Abstract:
A 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.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under Title 35, U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No. 60/014,417, entitled MICROPROCESSOR CONTROL FOR A HEAT PUMP WATER HEATER, filed on Mar. 29, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention generally relates to electronic control systems used in controlling liquid heating apparatus for raising the temperature of connected bodies of water. More particularly, the present invention relates to a control system which controls a heat pump which is coupled in heat exchange relationship to a domestic water heater, or other body of water to be heated such as a spa. Such a heat pump may be self-contained with a hot water retention tank provided therein and may be either an air-to-water type unit, a water-to-water type unit or a direct exchange (DX)-to-water type unit. 
     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&#39;s per hour of heating capacity, a typical heat pump type water heater has a much reduced heating capacity of 7,000 BTU&#39;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. 
     SUMMARY OF THE INVENTION 
     In particular, the invention relates to a microprocessor based control circuit which utilizes resident operational programs, variable signal inputs and contact closure type inputs and outputs to monitor and control heat pump water heater apparatus. The present invention microprocessor based control system is used for monitoring and controlling heat pump based water heating systems and for interfacing the water heating system with a centralized control source, such as an energy provider initiated enabling/disabling signal. In general, a conventional non-reversing heat pump is coupled to a conventional electric resistance type domestic water heater or hot water retention tank for the purpose of elevating the temperature of the water stored in such water heater or tank for providing domestic hot water. In the alternative, the heat pump may be coupled to any other body of water to be heated, such as a hot tub, spa or pool. A stand alone heat pump water heater having an integral hot water retention tank may be used and air-to-water, water-to-water, and DX-to-water type heat pumps are fully contemplated by the present invention. 
     Rather than utilizing the conventional thermostatic controls included with a domestic water heater to operate the heat pump, a microprocessor based control system is utilized for enhanced system operation and operator interface. A hot water temperature sensor, such as a thermistor, is placed in the hot water circuit and monitors hot water. Typically, the heat pump condenser is coupled with the domestic water heater so as to form a hot water circuit with a dedicated hot water pump interposed therein. Operational programs are downloaded and stored in memory associated with the microprocessor and include such routines as demand sampling, periodic sampling, on peak setback, quick heat recovery mode, liming parameter control, high evaporator temperature control, fault retry, loop pump slaving, testing and diagnostics. 
     To implement demand sampling, temperature sensors are placed in the cold water supply entering the domestic water heater and in the domestic hot water supply exiting the domestic water heater. In the event a decrease in temperature is sensed in the cold water supply and an increase in temperature is sensed in the domestic hot water supply, the control system energizes the dedicated hot water pump so as to cause hot water to circulate from the domestic water heater into the condenser and back into the domestic water heater. 
     In this manner the temperature of the hot water stored in the domestic water heater is sampled by the microprocessor control system and in the event of a call for heating, the microprocessor cycles the heat pump. If there is no demand for hot water, as sensed by the cold water supply sensor and the domestic hot water supply sensor, the microprocessor will cycle the hot water supply pump and sample the hot water temperature at preset periodic intervals, say every other hour. 
     In the alternative, the microprocessor control system may utilize a preset periodic sampling routine which energizes the dedicated hot water pump and samples the hot water temperature therein according to preset periodic intervals. For instance, every fifteen minutes the pump will be turned on and the temperature sampled to determine if the temperature of the water stored in the hot water retention tank has dropped below a preset hot water setpoint. A disadvantage associated with this alternative is that at a minimum the hot water pump is required to run at the beginning of each periodic sampling period just for determining demand use. 
     Another feature incorporated in the microprocessor control system of the present invention is on peak setback control. This permits an external signal, such as that generated by a centrally located energy source such as a utility, to disable all hot water heating operation during peak demand periods, i.e. those periods when overall energy use is high and the cost of energy is at a peak. Such a signal may be communicated via radio frequency or other communication medium and is generally recognized by the microprocessor controller in the form of a contact closure grounded signal. An override switch may be provided at the heat pump water heater unit to override the on peak disabling signal and to independently enable water heating operation. 
