Abstract:
A water heating system for controlling the heating of potable water in commercial or private dwellings with improved energy efficiency. The water heating system heats potable water in a tank by transferring excess heat generated in a refrigeration unit with a heat exchanger, and by extracting energy from insolation with a solar water heater unit. The system includes several control systems for regulating the operation of the heat exchanger, solar water heater unit, and refrigeration unit to provide increased energy efficiency and longevity to the various components of the system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefits from U.S. Provisional Patent Application No. 61/086,819, filed on Aug. 7, 2008, the contents of which are hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention is related to heating systems for potable water. More particularly, the present invention is related to water heating systems having a solar water heater unit, a heat recovery unit, and, if necessary, a conventional heating element. 
         [0004]    2. State of the Art 
         [0005]    Commercial and residential facilities and dwellings include various systems for heating potable water. Commonly, these water heating systems include a tank with a heating element that is configured to increase the temperature of water within the tank. The heating element can be an electrically powered element, a gas-burning element, an oil-burning element, or combinations of these elements. Unfortunately, the cost of fuel sources used by such conventional heating elements can reduce the economic feasibility of such water heating systems. 
         [0006]    Hot water heating systems that reduce the usage of such fuel sources may thus provide increased economic feasibility. 
       SUMMARY OF THE INVENTION 
       [0007]    A water heating system is provided for controlling the heating of potable water in commercial or private dwellings with improved energy efficiency. The water heating system includes a tank that stores potable water in fluid communication with a potable water source, a refrigeration unit that circulates refrigerant for air conditioning or other refrigeration purposes, a heat recovery unit (HRU) that transfers heat from the circulating refrigerant of the refrigeration unit to the water stored in the tank, and a solar water heater unit that extracts heat from insolation and transfers the extracted heat to the water stored in the tank. 
         [0008]    The refrigeration unit preferably includes a first fluid loop for circulating the refrigerant, a compressor coupled to the first fluid loop for compressing the refrigerant, a fan and an expansion valve coupled to the first fluid loop for cooling the refrigerant, and an evaporator section along the first fluid loop which absorbs heat from a refrigeration area to cool the refrigeration area. 
         [0009]    The heat recovery unit includes a first heat exchanger and a second fluid loop which circulates a first heat transfer medium between the tank and the first heat exchanger. The first heat exchanger has a first flow path which is part of the first fluid loop of the refrigeration unit, and a second flow path which is part of the second fluid loop and thermally coupled to the first flow path. Thus, the second fluid loop of the heat recovery unit is thermally coupled to the first fluid loop of the refrigeration unit at the heat exchanger, which allows the first heat transfer medium circulating in the second fluid loop to transfer heat from the refrigerant to the water stored in the tank. In the exemplary embodiment, the second fluid loop is in direct fluid communication with the water stored in the tank such that first heat transfer medium circulating through the second fluid loop is water from the tank. 
         [0010]    The solar water heater unit includes a solar collector which extracts energy from insolation, and a third fluid loop which circulates a second heat transfer medium between the solar collector and the tank to heat the potable water in the tank. 
         [0011]    The refrigeration unit, heat recovery unit, and solar water heater unit each include measuring means for measuring temperature, pressure, or other parameters at various locations in the system, and control means for controlling their operation based on the measured parameters to maximize the energy efficiency, hot water capacity, and longevity of the system while reducing the system&#39;s operational costs and fuel consumption. 
         [0012]    The refrigeration unit preferably includes a fan control means which operates to deactivate (turn off) the cooling fan of the refrigeration unit when the refrigerant is sufficiently cooled on account of the operation of the heat exchanger in transferring heat away from the refrigerant to the water in the tank, and operates to activate (turn on) the cooling fan of the refrigeration unit when additional cooling is needed. 
