Abstract:
A control system for a reversible heat pump includes a system controller which polls the heating or cooling demands of zone controllers associated with a series of heat exchangers downstream of the heat pump. The system controller is operative to configure the heat pump in response to which of the demands is dominant. The system controller is also operative to change the configuration of the heat pump in response to a change in dominant demand. The implemented change in heat pump configuration is preferably premised on the temperature of water returned to the heat pump being within a predefined range of temperature. The return water temperature requirement may be overriden by the system controller if an acceptable period of time for allowing the returning water temperature to reach an acceptable temperature level has elapsed.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to reversible heat pump systems which condition water that is to be circulated through one or more heat exchangers downstream of the heat pump system. 
     Heat pump systems which provide heated or cooled water to one or more heat exchangers are typically not required to switch from a heating mode to a cooling mode very often. This is in part due to the inherent inertia of a heat pump in trying to heat water that has been previously cooled. In this regard, the compressor within the heat pump must process a sufficient amount of refrigerant over time that can give up the necessary heat to the water that has been previously cooled. The compressor is presented with a similar heavy load situation when it is required to process a sufficient amount of refrigerant over time to absorb heat from previously heated water so as to produce cooled water. This inability to switch between heating and cooling or vice versa has previously led to switching the mode of operation of the heat pump system infrequently. For instance, changeovers would be implemented on particular calendar dates indicating normal change of seasonal weather conditions. On the other hand, a changeover might be implemented depending on a separately sensed outdoor air temperature indicating whether the heat pump system should be in either heating or cooling for the day. The above described changeover controls do not allow a heat pump system to respond to heating or cooling demands that may change throughout the day. The above described systems moreover do not respond to different demands for cooling or heating throughout a building on a given day. 
     OBJECTS OF THE INVENTION 
     It is an object of this invention to provide a heat pump system with the capability to automatically change from one operating mode to another operating mode at any time regardless of outdoor air temperature or calendar date. 
     It is another object of this invention to provide a heat pump system that will be responsive to different demands for cooling or heating throughout a building on a given day. 
     SUMMARY OF THE INVENTION 
     The above and other objects are achieved by providing a heat pump system controller with control logic, which continually polls the spaces or zones in which heating or cooling may be demanded so as to determine whether there is a predominance of either heating or cooling being demanded. The polling also checks to see whether a determined predominance of demand for either heating or cooling meets certain minimum demand requirements. In the event that minimum demand requirements are met, then a system demand is set reflecting the polling results. For instance, the system demand would be set for producing heated water if the predominance of polled spaces reflected that more spaces requested heating than requested cooling and that the number of spaces requesting heating exceeded some minimum number of spaces required to implement a changeover from cooling to heating. The system demand does not, however, allow for an immediate changeover to heating in the event that a changeover to heating is being requested by the polling results. In particular, the controller will first check to see whether the current mode of operation of the heat pump has been in effect for a minimum time period before stopping the then active compressor. The controller will preferably thereafter inquire as to whether a particular water temperature in the water return line to the heat pump is within a range of temperatures. This will allow the zone controllers associated with heat exchangers that are still demanding cooling to continue giving up heat to the circulating water so as to thereby increase the temperature of the returning water. During this time, the compressor will remain off so as to not be presented with an otherwise heavy load of trying to heat low temperature water. The controller may also override the requirement of raising the return water temperature to a desired temperature level in the event that a particular changeover period of time has elapsed since the compressor was turned off. It is only after the return water temperature is within range or the changeover time period has expired, if the latter is required, that the controller will proceed to actually authorize the changing of the valve position of a reversing valve within the heat pump system so as to configure the heat pump system to a heating mode of operation. The compressor will also be turned on at this point in time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the present invention, reference should now be made to the following detailed description thereof taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a schematic view of a heat pump system configured by a system controller to deliver heated water to heat exchangers associated therewith; 
     FIG. 2 is a schematic view of the heat pump system of FIG. 1 configured by the system controller to deliver cooled water to heat exchangers associated therewith; and 