     Another feature associated with the microprocessor control system of the present invention is quick heat recovery mode, wherein electric resistance heating elements of a conventional domestic hot water heater may be utilized to supplement the heating capacity of the heat pump water heater. Upon a substantial demand for hot water, it is extremely important for a water heating system to provide quick heat recovery for additional hot water usage to adequately satisfy the needs of the end users. 
     The quick recovery mode of operation may be disabled by a centrally initiated control signal in a manner similar to that described above relating to the on peak setback feature. The use of the electric resistance heating elements to supplement the heating capacity of the heat pump water heater may be disabled by a central control signal, such as generated by a utility company for various purposes. Again, the signal may be communicated via radio or other communication means and is generally recognized by the microprocessor controller in the form of a contact closure grounded signal. 
     As an example, with the hot water temperature more than say 50° F. below setpoint, i.e. 85° F. with a setpoint of 135° F., the quick heat recovery mode logic allows the electric resistance heating elements of the domestic water heater to cycle on until the hot water temperature reaches say 25° F. below setpoint, i.e. 110° F., for thirty continuous seconds. In this manner the effective water heating capacity of the system is effectively doubled so as to increase comfort during high hot water draw peak periods, such as multiple showers etc. The microprocessor control system is provided with random start logic so that after the on peak setback signal has changed states so as to permit heat pump water heater operation, the heat pump water heater units will be randomly started over a preset period of time to prevent excessive instantaneous energy demand during power up. 
     Another feature associated with the microprocessor control system of the present invention is condenser liming parameter control logic. When liming occurs the heat exchanger capacity decreases, effectively making the heat exchanger smaller and smaller such that eventually the compressor cannot maintain setpoint due to insufficient heat transfer. As mineral buildup increases at the condenser, head pressure will be proportionately elevated as the heat pump unit continues to maintain the preset temperature of delivery water. If left unchecked, premature compressor wear and damage will result. Rather than simply using a high pressure lock-out switch, the microprocessor control system of the present invention prevents premature compressor lock-outs and service calls by adjusting the hot water setpoint and implementing a series of retry logic routines. 
     In the event a high pressure situation occurs, a high pressure switch trips and signals a fault condition to the microprocessor. Control logic within the microprocessor discontinues heat pump unit operation, reduces the hot water setpoint by say 5° F. and initiates a five minute delay period. At the end of this delay period the control system restarts the water heating operation. If the high pressure switch does not trip, then the unit will continue to operate at the reduced setpoint and the microprocessor control system will begin flashing an LED service light which will alert the service technician at the next scheduled servicing of the unit that a liming condition exists. If after the delay period the high pressure switch again trips and signals a fault, then the control logic will again discontinue heat pump operation, reduce the setpoint an additional say 5° F. and initiate another say five minute delay period. If the fault condition persists after a given number of tries and the hot water setpoint is reduced to a preset minimum, then the water heating unit is locked out and an audible alarm is sounded. 
     Another feature included in the microprocessor control system is high evaporator temperature logic which disables the heat pump when loop temperature becomes excessive, this feature is primarily for use with water-to-water ground loop systems. In the case of an earth coupled ground loop system, particularly in southern regions during the summer, the heat source loop water may reach temperatures above 90° F. In such extreme conditions the suction pressure will be above the typical 90 PSIG compressor limit and damage to the compressor may occur. 
     Another feature of the microprocessor control system involves providing a loop pump slaving signal between multiple heat pump water heater control boards whereby a remote loop pump may be energized according to a slaving signal. If any one of the multiple heat pump water heater control boards calls for loop pump operation then the remote loop pump will be energized. An additional feature incorporated in the microprocessor control system relates to fault retry logic. The fault retry logic implements a retry routine whereby selected faults reported to the microprocessor are retried at least once before heat pump water heater operation is locked out. In addition, a 30 second fault recognition period is required before a fault signal will be recognized as a fault. This retry feature serves to reduce unnecessary nuisance service calls and to prevent unnecessary heat pump water heater operation shutdown. 