         [0013]    The heat recovery unit preferably includes HRU control means which operates to activate the heat recovery unit to circulate the first heat transfer medium in the second fluid loop when (1) the temperature of the water in the second fluid loop becomes so low that it is in danger of freezing; and (2) when the refrigerant between the compressor and the heat exchanger is above a predetermined temperature (e.g., 125° Fahrenheit) and the potable water in the tank is below a maximum tank temperature (e.g., 155° Fahrenheit). During normal operation, the temperature of the refrigerant between the compressor and the heat exchanger will generally be higher than the temperature of the water in the tank, and the water temperature in the tank will generally be below the maximum temperature desired. Thus, the heat exchanger operates to transfer energy from the refrigerant (which would otherwise need to be expelled to the atmosphere through the use of the fan) to the water in the tank, thereby reducing the fan&#39;s operation requirements. 
         [0014]    The solar water heater unit preferably includes solar control means which operates to activate the solar water heater unit to circulate the second heat transfer medium in the third fluid loop when two conditions are met: (1) the difference between the temperature of the second heat transfer medium at the solar collector exceeds the temperature of the potable water in the tank by a predetermined amount (e.g., 8-24° Fahrenheit); and (2) the temperature of the potable water in the tank is below the maximum tank temperature desired (e.g., below a maximum tank temperature that is within a range of 155-200° Fahrenheit). The first condition allows for the activation of the solar water heater unit when efficient heat transfer can take place. The second condition prevents the water in the tank from exceeding a maximum temperature. A relief valve is provided to allow for the removal of a portion of the second heat transferring medium from the third fluid loop in the event that the second heat transferring medium gets too hot at the solar collector. 
         [0015]    In other embodiments, an additional tank is utilized for storing the potable water. The additional tank is in fluid communication with both the tank (which operates as a preheater tank) and the potable water source, and bypass valves are provided which may be set to enable the potable water to bypass the tank and flow directly into the additional tank. 
         [0016]    Additional objects, advantages, and embodiments of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic depiction of an exemplary embodiment of a hybrid water heating system according to the present invention. 
           [0018]      FIG. 2  is a schematic depiction of another exemplary embodiment of a hybrid water heating system according to the present invention. 
           [0019]      FIG. 3  is a table describing the function of the fan control means of the refrigeration unit of the invention. 
           [0020]      FIG. 4  is a table describing the function of the HRU control means of the heat recovery unit of the invention. 
           [0021]      FIG. 5  is a table describing the function of the solar control means of the solar water heater unit of the invention. 
           [0022]      FIG. 6  is a schematic of the circuitry of an embodiment of the controller of the heat recovery unit of the invention. 
           [0023]      FIG. 7  is a schematic of the circuitry of an embodiment of the operational control of the fan of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Turning now to  FIG. 1 , a water heating system according to the present disclosure is shown and is generally referred to by reference numeral  10 . The system  10  includes a tank  12  in fluid communication with a source  14  of potable water such as, but not limited to, a well or a city water source. The tank  12  is configured to place water stored therein in a heat exchange relationship with a heat recovery unit  16 , a solar water heater unit  18 , and a heating element  20 . The system  10  is configured to heat the potable water in the tank  12  by using heat available from free sources (e.g., refrigeration and solar units) in conjunction with the conventional heating element  20  to provide an energy efficient hot water heating system. 
         [0025]    The heat recovery unit  16  of the system  10  is in a heat exchange relationship with a conventional vapor compression refrigeration unit  22  such as, but not limited to, an air conditioner, a refrigerator, a freezer, a heat pump, or equivalent refrigeration units known in the art. The heat recovery unit  16  includes a first circulating pump  24  which circulates water from the tank  12  through a flow loop  17 , a heat exchanger  26 , and a first controller  28 . When heat is available from the vapor compression refrigeration unit  22 , the first controller  28  is configured to activate the pump  24  to pump the water from the tank  12  through the heat exchanger  26  and back into the tank  12 . 
