Abstract:
A hybrid heating apparatus heats potable water with waste heat from heat recovery units and insolation from solar collectors. A single circulation pump circulates fluid between at least one heat exchanger and each of the heat recovery units and preferably the solar collector. A single controller receives sensor readings from the heat recovery units and the solar collector units and receives a demand to heat the potable water. To satisfy the demand, the controller determines the extent to which the demand may be satisfied from heat available from the heat recovery units and the solar collector units and sends command signals both to the circulating pump to circulate the fluid and to appropriate ones of valves at connections to those heat recovery units and solar collector units to allow fluid to circulate to be heated to flow to the heat exchanger for effecting heat exchange to heat the potable water.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 12/205,979 filed Sep. 8, 2008, whose contents are incorporated herein by reference and which in turn claims the benefit of priority 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, and a continuation-in-part of international patent application no. PCT/US2009/049741 filed Jul. 7, 2009, the contents of which are incorporated herein by referent and which in turn claims priority from U.S. patent application Ser. No. 12/205,878 filed Sep. 8, 2008 and provisional patent application Ser. No. 61/086,819 filed Aug. 7, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a hybrid heating apparatus to heat potable water via “free heat”, i.e., waste heat recovered by heat recovery units (such as refrigeration units) and heat from insolation units (such as by solar collection units). A single controller directs operation of a single pump to circulate fluid between at least one heat exchanger and each of the heat recovery units and insolation units and directs operation of valves to allow the fluid to be circulated to become heated before reaching the heat exchanger, which heat exchange with the potable water heats the same. 
     2. State of the Art 
     Commercial and residential facilities and dwellings include various systems for heating potable water. In generally, they primarily rely on a conventional water heater that includes either a fossil fuel (oil or natural gas) furnace or boiler or an electric water heater, although an increasing number of such facilities and dwellings have turned to a solar water heater to satisfy their demand for heating potable water to the extent feasible. If the solar water heater cannot meet the demand for potable water heating, then the conventional water heater is operated to satisfy the demand. 
     A solar water heater may be operated in either a closed loop system or an open loop system to heat potable water stored in a tank. In an open loop system, potable water to be heated is pumped from the tank directly to the solar water heater and back. In a closed loop system, glycol or other kind of fluid having a lower freeze temperature than that of water is pumped to the solar water heater for heating and pumped back to a heat exchanger for heating the potable water in the tank. In climates susceptible to freezing outdoor temperatures, the closed loop system for the solar water heater is used. In climates that are not susceptible to freezing outdoor temperatures, the open loop system may be used for the solar water heater. 
     In the case of a dedicated solar water heater, the piping may become cold when exposed to cold outdoor temperatures overnight when there is no insolation. At the time of sunrise (or later if they do not face the morning sun), the solar collectors can start again to heat fluid through insolation, but the solar water heater would be operating under a cold start and thus will need to heat the cold fluid circulating in the piping to a higher temperature before it can attempt to satisfy a demand for heating potable water. 
     Installation and operating costs affect the economic feasibility of incorporating a solar water heater into an existing commercial and residential facility and dwelling to satisfy needs to heat potable water. Thus, the need to heat circulating fluid in piping from a cold start adversely affects the economics of heating potable water by insolation since the conventional water heater will need to operate that much longer until the cold start condition is overcome. Further, the cost for installation and operation of a dedicated pump (and heat exchanger in the case of a closed loop) for the solar water heater adversely affect the economics of heating potable water by insolation. 
     JP2004012025 proposes an efficient hybrid system that improves the relationship between respective pieces of equipment in a solar system and a cogeneration system by reducing carrier power by inverter control. The hybrid system includes a solar heat collector, a heat storage tank, a heat exchanger to supply hot water, a hot water storage tank, an auxiliary boiler, a heat exchanger for collecting waste heat, a non-utility generator, an absorption type refrigerator, a refrigerating tower, a heater exchanger for heating, a system connection board for controlling the drive of each piece of equipment and a DC power supply board. The overall efficiency of operation is improved when both a solar heat collector and a refrigerant waste heat collector are used to heat water through respective heat exchangers as a supplement to a conventional boiler. However, the economics of such a system is adversely impacted by installing and running respective pumps and using respective heat exchangers for each water heating system, i.e., solar insolation, waste heat recovery, auxiliary boiler, etc. 
     It would be desirable to reduce the overall installation and operating costs to heat potable water that uses “free heat” from a solar water heater and refrigerant waste heat recovery units (HRU) by integrating them rather than keeping them as separate, stand-alone water heating systems. 