     An additional feature of the microprocessor control system relates to a test mode routine which through an operator interface is selectable by service personnel to achieve shortened time delays for faster diagnostics. In addition, a diagnostic routine is provided whereby all inputs, outputs, thermistor status, and dynamic sensor modes (real time display of sensor input faults) can be displayed via one or more LEDs for fast and simple control board diagnostics. A “soft” reset may be implemented by using a reset switch after fault lockout, whereby all applicable fault LED indicators will remain lit for easy troubleshooting by service personnel. Upon initiating a “hard” reset, such as by removing power, all fault indication is cleared. 
     The microprocessor control system of the present invention utilizes a temperature sensor which is located in the suction line between the evaporator and the compressor. In the event the suction line temperature sensor senses an entering temperature of say 58° F., which directly relates to the 90 PSIG suction pressure limit, the control system disengages the heat source loop pump. In this manner the heat source is drawn into the evaporator section in a segmented rather than continuous fashion, thereby effectively dropping the average loop water temperature to say 85° rather than the actual heat source temperature of say 97°. By dropping the average loop water temperature by say 12° F., the microprocessor control system allows effective compressor operation and avoids unnecessary lock-out. The heat source loop pump is reactivated when the temperature of the fluid entering the compressor has dropped to say 48° F. 
     In the case of air-to-water type heat pump units, the evaporator air coil may frost up or freeze if the coil gets below the freezing point of water. A freeze protection sensor is placed on the entering side of the evaporator air coil and provides a signal representative of that temperature to the microprocessor controller. Upon sensing an air coil condition of say 28° F., a fault condition is reported and the electric heating resistance elements are energized to satisfy any call for hot water heating. In addition, a freeze protection LED is caused to flash indicating a frost or low ambient condition and the operating mode transitions to emergency mode. Rather than completely shutting the unit down, retry logic as described above is utilized for a freeze fault condition. The time interval of the delay period may be extended based upon the temperature sensed and the unit may or may not be locked out after a given number of consecutive faults. If the temperature does not go above say 35° F., then the heat pump unit stays in the emergency mode allowing the electric resistance heat to satisfy any demand for hot water and the low ambient freeze protection LED continues to flash. 
     The microprocessor control system accepts an optional aquastat type control signal to directly control or augment the control of the water heating system. This is particularly useful when utilizing the heat pump water heating system in conjunction with a pool or hot tub type spa. 
     The microprocessor control system of the present invention includes numerous safety controls, high pressure switch, low pressure switch, freeze protection, audible alarms, LED&#39;s for diagnostic and fault condition indication and short cycle protection. In addition, testing, fault retry, diagnostics, startup, and random start routines are provided for enhanced system operation. A multiple pin dip switch is utilized for direct user interface to permit field selectable options such as service test mode, air/liquid/DX based unit selection, time sampling/demand sampling control selection, freeze protection setting, hot water temperature setting selection, and diagnostics routine selection. All of the above described features are advantages of the microprocessor control system of the present invention over prior art heat pump water heating apparatus. 