         [0026]    The refrigeration unit  22  includes a flow loop  19  for circulating refrigerant. A compressor  32  operably coupled to the flow loop  19  compresses the refrigerant and passes the compressed refrigerant to a condenser  34 . The condenser  34  is also operably coupled to the flow loop  19  and includes a cooling fan  36  to force outside air  38  across the condenser  34  to remove heat from the refrigerant within the flow loop  19 . Thus, the refrigeration unit  22  typically consumes electrical energy to operate the cooling fan  36  to expel waste heat to the outside air  38 . The compressed, condensed refrigerant is then expanded in an expansion valve  40  to a lower temperature, and then passed through an evaporator  42 . The evaporator  42  includes a blower unit  44  which blows inside air  46  from a conditioned space across the evaporator  42 . The refrigeration unit  22  thus provides conditioned air  46  to a conditioned space. 
         [0027]    The heat exchanger  26  of the heat recovery unit  16  is in heat exchange communication with the refrigerant in the flow loop  19  between the compressor  32  and the condenser  34 , which is generally at a high temperature. The heat exchanger  26  operates to transfer waste heat (which is typically removed from the refrigerant by the fan  36  in the prior art) to the water in tank  12 , which will generally be at a lower temperature than that of the refrigerant between the compressor  32  and the condenser  34 . The heat exchanger  26  includes a first flow path  19   a  which is part of the flow loop  19  of the refrigeration unit  16 , and a second flow path  17   a  which is part of the flow loop  17  of the heat recovery unit  16  and thermally coupled to the first flow path  19   a . The heat recovery unit  16  removes heat from the refrigerant in the flow loop  19  of the refrigeration unit  22  and transfers it to the potable water in the tank  12 , which also reduces the typical cooling requirements of the fan  36 . 
         [0028]    The operation of the controller  28  of the heat recovery unit  16  of the system  10  is best understood with reference to  FIGS. 1 ,  4 , and  6 . The controller (HRU control means)  28  activates the circulation pump  24  to circulate water from the tank  12  through the heat exchanger  26  when heat is available from the refrigeration unit  22 . For example, the controller  28  can receive a first input  48  indicative of a condition of the refrigerant in the refrigeration unit  22  such as, but not limited to, a temperature signal, a pressure signal, or other signals conveying information related to the refrigerant&#39;s properties. When the first input  48  reaches a predetermined level indicating that heat is available from the refrigeration unit  22 , the controller  28  may activate the circulation pump  24 . In one example, the first input  48  can be a temperature signal and the predetermined level might be 125 degrees Fahrenheit (F). 
         [0029]    The controller  28  is also preferably configured to deactivate the circulating pump  24  to cease circulating water from the tank  12  through the heat exchanger  26  when the water within the tank  12  reaches a predetermined temperature. For example, the controller  28  may receive a second input  50  indicative of the water temperature within the tank  12 . When the second input  50  reaches a predetermined level, the controller  28  deactivates the circulation pump  24 . In one example, the second input  50  may be a temperature signal and the predetermined level might be 155 degrees Fahrenheit (F). 
         [0030]    The controller  28  may also be configured to activate the circulating pump  24  when the temperature of the water in the second fluid loop  17  becomes so low that it is in danger of freezing. For example, the controller  28  may receive a third input  51  indicative of the water temperature within the second fluid loop  17 . When the third input  51  reaches a predetermined level, the controller  28  activates the circulation pump  24  to circulate water from the tank  12  through the second fluid loop  17  to prevent freezing therein. It is noted that if the refrigeration unit  22  is operational, then the circulating pump  24  will operate as discussed above to transfer heat from the refrigerant to the water at the heat exchanger  26 . But in the event that the refrigeration unit  22  goes down during the winter months, the operation of the circulating pump  24  to circulate water from the tank  12  through the second fluid loop  17  will help to prevent the water from freezing in the second fluid loop  17 . It is anticipated that other back-up sources of heat may be utilized with the system (such as gas or oil) to heat the tank  12  so that the tank  12  water will remain warm even during a long power outage. It is also anticipated that this anti-freezing operation of the controller  28  will be far less common, but will provide an important safety measure in the winter time to prevent the heat recovery unit  16  from freezing and increase its longevity. 