     FIGS. 3A,  3 B and  3 C present a flow chart of the method used by the system controller within FIGS. 1 and 2 to control the configuration of the heat pump system of FIGS.  1  and  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a reversible heat pump system  10  delivers hot or cold water via a pump  12  to fan coil heat exchangers  14 ,  16  and  18 . Each fan coil heat exchanger typically receives the heated or cooled water from the heat pump and conditions air flowing through the fan coil heat exchanger. The resultingly conditioned air is provided to a space that is to be heated or cooled. This space is often referred to as a “zone of heating or cooling”. Water from the heat pump system  10  flows through the fan coil heat exchanger  14  in the event that a zone controller  20  authorizes such a flow by positioning of a control valve  22 . The zone controller  20  may also divert any water flow around the fan coil heat exchanger  14  by a further positioning of the control valve  22 . It is to be appreciated that the fan coil heat exchanger  16  operates in a similar fashion in response to the positioning of a control valve  24  under the control of a zone controller  26 . It is furthermore to be appreciated that the last fan coil heat exchanger  18  will also be controlled by the positioning of a control valve  28  under the control of a zone controller  30 . Water flow to each heat exchanger within each corresponding fan coil can either fully bypass the heat exchanger, fully flow through the heat exchanger, or partially flow through the heat exchanger and bypass. The control valve position is determined by the zone controller and is a function of the zone&#39;s heating or cooling requirement and the operating mode of the water loop. Each zone controller  20 ,  26  and  30  is also connected to a corresponding temperature sensor such as  32 ,  34  and  36 , which senses the temperature in the respective zone serviced by the fan coil heat exchanger and provides such temperature information to the respective zone controller. Each zone controller will furthermore have a stored setpoint value for the particular zone. This may be a temperature that is arbitrarily defined by an individual either through a programmable thermostat or other device suitable for entering setpoint information. Each zone controller will either have a demand for heat or a demand for cooling or essentially a demand for neither heating or cooling depending on the sensed temperature in the zone versus the zone&#39;s stored setpoint. Each individual zone demand is provided to a system controller  38  via a bus  40 . The system controller  38  is operative to analyze the collected zone demands so as to determine whether the heat pump system  10  should be in a heating or a cooling mode of operation. 
     The heat pump system  10  is shown operating in a heating mode in FIG.  1 . In this mode, heat is extracted from air being drawn over a heat exchanger  42  by a fan  44 . It is to be appreciated that the heat exchanger  42  could also remove heat from a medium other than air. For instance, heat could be extracted from a medium circulating through piping buried in the earth. In any event, refrigerant flowing through this heat exchanger absorbs a large quantity of heat from whatever the heat exchange medium is and stores it in vapor form for later release. The refrigerant in vapor form flows from heat exchanger  42  to a four way reversing valve  46  via a line  48 . The four way reversing valve directs the refrigerant in vapor form to the suction inlet of a compressor  50  via a suction line  52 . The compressor  50  discharges the refrigerant vapor at a high pressure to the reversing valve  46  via a line  54 . The four way reversing valve directs the high pressure refrigerant vapor to heat exchanger  56  via a line  58 . The heat exchanger  56  functions as a condenser in the heating mode. The heat of condensation of the condensing refrigerant circulating through the heat exchanger  56  is absorbed by water returning from the fan coil heat exchangers  14 ,  16 , and  18  via a return line  60 . The water exits the heat exchanger  56  as hot water being drawn by the pump  12 . The refrigerant exits the heat exchanger  56  as a mixture of vapor and liquid refrigerant at high pressure and flows into a receiver  62  via a line  64 . The pool of high pressure, hot refrigerant liquid in the receiver  60  is preferably subcooled before passing out of the receiver on a line  66  connected to a thermal expansion valve  68 . The thermal expansion valve  68  allows the liquid refrigerant to expand to a lower pressure before entering the heat exchanger  42  wherein the liquid refrigerant evaporates absorbing heat from the air or other fluid medium as has been previously described. 
     Referring now to FIG. 2, the heat pump system  10  is illustrated in a cooling mode of operation. In the cooling mode, the four way reversing valve  46  directs hot refrigerant vapor discharged by the compressor  50  via line  54  to heat exchanger  42  via line  48 . The heat of condensation is preferably removed from the hot refrigerant vapor by air flowing over the heat exchanger  42  . This produces high pressure subcooled liquid refrigerant at the outlet end of the heat exchanger  42 . This high pressure subcooled liquid refrigerant flows into the thermal expansion valve  68  and is discharged at a lower pressure. The refrigerant passes through the receiver  60  and enters the heat exchanger  56  operating as an evaporator in this instance. Heat will be extracted from the water circulating through the heat exchanger  56 . The circulating water is the water returning from the fan coil heat exchangers  14 ,  16 , and  18  via return line  60 . The resulting chilled or cooled water is drawn out of the heat exchanger  56  by pump  12 . The low pressure refrigerant vapor is discharged from the heat exchanger  56  via line  58  and is directed by the four way reversing valve  12  to the suction inlet of the compressor  50  via line  52 . 
