Abstract: A forced air heating and cooling system employs a water source heat pump as the primary heating source and as a back-up or supplementary source of cooling. Supplementary heating is supplied by an electrical heater such as resistance heater, while primary cooling is provided by a water coil heat exchanger. Separate thermostats control the heat pump on the one hand, and the resistance heater and water coil heat exchanger on the other hand; the two thermostats can be combined in a simple unit. The thermostats automatically control two valves which control system water flow. For cooling, a first valve opens to permit water flow in series through first the water coil and then the heat pump. For heating, only the second valve is opened to bypass the water coil and deliver water directly to the heat pump. The water is ground water derived from a well, spring or lake. The thermostats automatically switch on the heat pump, heater and/or heat exchanger to effect the necessary heating or cooling.