         [0031]    The controller  28  can be embodied by a variety of control circuitry, such as a programmed controller or dedicated hardware logic (PLD, FPGA, ASIC) and supporting circuitry (e.g., thermistors for temperature sensing or pressure transducers for pressure sensing), one or more relays and supporting circuitry (e.g., thermostats for temperature sensing or pressure controllers for pressure sensing) or other suitable circuitry. An exemplary embodiment of controller  28  is illustrated in  FIG. 6 , which includes a first thermostat  601  coupled between one leg  602 A of line AC and the control path  605  of a double pole single throw relay  603  that extends to the other leg  602 B of line AC. The legs  602 A,  602 B of line AC are protected by corresponding fuses  604 A,  604 B, respectively. The relay  603  includes two switchable current paths  607 ,  609  that are selectively activated by the electrical signals of the control path  605 . The current path  607  extends to a red LED  611  series coupled between the relay  603  and leg  602 B of line AC. The current path  609  is connected to a green LED  613  coupled between the relay  603  and leg  602 B of line AC. Second and third thermostats  615 ,  617  are series coupled between leg  602 A of line AC and the green LED  613 . Like the green LED, one of the terminals of the circulating pump  24  is connected to leg  602 B of line AC while the other terminal is connected to the current path  609  from the relay  603  as well as the current path through the series coupled thermostats  615 ,  617 . 
         [0032]    The first thermostat  601  is configured to sense tank water temperature and provide a normally-off current path that is turned on when the temperature of the tank water or water within the second fluid loop  17  falls below a threshold temperature (e.g., 38° F.) that indicates that the heat recovery unit  16  is near freeze up. When the first thermostat  601  is on, current flows through the control path  605  of the relay  603  and turns ON the switchable current paths  607  and  609  through the relay  603 . Such operations produce current flowing between the two legs  602 A,  602 B of line AC that turns on the red LED  611  as well as turns on the green LED  613  and the circulating pump  24  for heating the water to prevent such freeze up in the heat recovery unit  16 . The current path of the first thermostat  601  is returned to the normally-off state when the temperature exceeds a predetermined temperature (e.g., 48° F.). In the normally-off state of the first thermostat  601 , there is no current flowing through the control path  605  of the relay  603  and thus the switchable current paths  607 ,  609  through the relay  603  are off, which dictate that the red LED  611  is turned OFF and allow for control of the circulating pump  24  by the second and third thermostats  615 ,  617 . 
         [0033]    The second thermostat  615  is configured to sense temperature of the water in the tank  12  and provide a normally-on current path that is turned off when the temperature of the tank water reaches a predetermined temperature (e.g., 155° F.). The third thermostat  617  is configured to sense temperature of the refrigerant of the fluid loop  19  and provide a normally-off current path that is turned on when the temperature of the refrigerant reaches a predetermined temperature (e.g., 125° F.). In this manner, two thermostats  615  and  617  provide current that flows from leg  602 A to the green LED  613  and the circulating pump  24  to activate both the green LED  613  and the circulating pump  24  when the temperature of the tank water is less than the predetermined temperature (e.g., 155° F.) and the temperature of refrigerant of fluid loop  19  is greater than the predetermined temperature (e.g., 125° F.). In the off state of the second or third thermostats  615 ,  617 , there is no current flowing through the thermostats  615 ,  617  to the green LED  613  and the circulating pump  24 , which allows for control of the circulating pump by the first thermostat  601  and relay  603  as described above. 
         [0034]    It is noted that in other embodiments, the controller  28  may be configured to activate the circulating pump  24  to use the water from the tank  12  to heat the refrigerant regardless of the water temperature in the tank  12  in the event that the temperature of the refrigerant in the flow loop  19  becomes low enough to potentially hinder the operation of the refrigeration unit  16  (e.g., input  48  may override input  50  in the event that the refrigeration unit  16  is in danger of freezing up). 