     Referring again to the system controller  38 , the system controller sends a heating or cooling signal to the four way reversing valve  46  via a line  70 . The four way reversing valve responds to a heating signal by switching to the valve positions shown in FIG. 1 thereby configuring the heat pump system into a heating mode. The four way reversing valve responds to a cooling signal by switching to the valve positions shown in FIG. 2 thereby configuring the heat pump system into a cooling mode. The system controller also sends a signal via a line  72  to a motor  74  for the compressor  50  so as to deactivate the motor  74  when the heat pump system is transitioning from heating to cooling or vice versa. The system controller preferably uses the same line  72  to activate the motor  74  when the transition from one mode to another has been completed. The system controller receives a temperature of the water returning to the heat pump system from a temperature sensor  76  located in the return line  60   
     Referring now to FIGS. 3A,  3 B and  3 C, a process utilized by a programmable microprocessor within the system controller  38  is illustrated. The process begins with an initialization step  100 , which sets the initial values of the following variables: “changeover timer”, “heat run timer”, “cool run timer”, and “system demand” and“system mode. The microprocessor within the system controller  38  will proceed to a step  102  and poll each of the zone controllers for their respective zone demands for heating or cooling. It is to be appreciated that this is preferably done by addressing each zone controller  20 ,  26  and  30  via the bus  40  and requesting the specific zone demand of the zone controller. The zone demand will of course be a function of the difference between setpoint and sensed temperature in the respective zone. The zone demands are stored in a memory associated with the microprocessor within the system controller  38  in a step  104 . The microprocessor proceeds to a step  106  and computes the percentage of the polled zone controllers that have heating demands. This is preferably done by first adding up the number of zone controllers having a heating demand and dividing this number by the total number of zone controllers associated with the heat pump system. The results are stored as “percent heating requirement”. The microprocessor within the system controller proceeds to a step  108  and computes the percentage of zone controllers having cooling demands in a similar fashion. In other words, the microprocessor first adds up the number of zone controllers having cooling demands and divides this number by the total number of zone controllers associated with the heat pump system and stores the result as “percent cooling requirement”. 
     The microprocessor proceeds to a step  110  and inquires whether the percent heating requirement computed in step  106  is greater than the percent cooling requirement computed in step  108  . The microprocessor within the system controller  38  will proceed to step  112  in the event that the percent heating requirement exceeds the percent cooling requirement. Referring to step  112 , the processor will inquire as to whether the percent heating requirement computed in step  106  is greater than a “minimum heat demand”. The minimum heat demand is preferably a stored percentage value in the memory associated with the microprocessor. This percentage value should be slightly less than the percentage of zone controllers that must be demanding heat in the system of FIG. 1 in order for the system to change over to providing heated water. When this percentage is exceeded, the microprocessor within the system controller will proceed in a step  114  to set “system demand” equal to heat. 
     Referring again to step  110 , in the event that the percent heating requirement does not exceed the percent cooling requirement, the processor proceeds to a step  116  and inquires as to whether percent cooling requirement is greater than percent heating requirement. In the event that the answer is yes, the processor will proceed to a step  118  and inquire as to whether the percent cooling requirement is greater than a minimum cooling demand for the heat pump system of FIG.  1 . This minimum cooling demand will be slightly less than the percentage of zone controllers that must be demanding cooling in order to have the processor proceed in a step  120  to set system demand equal to cool. 
     Referring again to step  116 , in the event that the percent cooling requirement is not greater than the percent heating requirement, then the processor will proceed to a step  122  and determine if both the percent cooling and the percent heating equal zero. If both are equal and zero, the processor will proceed to set the “system demand” equal to none in a step  124 . In the event that both demands are not equal to zero in step  122 , then the processor will proceed directly to a step  128 . 
     Referring to step  128 , it is to be appreciated that the processor will have proceeded from either step  114 , step  120  or step  124  to this step with a particular setting of system demand. The processor will also have proceeded to this step from step  122  without changing the present system demand established previously. For instance, if the “system demand” is “none” as a result of its initial setting in step  100 , then it will continue to be so after exiting step  122  along the “no” path. If on the other hand, the “system demand” were previously set in a prior execution of the logic, then that would be the system demand setting after exiting step  122  along the “no path”. 
