Abstract: An environmental control system has an evaporator section comprising an evaporator for extracting heat from (and dehumidifying) air destined for the controlled space or for exhaust--this air may be a controlled mixture of fresh and recycled air--and a chiller for extracting heat from a source such as well water and from a thermal store. The heat pump condenser section comprises a condenser for rejecting heat to air entering the space and a condenser receiver for rejecting heat to the store. The system includes a system controller (not shown) programmed to control operation in a manner tending to minimize the prime energy requirement.