         [0035]    The operational control of the fan  36  of the refrigeration unit  16 , is best understood with reference to  FIGS. 1 ,  3 , and  7 . A fan control  30  is provided in the form of a delay relay or controller in electrical communication with the fan  36 . During normal operation of the refrigeration unit  16 , the fan control  30  delays the operation of the fan  36  until a condition within the refrigeration unit  16  reaches a predetermined level. As discussed above, the heat recovery unit  16  removes heat from the refrigerant in the flow path  19   a  of the flow loop  19  of the refrigeration unit  22  that would otherwise need to be removed by the fan  36 . Thus, the fan  36  need not be operated until the heat recovery unit  16  can no longer remove enough heat from the refrigeration unit  22  to keep the refrigeration unit  16  operating in a desired manner. 
         [0036]    For example, in medium temperature refrigeration units such as those present in a restaurant, bar, or other commercial establishment, it is typically desired that the refrigerant exiting the condenser  34  be in a vapor condition with a desired temperature and/or pressure. The fan control  30  receives a fourth input  52  from the refrigeration unit  22  which is indicative of the temperature of refrigerant within the flow loop  19  of the refrigeration unit  16 . The fan control  30  maintains the fan  36  in an off condition until the fourth input  52  reaches a predetermined level, at which time, the fan control  30  activates the fan  36  to expel heat from the refrigerant to the ambient air  38  at the condenser  34 . 
         [0037]    In one preferred embodiment, the fourth input  52  is a pressure input from a pressure transducer  52 - 1  positioned in the flow loop  19  of the refrigeration unit  22  between the heat exchanger  26  and the condenser  34 . If the pressure of the refrigerant in the flow loop  19  exceeds a predetermined limit after passing through the heat exchanger  26 , then insufficient heat has been removed from the refrigerant by the heat exchanger  26 . Typically, this results from the water in the tank  12  being of a sufficiently high temperature from the heat already collected by the heat recovery unit  16  and/or the solar collection unit  18  (further discussed below). 
         [0038]    When the pressure of the refrigerant in the flow loop  19  exceeds a predetermined limit after passing through heat exchanger  26 , the fan control  30  activates the cooling fan  36  to expel waste heat from the refrigerant to the outside air  38 . Conversely, when the pressure of the refrigerant in the flow loop  19  is below the predetermined limit after passing through heat exchanger  26 , the fan control  30  maintains the cooling fan  36  in a normally deactivated state. In embodiments of the invention in which the refrigeration unit  22  is a medium temperature refrigeration unit, the predetermined pressure limit at transducer  52 - 1  could be approximately 200 pounds per square inch (PSI). 
         [0039]    The controller  30  can be embodied by a variety of control circuitry, such as a programmed controller or dedicated hardware logic (PLD, FPGA, ASIC) and supporting circuitry (e.g., thermistors for temperature sensing or pressure transducers for pressure sensing), one or more relays and supporting circuitry (e.g., thermostats for temperature sensing or pressure controllers for pressure sensing) or other suitable circuitry. An exemplary embodiment of controller  30  is shown in  FIG. 7 , which includes a pressure control unit  701  electrically coupled between one leg  702 A of line AC and one of the terminals of the condenser fan  36  as shown. The other terminal of the condenser fan is connected to the other leg  702 B of line AC. A capillary tube  703  is fluidly coupled to the fluid loop  19 , preferably at a point downstream of the heat recovery unit  26  and upstream of the condenser  34  (e.g., preferably at  52 - 1  as shown, but may optionally be placed anywhere along the length of the condenser) in order to sample the pressure of the refrigerant in the fluid loop  19 . The pressure control unit  701  measures the sampled pressure of the refrigerant of the fluid loop  19  and provides a normally-off current path between leg  702 A and the terminal of the condenser fan  36  that is turned on when the sampled pressure reaches a predetermined cut-in pressure. This current path is then returned to the normally-off state when the pressure falls below a predetermined cut-off pressure. In the preferred embodiment, the cut-in and cut-out pressures are set by user input (for example, by user adjustment of dials for setting such cut-in and cut-out pressures). In the preferred embodiment, the pressure control unit  701  is realized by a unit (e.g., the 016 Single Pressure Control unit) sold commercially by Ranco Controls of Delaware, Ohio. 