     It is noted that the processor inquires as to whether the system demand equals none in step  128 . Assuming the system demand is heat as a result of step  114 , the processor will proceed along the no path out of step  128  to a step  130  and inquire as to whether the value of system demand equals the value of “system mode”. Since the processor will be operating immediately after initialization, the system mode value will be none prompting the processor to proceed along the no path to a step  132 . 
     Referring to step  132 , the processor will inquire whether the value of system mode is equal to none. Since system mode will be equal to none initially, the processor will proceed along the yes path to a step  134  and read the water temperature from sensor  52  in the return line  60 . The processor proceeds in a step  136  to inquire as to whether the water temperature read in step  134  is greater than ten degrees Centigrade and less than thirty-two degrees Centigrade. Since the heat pump system is not recovering from any previous heating or cooling mode of operation, the water temperature in the return line should be within this range of temperatures. This will prompt the processor to proceed along the yes path to a step  138  wherein inquiry is made as to whether system demand is equal to cool. Since the system demand was set equal to heat in step  114  , the processor will proceed out of step  138  along the no path to a step  140  and set the four way reversing valve  46  to heating. The processor will start the compressor motor  74  in a step  142 . 
     The processor proceeds to set “system mode” equal to heat in a step  144 . The processor will proceed from step  144  to a step  146  and send the system mode setting of “heat” to the zone controllers  20 ,  26 , and  30 . Each zone controller will use the communicated setting to determine how to position its control valve. In this regard, if the local demand is for heating, then the control valve will be positioned by the zone controller so as to deliver hot water from the boiler to the fan coil heat exchanger. If the local demand is however for cooling, then the hot water from the boiler will bypass the fan coil heat exchanger. It is to be appreciated that the above assumes that the local zone controller is not able to independently determine whether the water being delivered is hot or cold. In the event that the zone controllers possess the capability of independently determining the temperature of the water being delivered, then they will implement the positioning of their respective control valves without the need to receive the system mode setting from the system controller  38 . The processor will proceed from step  146  to a step  148  wherein a predefined time delay will be implemented before returning to step  102 . It is to be appreciated that the amount of time delay will be an arbitrarily defined amount of time so as to delay the system controller before it again polls the zone controllers in step  102 . 
     Referring again to steps  102 - 124 , the processor within the system controller will poll the zone controllers and thereafter compute the percentages of zone controllers having heat demands and the percentage of zone controllers having cooling demands before again determining whether or not the percentage heating requirement is greater than the percentage cooling requirement in a step  110 . Assuming that the zone controllers continue to have essentially the same demands, then the percent heating requirement will continue to exceed the percent cooling requirement so as to thereby prompt the processor to proceed from step  110  to step  112  and again inquire as to whether the minimum heat demand has been exceeded before again setting the system demand equal to heat in step  114 . The processor will proceed to step  128  and again inquire as to whether the system demand is equal to none. Since the system demand will be equal to heat, the processor will proceed to step  130  and inquire as to whether system demand equals system mode. Since system mode will now be equal to heat, the processor will proceed along the yes path to a step  150  and inquire as to whether system mode equals heat. Since system mode will be equal to heat, the processor will proceed to a step  152  and increment a “heat run timer”. The heat run timer will be incremented for the first time since the heat run timer was initially set equal to zero. It is to be appreciated that the amount by which the heat timer will be incremented will preferably be the same as the amount of delay set forth in step  146  between successive executions of the control logic. The processor will proceed from step  152  to step  148  wherein the delay will be again implemented before returning to step  102 . 
     It is to be appreciated that the processor within the system controller will continue to execute the control logic in the manner that has been previously discussed until there has been a change in the demands of the zone controllers so as to cause a change in the percentage heating requirement and percentage cooling requirements as computed in steps  106  and  108 . Assuming that the results produce a higher cooling requirement than heating requirement, then the processor will proceed out of step  110  to step  116  and hence to step  118  since the percentage cooling requirement will now exceed the percentage heating requirement. This will prompt the processor to inquire as to whether the percentage cooling requirement is greater than the minimum cooling demand required in step  118 . Assuming that the minimum cooling demand percentage has been met, the processor will proceed to set system demand equal to cool in step  120 . It is hence to be appreciated that the polling logic of steps  102  through  124  will have recognized a change in the zone controller demands sufficient to prompt the change of system demand from heat to cool. 