     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. A first sensor for sensing the temperature of water in the hot water circuit and for generating a first output signal representative of such temperature. A second sensor for sensing the temperature of the cold water supply at the inlet of the hot water retention tank and for generating a second output signal representative of such temperature. A third sensor for sensing the temperature of the domestic hot water at the output of the hot water retention tank and for generating a third output signal representative of such temperature. A pump for circulating water from the hot water retention tank, through the condenser and back to the hot water retention tank. A microprocessor receives the first, second and third sensor output signals and, upon detecting a drop in the temperature of the water at the receiving means and a rise in the temperature of the water at the discharge means energizes the pump and samples the hot water circuit temperature. The microprocessor cycles the heat pump water heater to maintain a predefined hot water setpoint. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic diagram of the heat pump water heating system of the present invention utilizing a water-to-water heat pump coupled to a conventional electric resistance domestic water heater; 
     FIG. 2 is a schematic diagram of the heat pump water heating system of the present invention utilizing a water-to-water heat pump with an integral hot water retention tank; 
     FIG. 3A is a schematic diagram of the microprocessor based electronic control system of the heat pump water heating system of FIG. 1; 
     FIG. 3B is a schematic diagram showing a typical optional external solenoid valve which may be operated by the control system of FIG. 3A; 
     FIG. 4A is a partial schematic diagram showing an alternative evaporator section of the heat pump water heating system of FIG. 1 utilizing an air-to-water type heat pump unit; and 
     FIG. 4B is a partial schematic of an alternative arrangement of the heat pump water heating system of FIG. 1 utilizing a DX-to-water type heat pump unit. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1 the present invention is shown having microprocessor based control system  20 , illustrated in FIG. 3A, for monitoring and controlling heat pump based water heating system  22  and for interfacing water heating system  22  with centralized control source  24 , such as energy provider initiated enabling/disabling signals. Conventional non-reversing heat pump  26  is coupled to electric resistance type domestic water heater  28  or integral hot water retention tank  30 , as shown in FIG. 2, for elevating the temperature of water stored in water heater  28  or tank  30 , typically used for providing domestic hot water. In the alternative, heat pump  26  may be coupled to any other body of water to be heated, such as a hot tub, spa or pool. FIG. 2 illustrates stand alone heat pump water heater  32  having integral hot water retention tank  30 . 
     Rather than utilizing thermostatic controls associated with conventional domestic water heater  28  to operate heat pump water heater system  22 , microprocessor based controller  20  is utilized for enhanced system operation and interface. Selection of temperature setpoint may be accomplished via a DIP switch on controller  20 . Hot water temperature sensor  34 , such as a thermistor, is placed in hot water circuit  36  to monitor hot water circuit temperature and generates a signal which is input to controller  20 . Heat pump condenser  38  is coupled with domestic water heater  28  so as to form hot water circuit  36  with dedicated hot water pump  40  interposed therein. Operational programs are downloaded into controller  20  and include such routines as demand sampling, periodic sampling, on peak setback, quick heat recovery mode, liming parameter control, and high evaporator temperature control. An external thermostat may be utilized via controller  20  to initiate compressor/pump/fan operation to satisfy a demand for hot water. 
     In implementing demand sampling, temperature sensors  42 ,  44  are respectively placed in the cold water supply entering the domestic water heater and in the domestic hot water supply exiting the domestic water heater. In the event a decrease in temperature is sensed at the cold water supply and an increase in temperature is sensed at the domestic hot water supply, a demand flag transitions to an active state and controller  20  energizes the dedicated hot water pump  40  so as to cause hot water to circulate from domestic water heater  20  into condenser  38  and back into the domestic water heater. In this manner the temperature of the water stored in domestic water heater  28  is sampled by controller  20  and in the event of a call for heating, controller  20  cycles heat pump  26  to maintain setpoint. In the event no hot water demand is sensed by cold water supply sensor  42  and domestic hot water supply sensor  44 , controller  20  will sample the temperature of hot water circuit  36  at preset periodic intervals, such as once every other hour. 
     In the alternative, controller  20  may utilize a preset periodic sampling routine which energizes dedicated hot water pump  40  and samples the hot water circuit temperature at preset periodic intervals. For instance, every fifteen minutes pump  40  will be turned on and the hot water circuit temperature sampled by sensor  34  to determine if the temperature of the water in domestic water heater  28  has dropped below a preset hot water setpoint. 
     Microprocessor controller  20  stores historical information relating to usage in memory and utilizes trend routines to effectively “learn” demand usage patterns. In this manner controller  20  can predict periods and patterns of heightened hot water demand and, in advance of such periods, raise hot water temperature to a desirable level. 
     Controller  20  utilizes on peak setback control programming which permits external enabling/disabling signal  24 , such as that generated by a centrally located energy source, such as a utility, to disable hot water heating operation during peak demand periods, i.e. when overall energy use is high and the cost of energy is at a peak. Such a signal may be communicated via radio frequency or other means of communication and is generally recognized by controller  20  in the form of a contact closure grounded signal. Override switch  46  may be provided at the heat pump water heater unit to override the on peak disabling signal and to independently enable water heating operation. 