         [0040]    Thus, system  10 , through the operation of the fan control  30  of the refrigeration unit  22 , maximizes the amount of heat recovered by the heat recovery unit  16  by eliminating the expulsion of heat from the refrigerant to the ambient air when such expulsion not needed. Further, system  10  minimizes energy usage by leaving fan  36  in a normally “off” state until such time as the heat recovery unit  16  no longer has sufficient capacity to remove enough heat from the refrigerant in the flow loop  19  to keep the refrigeration unit  22  operating as desired. 
         [0041]    The system  10  of the present invention also preferably incorporates the solar water heater unit  18  and uses it in conjunction with the heat recovery unit  16 . The solar water heater unit  18  and its operational control is best understood with reference to  FIGS. 1 and 5 . 
         [0042]    The solar collection unit  18  provides heat captured from solar energy to the water in the tank  12 . Thus, the water in tank  12  is heated not only by the heat recovery unit  16 , but also by the solar collection unit  18 . As such, the ability of the water in tank  12  to remove sufficient heat from the refrigeration unit  22  can be reduced when the solar collection unit  18  is operating. The fan control  30  protects the refrigeration unit  22  from damage due to overheating and maintains the refrigeration unit  22  in a desired operating condition when a large amount of heat is added to the water in the tank  12  by both the heat recovery unit  16  and solar collection unit  18 . 
         [0043]    The solar collection unit  18  includes a second circulating pump  54  which circulates a second heat transfer medium through a flow loop  21 . A solar collector  56  and second heat exchanger  60  are operably coupled to the flow loop  21  as shown in  FIG. 1 . A second controller  58  is provided for selectively activating and deactivating the second circulating pump  54  of the solar collection unit  18 . When heat is available from solar energy, the second controller  58  is configured to activate the circulating pump  54  to pump a heat-transfer fluid such as, but not limited to, propylene glycol through the solar collector  56  and the heat exchanger  60  via the fluid loop  21 . The solar collector  56  thus heats the heat-transfer fluid, and the heat from the heat-transfer fluid is used to indirectly heat the water in the tank  12  via the heat exchanger  60 . 
         [0044]    The fluid loop  21  of the solar collection unit  18  is shown by way of example as an indirect or closed-loop circulation system where the circulating pump  54  circulates the heat-transfer fluid through the solar collector  56  and the heat exchanger  60  to indirectly heat the water in the tank  12 . However, the solar collection unit  18  may also be a direct or open-loop circulation system in which the pump  54  circulates the potable water from the tank  12  directly through the solar collector  56  and back into the tank  12 . 
         [0045]    Conversely, while the fluid loop  17  of the heat recovery unit  16  is shown by way of example as a direct or open-loop circulation system where the pump  24  circulates the water from the tank  12  through the heat exchanger  26  and back into the tank  12 , the fluid loop  17  may instead be an indirect or closed-loop circulation system fluidly isolated from the water in the tank  12  in which the pump  24  circulates a heat-transfer fluid through the heat exchanger  26  and through an additional heat exchanger (not shown) in a heat exchange relationship with the water in tank  12  to indirectly heat the water in the tank. 
         [0046]    In addition, the heat exchanger  60  disposed at the tank  12  is shown by way of example only as a flat heat exchanger in tank  12 . However, it is contemplated that the heat exchanger  60  may be any device sufficient to place the heat-transfer fluid of the solar collection unit  18  in a heat exchange relationship with the water in the tank  12 . The tank  12  may also be a jacketed tank in which the heat exchanger  60  forms a heat exchange jacket around the outer surface of the tank  12 . 
         [0047]    The solar collector  56  can be any device sufficient to collect heat from solar energy. For example, the solar collector  56  can be a glazed flat-plate collector, an un-glazed flat-plate collector, an evacuated-tube solar collector, a photo-voltaic module, a drain-back system, and any combinations thereof. 