     The processor proceeds from step  120  to a step  128  and inquires as to whether system demand equals none. Since system demand will now be equal to cool, the processor will proceed along the no path to step  130  and inquire as to whether system demand still equals the value of system mode. Since system demand will have changed from heat to cool, the processor will proceed along the no path to step  132  and inquire as to whether system mode equals none. Since system mode will still be equal to heat, the processor will proceed along the no path to a step  154  and inquire as to whether system mode equals heat. Since system mode will still be equal to heat, the processor will proceed to a step  156  and inquire as to whether heat run timer is greater than minimum heat run. It will be remembered that the heat run timer will have been successively incremented in step  152  each time the processor within the system controller executes the control logic of FIG.  2 . Assuming that the heat pump system has been in a heating mode of operation for a considerable period of time, the heat run timer will normally exceed any minimum amount of time established for a heat run of the heat pump system of FIG.  1 . It is to be appreciated that this particular time value for minimum heat run will be stored in memory for use by the processor within the system controller. Assuming that the heat run timer has exceeded this minimum heat run value, the processor will proceed to a step  158  and issue a stop signal on the line  72  to deactivate the compressor motor  74 . 
     The processor will proceed from step  158  to a step  160  and set the changeover timer. The change over timer will be set equal to a predetermined changeover time period, “T” that the heat pump system of FIG. 1 must experience before it can be switched from heating to cooling. This changeover time period will have been stored in memory associated with the processor. The processor will proceed in a step  162  to set system mode equal to none and both heat run timer and cool run timer equal to zero. The processor will then proceed to step  148  and again implement the prescribed amount of delay before the next execution of the control logic. 
     At such time as the next execution occurs, the processor will again poll the zone controllers in a step  102  and compute the percentage heat requirement and cooling requirement in steps  106  and  108 . Assuming that the percentage cooling requirement continues to now exceed percentage heating requirement, the processor will again execute steps  110 , and  116  through  120  and again set the system demand equal to cool. This will prompt the processor to proceed through step  128  to step  130  since system demand will be equal to cool. Since system demand will not equal system mode at this time, the processor will proceed along the no path to step  132  to inquire whether system mode equals none. Since system mode will have been previously set equal to none in step  162 , during the previous execution of the control logic, the processor will proceed along the yes path to step  134  and read the water temperature from the water temperature sensor  76  in the return line from the fan coil heat exchangers. The processor will proceed to inquire as to whether the water temperature read from sensor  76  is between the range of temperatures set forth in step  136 . Since the compressor motor  74  will have just recently been turned off, the water temperature in the return line should be above thirty two degrees Centigrade so as to prompt the processor to proceed along the no path out of step  136  to a step  164  and inquire as to whether the changeover timer set in step  160  is equal to zero. The changeover timer will have just been set equal to a predetermined changeover time in the previous execution of the control logic. This will prompt the processor to proceed along the no path to a step  166  and decrement the changeover time previously loaded into the change over timer. It is to be appreciated that the amount of time thereby decremented will be essentially the delay time defined by step  148  between successive executions of the control logic. The processor proceeds from step  166  to step  148  wherein the delay is again implemented before the next successive execution of the control logic. 
     It is to be appreciated that successive executions of the control logic will occur as long as the zone controllers continue to indicate a higher percentage cooling requirement than heating requirement and that this higher percentage cooling requirement remains greater than the minimum cooling demand. At some point during the successive executions of the control logic, the processor may note in step  136  that the water temperature in the return line is within the range of the temperatures set forth in step  136 . On the other hand, the processor may note that the changeover timer has been decremented to zero in step  164  before the water temperature in the return line is within range. In either case, the processor will proceed from step  136  or step  164  to step  138  and inquire as to whether the system demand equals cool. Since the system demand will have been continually set equal to cool each time step  120  is encountered, the processor will proceed to step  168  and set the four way reversing valve  46  to a cooling position. This will prompt the heat pump system  10  to assume the configuration of FIG.  2 . The processor will thereafter proceed to step  170  and activate the compressor motor  74 . The processor will then proceed to a step  172  and set the system mode equal to cool. The processor will proceed to step  174  and send the system mode setting of “cooling” to the zone controllers  20 ,  26 , and  30 . Each zone controller will use the communicated setting to determine how to position its control valve. In this regard, if the local demand is for cooling, then the control valve will be positioned by the zone controller so as to deliver cooled water from the chiller to the fan coil heat exchanger. If the local demand is however for heating, then the cooled water from the chiller will bypass the fan coil heat exchanger. It is to be appreciated that the above assumes that the local zone controller is not able to independently determine whether the water being delivered is hot or cold. In the event that the zone controllers possess the capability of independently determining the temperature of the water being delivered, then they will implement the positioning of their respective control valves without the need to receive the system mode setting from the system controller  38 . 
