Heat pump water heater control circuit
BACKGROUND OF THE INVENTION 
     Because of the better coefficient of performance of heat pumps as compared to electrical resistive heating elements, there has been a movement in recent years to purchase electric resistance hot water heaters and then connect the water heater tank to a heat pump so that the heating was actually performed by the heat pump rather than the electric resistance elements in the water heater itself. This produced a good coefficient of performance while at the same time retained the electric resistance heating elements as a standby. One such arrangement is illustrated in U.S. Pat. No. 4,740,673, issued May 26, 1988, entitled &#34;Dual Control Thermostat Circuit&#34; that allows the conventional upper and lower thermostats to control the operation of the external heat pump. 
     One of the problems associated with heat pump heaters for water heaters is that the heating rate of the heat pump is typically slower than the heating rate associated with the resistance heating elements that come in the water heater. As a result, the heat recovery rate was less than desirable when there was a large drain of hot water from the tank and the upper section of the tank started to cool. 
     SUMMARY OF THE INVENTION 
     These and other problems and disadvantages associated with the prior art are overcome by the invention disclosed herein by providing an external circuit that works in conjunction with the existing tank thermostatic controls to control both the heat pump and the existing resistance elements by allowing the water heater to switch to the resistance heating mode automatically when there is a large drain of hot water and the upper section of the tank begins to cool. The heat pump and resistance modes may operate nonsimultaneously or simultaneously as desired. 
     The heat pump water heater control circuit of the invention includes an impedance and a thermostatically operated bypass switch in parallel with each other connecting the existing tank circuit to line voltage through the existing thermostatic switch in the water heater. The impedance has a sufficiently high impedance to reduce the current flow through the resistive heating elements to the level that substantially prevents the heating capability of the electric resistive heating elements when the existing thermostatic switch network in the hot water tank normally associated with the electrical resistive heating elements closes. The impedance controls an operative element to connect the heat pump circuitry to line voltage to operate the heat pump when line voltage is imposed across the impedance. The bypass switch closes when a sufficiently large volume of hot water is withdrawn from the water tank to cause the temperature in the upper portion of the water tank to drop. This re-enables the existing upper resistive heating element and disables the impedance until the temperature in the upper portion of the tank is raised back to the desired temperature. In those instances where both the heat pump and resistance element can be simultaneously operated, a latching relay is used to keep the heat pump working when the bypass switch closes. 
    
    
     These and other features and advantages of the invention will become more clearly understood upon consideration of the following detailed description and accompanying drawings wherein like characters of reference designate corresponding parts throughout the several views and in which: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an electrical schematic illustrating the control circuit of the invention shown connected to a conventional dual element electric water heater; 
     FIG. 2 is a diagrammatic view of a water heater with the invention installed; and, 
     FIG. 3 is an electrical schematic illustrating a second embodiment of the invention. 
     These figures and the following detailed description disclose specific embodiments of the invention, however, is to be understood that the inventive concept is not limited thereto since it may be embodied in other forms. 
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Referring to FIG. 1, the heat pump water heater control circuit 10 is connected to an existing tank circuit 11. This allows the circuit 10 to be connected to the water heater WH (FIG. 2) without having to modify the existing tank circuit 11. 
     The existing tank circuit 11 includes a conductor 12 which is connected to a terminal L 2  adapted to be connected to one side of line voltage. Conductor 12 is connected to one end of both electrical resistance heating elements H 1  and H 2 . The other side of the upper heating element H 1  is connected to contact C 1  of a single pole double throw (SPDT) thermostatically operated switch SW 2  located on the upper portion of the water tank. The other side of the lower heating element H 2  is connected to one side of thermostatically operated switch SW 3  located on the lower portion of the water tank which in turn is connected to the contact C 2  of SPDT switch SW 2 . The common side C 3  of the switch SW 2  is connected to conductor 14 which in turn is connected to a terminal L 1  &#39; adapted to be connected to the other side of line voltage in its originally intended usage. Typically, line voltage is 230 volts although different line voltages would work just as well. 
     The heat pump water heater control circuit 10 is designed to connect the existing tank circuit 11 to line voltage so that the circuit 10 effectively controls the water heating operation. The circuit 10 includes wire 20 which is connected to one side of line voltage, typically at terminal L 2 . That side of the circuit 11 still remains connected to one side of line voltage at terminal L 2 . Terminal L 1  &#39; in existing circuit 11 is disconnected from line voltage and connected to the circuit 10 as will become more apparent. 
     The heat pump water heater control circuit 10 also includes wire 21 which is connected to conductor 14 at terminal L 1  in the existing tank circuit 11. Wire 21 is connected to one side of a high impedance relay coil RC 1  of relay R 1 . The opposite side of the coil RC 1  is connected to one side of a high pressure cutout switch SW 4 . Switch SW 4  is a safety switch which opens to disable the heat pump circuit when the refrigerant pressure becomes too high. 
     As explained in U.S. Pat. No. 4,740,673, the resistance of the relay coil RC 1  is selected so that the current flow therethrough required to operate the relay will be minimal when line voltage is imposed across it. In the particular example, the relay R 1  is selected with a 10 watt coil which allows a current flow of about 1/20 amp when 230 volts is imposed across it. Typically, the current flow through an existing tank circuit to cause the heating elements H 1  and H 2  to heat produces current flow in the neighborhood of 10-25 amps. Thus, when relay coil RC 1  is connected in series with either resistance elements H 1  and H 2 , negligible heat is generated in the water but there will be sufficient current flow to operate relay R 1 . 