         [0048]    The term “glazed flat-plate collectors” used herein refers to collectors having an insulated, weatherproofed box that contains a dark absorber plate under one or more glass or plastic covers. The term “unglazed fiat-plate collectors” used herein refers to collectors having a dark absorber plate, made of metal or polymer, without a cover or enclosure. The term “evacuated-tube solar collectors” used herein refers to collectors having parallel rows of transparent glass tubes where each tube contains a glass outer tube and a metal absorber tube attached to a fin. The fin&#39;s coating absorbs solar energy but inhibits radiative heat loss. The term “photo-voltaic module” used herein refers to collectors having an array of photo-voltaic cells that convert solar energy into electrical potential. The electrical potential can be used to provide current to an electrical heating element, which heats the water in the tank  12 . 
         [0049]    The controller  58  of the solar water heater unit  18  controls the circulating pump  54  to circulate the heat-transfer fluid from the heat exchanger  60  in the tank  12  through the solar collector  56  only when heat is available at the solar collector  56 . For example, the controller  58  may receive a fifth input  66  indicative of a condition of the solar collector  56 . The fifth input  66  may include, but is not limited to, a temperature signal indicative of the temperature of the heat-transfer fluid at the solar collector  56 . When the fifth input  66  reaches a predetermined limit indicating that sufficient heat is available from the solar collector  56 , the controller  58  activates the circulation pump  54 . 
         [0050]    The controller  58  is preferably configured to deactivate the circulating pump  54  to cease circulating the heat-transfer fluid through the solar collector  56  and the heat exchanger  60  when the water within the tank  12  reaches a predetermined temperature. For example, the controller  58  can receive a sixth input  68  indicative of a temperature of the water within the tank  12 . When the sixth input  68  reaches a predetermined limit, the controller  58  deactivates the circulating pump  54 . The circulating pump  54  can be an electrically powered pump, powered by a standard 115-volt power source. The pump  54  may also be powered by electricity collected by a photo-voltaic solar collector (not shown). 
         [0051]    The controller  58  is described by way of example as operating based on a first temperature limit (e.g., sensed from fifth input  66 ) and a second temperature limit (e.g., sensed from sixth input  68 ). However, as discussed in  FIG. 5 , the controller  58  may also operate as a differential controller in which the controller  58  activates the circulating pump  54  when the fifth and sixth inputs  66 ,  68  are indicative of a temperature differential of at least a predetermined value. For example, the controller  58  can be configured to activate the circulating pump  54  when the fifth and sixth inputs  66 ,  68  are indicative of at least approximately 8 degrees Fahrenheit (F) and can deactivate the pump when the temperature differential is less than approximately 8 degrees Fahrenheit (F). Similarly, the controller  28  of the heat recovery unit  16  ( FIGS. 1 and 4 ) may be configured to operate as a differential controller in which the controller  28  only activates the circulating pump  24  when the first and second inputs  48 ,  50  are indicative of at least a predetermined value. The controller  58  can also operate to deactivate the circulating pump  54  upon the fifth input  66  exceeding a third temperature limit indicative that the solar collector is at a maximum temperature for preventing damage to system components. A relief valve (not shown) is operably coupled to the flow loop  21  for lowering the pressure within the flow loop  21  in the event that the fifth input  66  exceeds the third temperature limit. In an open configuration of the relief valve, the second heat transferring medium is drained from the flow loop  21  in gas or liquid form to lower the pressure therein. 
         [0052]    The controller  58  can be embodied by a variety of control circuitry, such as a programmed controller or dedicated hardware logic (PLD, FPGA, ASIC) and supporting circuitry (e.g., thermistors for temperature sensing or pressure transducers for pressure sensing), one or more relays and supporting circuitry (e.g., thermostats for temperature sensing or pressure controllers for pressure sensing) or other suitable circuitry. In an exemplary embodiment, the controller  58  is realized by a programmed controller adapted for differential temperature control of solar energy systems, such as the GL-30 module sold commercially by Goldline Controls Inc of East Greenwich, R.I. 
