Control of space heating and water heating using variable speed heat pump

In an integrated heating system for simultaneously heating a space and hot water including a compressor with a variable speed drive for compressing a heat transfer fluid, the compressor having a suction port and a discharge port, a water heat exchanger and indoor heat exchanger operatively connected in serial arrangement to the discharge port of the compressor, an outdoor heat exchanger operatively connected to the suction port of the compressor, and expansion device operatively connected to the indoor and outdoor heat exchangers, the indoor and outdoor heat exchangers each having a variable speed fan associated therewith for moving air over each heat exchanger for transferring heat to and from the heat exchangers, a thermostat device for presetting a desired space temperature and comparing the desired space temperature to the actual space temperature for determining a space heating load, and a controller having outputs for controlling the speed of the compressor variable speed drive and the indoor variable speed fan, and inputs for receiving signals from the thermostat device for the space heating load, from the indoor variable speed fan for the speed of the indoor fan, and from an outdoor air temperature sensor for the outdoor air temperature, a method of operating the heating system comprising the steps of:

• sensing the space heating load for the space to be heated;
• controlling the speed of the variable speed indoor fan in response to the space heating load;
• sensing the outdoor temperature;
• comparing said outdoor temperature with said space heating load;
• controlling the speed of the compressor variable speed drive as a predetermined function of said outdoor temperature and said space heating load;
• and
• limiting the operation of the the expansion valve as a predetermined function of said outdoor temperature and the speed of the compressor variable speed drive.