     The other side of the switch SW 4  is connected to the heat pump contact C HP  of a manually operated selector switch SS M  through fuse F and wire 22. The common side of selector switch SS M  is connected to line voltage at terminal L 1 . The resistance heating contact C RES  of switch SS M  is connected directly to the original terminal L 1  &#39; while the &#34;off&#34; contact C OFF  is left unconnected. The selector switch SS M  allows the user to manually select heat pump operation when contact C HP  is closed, to select only electrical resistance heating when contact C RES  is closed or to turn the water heater off when contact C OFF  is closed. 
     When the contact C HP  is closed, line voltage at terminal L 1  is supplied through fuse F and contacts C c  of the relay R 1  to a heat pump circuit 24. The circuit 24 is connected to the other side of line voltage at terminal L 2  through fuse F and wire 20. The heat pump circuit 24 includes a water pump motor M 1 , a fan motor M 2  and a compressor motor M 3  connected in parallel with each other between the wires 20 and 22 through the contacts C c  of relay R 1 . 
     To take advantage of the higher recovery rate of the resistance heating element H 1  when a large volume of hot water is withdrawn from the water tank, a thermostatically operated switch SW 5  is connected in parallel across the coil RC 1  so that closing switch SW 5  shorts out coil RC 1  to effectively disable relay R 1  and the heat pump while at the same time re-enabling the heating element H 1  as will become more apparent. The switch SW 5  is preferably a bulb type thermostat so that the capillary tube 25 and sensing bulb 26 thereof can be inserted into the water tank WT through the hot water return line RL from the heat pump HP as diagrammatically shown in FIG. 2. This permits the bulb 26 to be located at about the same height in the water tank WT as the upper thermostatic switch SW 2 . An appropriate coating such as plastic is applied to the capillary tube 25 and the bulb 26 to protect them and isolate the operating fluid in the switch from the potable water. The set points of the thermostatic switches SW 2  and SW 5  are selected to be the same so that switch SW 5  closes when the contact C 1  of the switch SW 2  closes so that the upper heating element H 1  is energized at the same time the relay R 1  is disabled. 
     Because the heating capacity of the heat pump HP is undesirably reduced when the temperature of the ambient air from which the heat pump is extracting heat falls below a certain range, an ambient thermostatic bypass switch SW 6  is also connected in parallel across the coil RC 1  of the relay R 1 . Thus, the heat pump circuit 24 is also disabled and the resistance heating elements H 1  and H 2  re-enabled when the switch SW 6  closes. Typically, the set point of the switch SW 6  is about 45° F. 
     When either thermostatic switch SW 2  or SW 3  closes to call for water heating, coil RC 1  of relay R 1  is energized to close contacts C c  and energize the compressor CP, pump PP and fan FN. The small current through the coil RC 1  produces a negligible voltage drop through the particular heating element H 1  or H 2  connected in the circuit. Water is circulated through and heated by the heat pump HP until both tank thermostatic switches SW 2  and SW 3  are satisfied. The coil RC 1  is de-energized when the switches SW 2  and SW 3  are satisfied to turn off the heat pump circuit 24. 
     If a sufficiently large amount of hot water is withdrawn from the water tank to cause the contact C 1  of the upper thermostatic switch SW 2  to close and the switch SW 5  to close due to the drop of water temperature, the coil RC 1  of contactor relay R 1  is bypassed allowing full current to be supplied to the heating elements, normally to the upper element H 1  since thermostatic switch SW 2  will be closed when the upper section of the tank begins to cool. Thermostatic switches SW 2  and SW 5  should be set at approximately the same temperatures. When thermostatic switch SW 5  is satisfied and opens, the system reverts back to the heat pump mode until the lower thermostatic switch SW 3  is satisfied. This configuration does not allow simultaneous operation of the resistance and heat pump modes. 
     When electric codes allow both the heat pump and the electrical resistance heating elements to operate at the same time, a second embodiment of the invention seen in FIG. 3 can be used. The heat pump water heater control circuit 110 has the same components as the first embodiment 10 with the same reference numbers applied thereto. Additionally, a relay coil RC 2  of a holding relay R 2  in series with a normally closed temperature or pressure switch SW 7  is connected between the wires 20 and 22 in parallel with the heat pump circuit 24 so that closing of contacts C c  serves to energize the relay R 2 . The holding contacts C h  of the relay R 2  are connected in parallel across the contacts C c  so that the contacts maintain the heat pump circuit 24 energized even if the relay R 1  is de-energized. Switch SW 7  may be a thermostat which senses the temperature of the water circulating through the heat pump or a pressure switch which senses refrigerant condensing pressure in the heat exchanger. When the circulating water temperature or refrigerant condensing pressure reaches a preset level, switch SW 7  opens causing relay R 2  to be de-energized and contacts C h  to open until the next cycle. 
     The circuit 110 operates like the circuit 10 up to the time the contacts C c  close. The relay R 2  causes the contacts C h  to close so that the heat pump HP continues to operate regardless of the activation state of relay R 1 . At the same time, the upper heating element H 1  has full power applied thereto so that it also heats the water. This configuration gives the very fastest recovery rate of hot water. Once the water temperature at the upper portion of the water tank opens the switch SW 5 , the circuit 110 causes the system to revert back to heat pump heating only.