     SUMMARY OF THE INVENTION 
     A water heating apparatus 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 via a heat exchanger, a solar water heater unit that extracts heat from insolation and transfers the extracted heat to the water stored in the tank preferably also via the same heat exchanger, and at most one circulating pump to circulate fluid between the heat exchanger and each of the HRU and preferably also the solar water heater (if in a closed loop system). 
     The refrigeration unit preferably includes circulating refrigerant, a compressor for compressing the refrigerant, a fan and an expansion valve for cooling the refrigerant, and an evaporator section that absorbs heat from a refrigeration area to cool the refrigeration area. 
     A single circulating pump is operated to circulate a heat transfer fluid between a heat exchanger and each of the heat recovery units. The heat exchanger exchanges heat with potable water stored in a tank. 
     The solar water heater unit includes a solar collector that extracts energy from insolation. If the solar water heater unit is in a closed loop, as are the heat recovery units, then the same circulating pump is operated to circulate the heat transfer fluid to the solar water heater to heat the heat transfer fluid as is used to circulate the heat transfer fluid between the heat exchanger and each of the heat recovery units. Otherwise, the solar water heater unit is in an open loop in the sense that potable water is circulated directly from the tank to the solar water heater to effect heating of the potable water directly. 
     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. 
     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. 
     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 in an open loop situation (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 difference between the temperature of the second heat transfer medium at the HRU exceeds the temperature of the potable water in the tank by a predetermined amount (e.g., 8-24.degrees. Fahrenheit). During normal operation, the temperature of the refrigerant between the HRU 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. 
     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 degrees 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 less than 200 degrees Fahrenheit). The first condition allows for the activation of the solar water heater unit when efficient heat transfer can take place. The second condition is when tank temperature is above 185 degrees Fahrenheit controller  58  activates 3-way valve  72 A to dissipate the heat to  71  (heat dump), until the tank temp is below 175 degrees Fahrenheit. The third 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. 
     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. 
     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 
         FIG. 1  is a schematic depiction of an exemplary embodiment of a water heating system according to the present invention. 
         FIG. 2  is a table describing the function of the fan control means of the refrigeration unit of the invention. 
         FIG. 3  is a table describing the function of the water heating system control means of the solar water heater unit and HRU of the invention. 
         FIG. 4  is a schematic of the circuitry of an embodiment of the operational control of the fan of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to  FIG. 1 , a water heating apparatus or system  110  of the present invention 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 conventional heating tank  112 - 2  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  112 - 1  is configured to place water stored therein is fluid communication with a heat recovery unit  16 , a solar water heater unit  18 , and a heating element  120 . 
     The system is configured to heat the potable water in the pre-heat tank  112 - 1  by using heat available from free sources (e.g., refrigeration and solar units) in conjunction with the conventional heating element  120  to provide an energy efficient hot water heating system  110 , a conventional heating tank  112 - 2 , and a bypass system  180 . The conventional 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 water heater or solar collection unit  18 . 
     The heat recovery unit  16  of the system 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 circulating pump  54  and a valve  74 A, which circulates fluid medium from the tank  112 - 1  through a flow loop  17 , a heat exchanger  26 , and a first controller  58 . When heat is available from the vapor compression refrigeration unit  22 , a controller  58  is configured to activate the pump  54  and a valve  74 A, to pump the fluid medium from the tank  112 - 1  through the heat exchanger  26  and back into the tank  112 - 1 . 
     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 to a conditioned space. 
     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  112 - 1 , 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 in fluid communication with 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 in fluid communication with the potable water in the tank  12 , which also reduces the typical cooling requirements of the fan  36 . 
     The operation of the controller  58  of the heat recovery unit  16  of the system is best understood with reference to  FIG. 1 . The controller  58  activates the circulation pump  54  and a valve  74 A, to circulate a fluid medium (heat transfer fluid) from the tank  112 - 1  through the heat exchanger  26  when heat is available from the refrigeration unit  22 . For example, the controller  58  can receive a first sensor input  69  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  69  reaches a predetermined level indicating that heat is available from the refrigeration unit  22 , the controller  58  may activate the circulation pump  54  and a valve  74 A. 
     The controller  58  is also preferably configured to deactivate the circulating pump  54  and a valve  74 A, to cease circulating fluid medium from the tank  112 - 1  through the heat exchanger  26  when the water within the tank  112 - 1  reaches a predetermined temperature. For example, the controller  58  may receive a second sensor input  68  indicative of the water temperature within the tank  112 - 1 . When the second sensor input  68  reaches a predetermined level, the controller  58  deactivates the circulation pump  54  and a valve  74 A. In one example, the second sensor input  68  may be a temperature signal and the predetermined level might be 155 degrees Fahrenheit (F). 