     Controller  20  also utilizes a quick heat recovery routine, wherein electric resistance heating elements  48  of conventional domestic hot water heater  28  may be utilized to supplement the heating capacity of heat pump water heater  26 . This quick heat recovery mode of operation is important when a substantial demand for hot water is experienced. In this manner the overall water heating capacity of water heating system  22  is increased to adequately satisfy the needs of the end users. 
     The quick heat recovery mode of operation may be disabled by centrally initiated control signal  24  in a manner similar to that described above relating to the on peak setback feature. In addition, quick heat recovery mode may be locally disabled via a DIP switch provided on controller  20 . The use of electric resistance heating elements  48  to supplement the heating capacity of heat pump water heater  26  may be disabled by central control signal  24  which may be utilized by a utility company for various purposes. Again, signal  24  may be communicated via radio or other communication means and is generally recognized by controller  20  in the form of a contact closure grounded signal. As an example, with the hot water temperature more than say 50° F. below setpoint (say 135° F.) the quick heat recovery mode logic allows electric resistance heating elements  48  of domestic water heater  28  to become energized until the hot water temperature reaches say 25° F. below setpoint for say thirty continuous seconds. In this manner, the effective water heating capacity of system  22  is effectively doubled to increase comfort during high hot water draw peak periods, such as multiple showers etc. Controller  20  is provided with random start logic so that after the on peak setback signal has changed states so as to permit heat pump water heater operation. Where multiple heat pump water heating systems are enabled/disabled by a single central control signal  24 , the heat pump water heater units will be randomly started over a preset period of time to prevent excessive instantaneous energy demand. 
     Controller  20  also utilizes condenser liming parameter control logic. As mineral buildup increases at condenser  38 , head pressure will be proportionately elevated to maintain the temperature of the delivery water. If left unchecked eventually premature compressor wear and resulting damage will result. When liming occurs the heat exchange capacity of heat pump  26  decreases, effectively making the heat exchanger smaller and smaller such that eventually compressor  50  cannot maintain setpoint due to insufficient heat transfer. Rather than simply using a high pressure lock-out switch, controller  20  prevents premature compressor lock-out and service call situations by adjusting the hot water setpoint and implementing a series of retry logic routines. 
     In the event a high pressure situation occurs, high pressure switch  52  trips and signals a fault condition to controller  20 . Control logic associated with controller  20  discontinues the operation of heat pump unit  26 , reduces the hot water setpoint by say 5° F. and initiates a five minute delay period. At the end of this delay period controller  20  restarts the water heating operation. If high pressure switch  52  does not trip, then the unit will continue to operate at the reduced setpoint and the controller  20  will begin flashing LED service light  54  to alert a service technician at the next scheduled servicing of unit  22  that a liming condition exists. If high pressure switch  52  again trips and signals a fault, then the control logic will again discontinue heat pump operation, reduce the setpoint an additional say 5° F. and initiate another say five minute delay period. If the fault condition persists after a given number of tries and the hot water setpoint is reduced to a preset minimum, then water heating unit  22  is locked out and audible alarm  56  is sounded. 
     Controller  20  is also provided with high evaporator temperature logic which disengages heat source loop pump  64  when loop temperature becomes excessive, this feature is primarily for use with water-to-water ground loop systems. In the case of an earth coupled ground loop system, particularly in southern regions during the summer, the temperature of liquid heat source loop  58  may be above 90° F. In such conditions the suction pressure associated with compressor  50  will be above the typical 90 PSIG limit and damage to compressor  50  may occur. Controller  20  receives an input from suction side temperature sensor  60 , which is located in the suction line between evaporator  62  and compressor  50 . Upon sensing an entering temperature of say 58° F., which directly corresponds to the 90 PSIG suction pressure limit, controller  20  disengages heat source loop pump  64 , which may be a remotely located central pump. In the alternative, an air-to-water type heat pump unit, as shown in FIG. 4A, or a DX-to-water type heat pump, as shown in FIG. 4B, may be used in lieu of the water-to-water ground loop type system. 
