Lowering space temperature at night is recognized as an effective method of reducing building heat loss and fuel consumption of both fossil fuel-fired and electrical resistance heating systems. With the rapid growth in the number of heat pump installations in recent years, there is increasing interest in methods o~ reducing energy consumption and operating costs of heat pump systems.

In this study, a hybrid computer simulation was 'used to determine whether or not setback of a heat pump system would reduce energy consumption and operating costs. The simulation considered the effects of indoor and outdoor temperatures on both heat pump capacity and heat pump energy consumption, and considered several methods for control of the auxiliary heat elements.

No attempt was made to determine the effect of relative size of heat pump and auxiliary heat on night setback economics, nor was any attempt made to analyze the effects of variations in building construction, level of insulation or structure thermal storage. Also, this study has made no attempt to assess the cost of providing capacity for the morning pick ~p load which may be imposed on the electric utility. In addition, changes in utility rate structures, i.e., time of day metering, peak load pricing, possible future demand rates, etc. would influence operational costs and must be considered in assessing the overall impact of heat pump setback programming.

All of these variables can effect heat pump setback economics in varying degrees. Thus, additional studies are necessary before final conclusions can be reached.