Abstract: Apparatus are disclosed for conditioning the air within an indoor space and include first heat transfer means for selectively heating air passing to the space and a heat pump selectively operable in a heating mode, also for heating air passing to the indoor space. Control means are provided for effecting operation of the first heat transfer means and the heat pump which include means for sensing the demand for heat within the indoor space and for distinguishing between a relatively low demand and a relatively high demand. Operation of the heat pump is effected in response to a relatively low demand for heat until such demand is satisfied. In the event that a relatively high demand for heat is sensed, operation of the heat pump is terminated and the first heat transfer means is operated until such time as both the relatively high and low demands for heat are satisfied.