A heating system of the heat pump type is disclosed in which the outside coil, employed to reject heat in air conditioning mode or to absorb heat in heating mode, is supplied with heat independently of that provided by ambient air when the efficiency of the system falls off due to low ambient air temperature. The amount of heat thus supplied is controlled to increase the efficiency of the system sufficiently to produce a significant net decrease in operating cost. In a conventional system, the outside air coil-cooling face is turned off and augmenting heat is supplied to the coil when the ambient air temperature is moderate (e.g., 32.degree.-38.degree. F.) for the system, and the augmenting heat is supplied at a rate which is at least sufficient to restore the efficiency of the system to that inherent with a much higher ambient air temperature. At the same time, the outside coil temperature is monitored and the augmenting heat is temporarily discontinued when the coil temperature reaches a selected value (e.g., 70.degree. F.).