         [0053]    When heat is unavailable from either the heat recovery unit  16  or the solar collection unit  18 , the system  10  utilizes a conventional heating element  20  to heat the water in the tank  12 . Heating element  20  may be an electrically powered element, a gas-burning element, an oil-burning element, and combinations thereof. 
         [0054]    The hybrid hot water heat system  10  of the present invention thus combines three heating sources, two of which are available without consuming additional energy. Additionally, the fan control  30  of the hybrid hot water heat system  10  of the present invention selectively activates and deactivates the fan  36  of the vapor compression refrigeration unit  22  to minimize the available heat expelled to the ambient air  38 . The fan control  30  also maximizes the amount of heat recovered by the heat recovery unit  16  and minimizes the amount of energy used while protecting the vapor compression refrigeration unit  22  from being damaged. 
         [0055]    An additional preferred embodiment of the hybrid hot water heating system  10  according to the present invention is shown in  FIG. 2  and is generally referred to by reference numeral  110 . System  110  is substantially similar to system  10 , and, for clarity, only those components that differ from system  10  are described below. 
         [0056]    System  110  is a two-tank system that includes a pre-heat tank  112 - 1 , a conventional heating tank  112 - 2 , and a bypass system  180 . The pre-heat tank  112 - 1  is in a heat exchange relationship with the heat recovery unit  16  and the solar collection unit  18  in the manner described above with respect to system  10 . The heating tank  112 - 2  includes a conventional heating element  120 , which may be an electrically powered element, a gas-burning element, an oil-burning element, and combinations thereof. The combination of the pre-heat tank  112 - 1  with the heating tank  112 - 2  allows the system  110  to maximize the collection and storage of heat from the heat recovery unit  16  and the solar collection unit  18 . 
         [0057]    The bypass system  180  allows a user to divert incoming water from the water source  14  to bypass the pre-heating tank  112 - 1  to flow directly into the heating tank  112 - 2 . In the illustrated embodiment of  FIG. 2 , the bypass system  180  includes a first valve  182 , a second valve  184 , and a third valve  186 , each being a two-way valve having an open state and a closed state. When an operator desires the use of the pre-heating tank  112 - 1 , the first and second valves  182 ,  184  can be moved to the open state while the third valve  186  is moved to the closed state. In this configuration, water from the water source  14  flows through the first valve  182  into the pre-heat tank  112 - 1  and from the pre-heat tank  112 - 1  to the heating tank  112 - 2  through the second valve  184 . 
         [0058]    Conversely, when an operator desires to bypass pre-heating tank  112 - 1 , the first and second valves  182 ,  184  can be moved to the closed state while the third valve  186  is moved to the open state. In this configuration, water from the water source  14  flows through the third valve  186  directly into the heating tank  112 - 2  without passing through pre-heating tank  112 - 2 . 
         [0059]    The bypass system  180  is described above by way of example as a manually activated system in which the operator moves the valves  182 ,  184 ,  186  between the open and closed states. However, it is contemplated that the valves of bypass system  180  may be automatically controlled between the open and closed states based on the availability of heat from either the heat recovery unit  16  or the solar collection unit  18 . 
         [0060]    Additionally, the bypass system  180  is described above by way of example with respect to the three separate two-way valves  182 ,  184 , and  186 . However, it is contemplated that the bypass system  180  may include any combination of valves sufficient to selectively place the pre-heating tank  112 - 1  in fluid communication with the water source  14  and the heating tank  112 - 2 . For example, it is contemplated that the bypass system  180  may include one three-way valve that replaces the first and third valves  182 ,  186 . 
         [0061]    It should also be noted that the terms “first”, “second”, “third”, “upper”, “lower”, and the like may be used herein to modify various elements. These modifiers do not imply a spatial, sequential, or hierarchical order to the modified elements unless specifically stated. 
         [0062]    While the present disclosure has been described with reference to one or more exemplary embodiments, it is not intended that the invention be limited thereto, and it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated, but that the disclosure will include all embodiments.