     Where there are multiple heat pump water heaters thermodynamically connected to a central heat source loop system, any one controller  20  may energize the central pump. In this manner, the heat source is drawn into evaporator  62  in a segmented rather than continuous fashion, thereby effectively dropping the average loop water temperature to say 85° F. rather than the actual heat source temperature of say 97° F. By dropping the average loop water temperature by say 12° F., continuous and effective compressor operation is achieved. Heat source loop pump  64  is restarted when the temperature in the suction line of compressor  50  has dropped to a reading of say 48° F. 
     An example of the freeze protection routine operation is as follows. In the case of an air-to-water type heat pump unit, as shown in FIG. 4A, evaporator air coil  66  may frost up or freeze should the coil get below the freezing point of water. Freeze protection sensor  68  is placed on the leaving side of the evaporator air coil and provides a signal representative of that temperature to controller  20 . Upon sensing an air coil condition of say 15° F. a fault condition occurs and electric heating resistance elements  48  are energized as needed to satisfy any call for hot water heating. During a freeze condition, LED  54 , which may be multiple LEDs, indicates the fault condition, such as by flashing. Controller  20  utilizes retry logic as described above in the event of a freeze fault condition rather than simply shutting the unit down. The time interval of the retry logic delay period may be extended based upon the temperature sensed by sensor  68 . 
     Heat pump unit  26  may or may not be locked out after a given number of consecutive faults. If the sensed temperature does not rise above say 35° F., then heat pump unit  26  stays in the emergency mode, whereby electric resistance heat satisfies any demand for hot water and LED  54  continues to flash. LED  54  flashes in predefined patterns which are distinguishable one from the other depending upon the fault condition(s) that exist(s). In the alternative, multiple LEDs may be used with each having a specific function and such LEDs may flash or remain on (solid) for fault indication. 
     Controller  20  will accept an optional aquastat type control signal to directly control or augment the control of water heating system  22 . This is particularly useful when utilizing the heat pump water heating system in conjunction with a pool or hot tub type spa which may be used in lieu of domestic water heater  28 . 
     The microprocessor based control system, illustrated in FIG. 3A, incorporates numerous safety controls including high pressure switch  52 , low pressure switch  72 , freeze protection  68 , audible alarms  56 , LED&#39;s  54  for diagnostic and fault condition indication and short cycle protection. In addition, testing, fault retry, diagnostics, startup, and random start routines are provided for enhanced system operation. A multiple pin dip switch SWI,  55 , is utilized for direct user interface to permit field selectable options such as service test mode, air/liquid/DX source based unit selection, time sampling/demand sampling control selection, freeze protection setting, hot water temperature setting selection, and diagnostics routine selection. 
     Microprocessor controller  20  provides a loop pump slaving signal between multiple heat pump water heater control boards whereby a remote loop pump may be energized according to the slaving signal. If any one of the multiple heat pump water heater control boards calls for loop pump operation then the remote loop pump will be energized. An additional feature incorporated in microprocessor controller  20  is fault retry logic. The fault retry logic implements a retry routine whereby selected faults reported to microprocessor  20  are retried at least once before heat pump water heater operation is locked out. In addition, a 30 second fault recognition period is required before a fault signal will be recognized as a fault. This retry feature serves to reduce unnecessary nuisance service calls and prevent unnecessary heat pump water heater operation shutdown. 
     Microprocessor controller  20  provides a test mode routine which through an operator interface is selectable by service personnel to achieve shortened time delays for faster diagnostics. A diagnostic routine is provided whereby all inputs, outputs, thermistor status, and dynamic sensor modes (real time display of sensor input faults) can be displayed via LEDs for fast and simple control board diagnostics. A “soft” reset may be implemented by using a reset switch after fault lockout, whereby all applicable fault LED indicators will remain lit for easy troubleshooting by service personnel. Upon initiating a “hard” reset, such as by removing power, all fault indication is cleared. 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.