     It is hence to be appreciated that the control logic will have implemented a changeover from heating to cooling in the event that the changeover time as defined by the changeover timer elapses or in the event that the water temperature sensor is within the predefined range of water temperatures in step  136 . It is furthermore to be appreciated that the control logic can possibly implement a changeover from cooling back to heating when the percentage heating requirement exceeds the percentage cooling requirement at some point during the successive executions of control logic. At such time, the system demand will be set equal to heat in step  114  prompting the processor to proceed through steps  128 ,  130 ,  132  to step  154  to inquire whether the system mode is equal to heat. Since the system mode will still be equal to cool, the processor will proceed from step  154  along the no path to step  174  to inquire whether the system mode is equal to cool. Since system mode will still be equal to cool, the processor will proceed to a step  176  to inquire whether the cool run timer is greater than the minimum cool run time. If the cool run timer has not been sufficiently incremented so as to exceed the minimum cool run time, the processor will proceed to step  178  and increment the cool run timer before returning to step  148 . The processor will again execute the aforementioned logic steps of  114 ,  128 ,  130 ,  132 ,  154 ,  174  and  176  until the cool run timer exceeds the minimum cool run time. At this point, the processor will proceed to stop the compressor motor  74  before setting the changeover timer equal to “T” in step  160 . The processor will proceed to step  162  and set system mode equal to none and heat run timer and cool run timer equal to zero. The processor will proceed to step  148  and implement the delay before again polling the zone controllers in step  102 . Assuming that the polling continues to indicate that heating requirements exceed cooling requirements, the processor will proceed though steps  110 - 114 ,  128  to step  132 . Since the system mode is now equal to none, the processor will proceed to implement steps  134 ,  136 , and steps  164 - 166  and then  148  until such time as the water temperature read in step  134  is within range or the changeover timer has been decremented to zero. At such time, the processor will proceed to step  138  and hence to steps  140 - 146  so as to change the heat pump system to a heating mode of operation. 
     Referring again to step  116 , it is to be noted that there may a situation wherein the particular polling by the processor will indicate that there is neither a predominance of heating or cooling being required by the zone controllers. In this case, the processor will proceed to step  122  and inquire as to whether the percent cooling requirement and the percent heating requirement are both equal to zero. If this is the case, the processor proceeds to set the system demand equal to none in a step  124  prompting the processor to proceed to step  128 . Depending upon the previous system mode setting, the processor will proceed through either step  154  or step  174  in order to stop the compressor motor  74  and set the system mode equal to none. The processor will proceed through step  148  before again implementing the aforementioned logic as long as the polling requirements remain unchanged. 
     Referring again to step  122 , in the event that the percent cooling requirement and percent heating requirement do not equal zero, the processor will proceed to step  128 . Since the system requirements and system mode will be whatever was previously determined, the processor will proceed to step  130  where it will then proceed along the yes path and increment the appropriate run timer for whatever mode it is currently in. 
     It is to be appreciated that the control logic of FIGS. 3A,  3 B and  3 C allow the system controller  38  to potentially initiate a changeover from either heating to cooling or vice versa in response to the polling of the zone controllers  20 ,  26 , and  30 . This changeover will actually occur only when certain requirements are met. Specifically, the heat pump system must have been running in what ever mode it is presently in for a minimum time. Secondly, the water temperature must be within the predefined temperature range or the changeover timer must have expired indicating that the change over time has been exceeded. It is only after such events have occurred that the system controller will authorize the repositioning of the four-way reversing valve  46  and activate the compressor motor  74 . 
     It is to be appreciated that preferred embodiments of the invention have been disclosed. Alterations or modifications may occur to one of ordinary skill in the art. For instance, the control logic may be altered so as to not require a sensing of water temperature in the return line. In this case, the changeover time would be the governing factor as to whether a changeover would be allowed to occur. It will be appreciated by those skilled in the art that further changes could be made to the above-described without departing from the scope of the invention. Accordingly, the foregoing description is by way of example only and the invention is to be limited only by the following claims and equivalents thereto.