that has accumulated on the coil is melted. Before defrost occurs, however, changing weather conditions (namely, changing outdoor temperature and/or changing outdoor relative humidity) may effectively invalidate the previously determined defrost temperature split or DV, and a frost condition may be reached at a substantially different temperature split, either greater or smaller than that previously calculated. To ensure that the heat pump is switched to a defrost mode only and always when defrost is needed, the defrost control system continually monitors the outdoor ambient and outdoor coil temperatures and from those temperatures any significant weather condition change may be detected and a new defrost temperature split, that will exist when defrosting becomes necessary under the new weather conditions, will be calculated from the sensed temperatures. When the new defrost temperature split or DV is attained, defrost takes place. Hence, the temperature differential, where defrosting will be required, is effectively updated or adjusted between defrost modes in response to changing weather conditions, thereby optimizing the efficiency of the heat pump and conserving energy.