Microprocessor control for a heat pump water heater

Microprocessor based control system monitors and controls a heat pump water heater system and interfaces the water heating system with an external centralized control system. The microprocessor control system includes sensors, safety switches, device interface relays, user interface devices and precanned software to provide versatile monitoring and control over a heat pump type water heating system.

The system generally involves a typical heat pump coupled with a domestic water heater, hot water retention tank or other body of water. The control system provides operational control over the heat pump to maintain the hot water stored in the domestic water heater in a predetermined setpoint and controls the use of electric resistance heating elements in the domestic water heater for added heating capacity for quick heat recovery type operation. The control system receives a centralized signal typically from a utility company to disable heat pump water heating operation during peak demand time periods. Control logic is provided to carry out effective liming parameter control, high evaporator temperature control, and defrost/anti-freeze protection control.

It is known to replace or augment conventional electric resistance water heaters with heat pump water heaters as a more efficient means of producing domestic hot water. One prior art method of controlling such heat pump water heaters has been to use two-position bimetallic type thermostats which are generally provided in domestic water heaters as the primary operating control. An example of such a prior art heat pump water heater control circuit may be found at U.S. Pat. No. 5,255,338 (Robinson, Jr. et al). One advantage of this type of arrangement is that the hot water thermostat is located directly in the hot water tank.

A disadvantage associated with the above described method is that two-position bimetallic type thermostats are not as versatile as full range type sensors, such as thermistors, and are not as effective when used with microprocessor type controls. Another disadvantage is that tying into the water heater thermostat and control wiring often results in voiding UL or other industry certifications. Other prior art systems have placed sensors directly in the hot water tank, which results in the disadvantage of added retrofit labor and material costs and, again, the possibility of voiding certifications.

While the comparison of energy costs between heat pump type water heaters versus electric resistance type water heaters favors the use of the heat pump, one detraction from the use of heat pump type water heaters is the issue of quick heat recovery. In keeping the costs of heat pump type water heaters comparable with the costs of electric resistance type water heaters, manufacturers have tried to minimize the size of the compressor used in heat pump type water heaters. An unfortunate result of this is the reduced heating capacity of the heat pump water heater unit. While a typical electric resistance type water heater will deliver 16,000 BTU's per hour of heating capacity, a typical heat pump type water heater has a much reduced heating capacity of 7,000 BTU's per hour. Accordingly, when a large demand for hot water consumes the hot water stored in the hot water retention tank, the electric resistance type water heater is able to more rapidly heat the replacement cold water than a typical heat pump type water heater.

For consumer satisfaction, a quick heat recovery rate is essential. For this reason heat pump type water heaters are most often used in conjunction with conventional electrical resistance type water heaters. The electric resistance heating elements are generally used to compliment the heat pump water heating capacity during periods of large demand. Another problem typically associated with heat pump water heaters is that of liming, which effectively reduces the capacity of the unit and may eventually lead to compressor damage. A prior art method of preventing compressor damage due to liming was to include a high pressure switch to terminate compressor operation upon excessive pressure being exhibited in the heat pump system. A draw back associated with this is that the compressor is shut down, often prematurely, with no advance warning and a service call is required to place the unit in an operating condition.

Another condition associated with heat pump operation is that of high evaporator temperature, which corresponds to high suction pressure. Generally, the suction side high pressure limit for a heat pump water heater type compressor is 90 PSIG, this corresponds to an evaporator refrigerant discharge temperature of 62°.

In the case of an earth ground loop system operating under summer conditions, the loop temperature will often be in the range of 80°-100° F. or above. This results in elevating the evaporator refrigerant discharge temperature above 62° F. and tripping the high suction pressure limit switch. Prior art heat pump water heaters coupled to a ground loop system simply lock out compressor operation based upon a high pressure limit switch located on the suction side of the compressor. In the case of air-to-water type heat pump units, freeze protection on the evaporator coil is of prime importance. Prior art heat pump water heaters utilize a two-position bimetallic type thermostat which locks out compressor operation upon experiencing a freeze condition at the intake of the evaporator coil. To place the heat pump unit in a condition for operation, a service call was necessary or at least a resetting of the freeze-stat by maintenance personnel.

In one embodiment the invention provides an electronic control system for controlling a heat pump water heater including a compressor, an evaporator, and a condenser coupled with a hot water retention tank to form a hot water circuit. The hot water retention tank includes means for receiving water from a supply and provides domestic hot water. The heat pump water heater control system consists of the following components.