     The controller  58  may also be configured to activate the circulating pump  54  and a valve  74 A, when the temperature of the fluid medium in the second fluid loop  17  becomes so low that it is in danger of freezing. For example, in an Open Loop configuration the controller  58  may receive a first sensor input  69  indicative of the fluid medium temperature within the second fluid loop  17 . When the first sensor input  69  reaches a predetermined level, the controller  58  activates the circulation pump  54  and a valve  74 A, to circulate water from the tank  112 - 1  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  54  will operate as discussed above to transfer heat from the refrigerant to the fluid medium at the heat exchanger  26 . 
     In the event that the refrigeration unit  22  goes down during the winter months, the operation of the circulating pump  54  and a valve  74 A, to circulate fluid medium from the tank  112 - 1  through the second fluid loop  17  will help to prevent the fluid medium 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  112 - 1  so that the tank  112 - 1  water will remain warm even during a long power outage. It is also anticipated that this anti-freezing operation of the controller  58  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. 
     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. 
     The operational control of the fan  36  of the refrigeration unit  16  is best understood with reference to  FIGS. 1 ,  2  and  4 . 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. 
     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 . 
     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  112 - 1  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). 
     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). 
     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. 4 , which includes a pressure control unit  701  in electrical connection 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 in fluid communication with 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. 
     Thus, system  110 , 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  110  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. 
     The system  110  of the present invention also preferably incorporates in one fluid medium loop of a hybrid water heating system, the solar water heater unit  18 , and uses it in conjunction with the heat recovery unit  16 . The solar water heater unit  18  and HRU  16  and its operational control is best understood with reference to  FIG. 1 . 
     The solar collection unit  18  provides heat captured from solar energy to the water in the tank  112 - 1 . Thus, the water in tank  112 - 1  is heated not only by the heat recovery unit  16 , but also by the solar collection unit  18 . 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  112 - 1  by both the heat recovery unit  16  and solar collection unit  18  thru one Solar and HRU fluid medium Loop. 
     The solar collection unit  18  includes a circulating pump  54 , which circulates a heat transfer medium through a flow loop  17 . A solar collector  56  and a heat exchanger  60 A or B are operably coupled via three-way valve  72 B to the flow loop  17  as shown in  FIG. 1 . A controller  58  is provided for selectively activating and deactivating the circulating pump  54  of the solar collection unit  18 . When heat is available from solar energy the 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 A or B via the fluid loop  17 . 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 A or B. 
     The fluid loop  17  of the solar collection unit  18  and HRU  16  is shown ( FIG. 1 ) 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 HRU  16  in fluid communication with the heat exchanger  60 A or B to indirectly heat the water in the tank  112 - 1 . 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  112 - 1  directly through the solar collector  56  and HRU  16  back into the tank  112 - 1 . 
     Conversely, while the fluid loop  17  of the heat recovery unit  16  is shown ( FIG. 1 ) by way of example as a indirect or closed-loop circulation system where the pump  54  circulates the fluid medium from the tank  112 - 1  through the heat exchanger  26  and back into the tank  112 - 1 , the fluid loop  17  may instead be an indirect or closed-loop circulation system isolated from the water in the tank  112 - 1  in which the pump  54  circulates a heat-transfer fluid through the heat exchanger  26  and through an additional heat exchanger  60  A or B in a heat exchange relationship with the water in tank  112 - 1  to indirectly heat the water in the tank. 
     In addition, the heat exchanger  60 A or B disposed at the tank  112 - 1  is shown by way of example only as a flat heat exchanger in tank  112 - 1 . 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  112 - 1 . The tank  112 - 1  may also be a jacketed tank in which the heat exchanger  60  forms a heat exchange jacket around the outer surface of the tank  112 - 1 . 
     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. 
     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 . 
     The controller  58  of the solar water heater unit  18  controls the circulating pump  54  and a valve  74 B, to circulate the heat-transfer fluid from the heat exchanger  60  in the tank  112 - 1  through the solar collector  56  only when heat is available at the solar collector  56 . For example, the controller  58  may receive an input  66  indicative of a condition of the solar collector  56 . The 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 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  and a valve  74 B. 
     The controller  58  is preferably configured to activate the circulating pump  54  and a valve  74 B, to cease circulating the heat-transfer fluid through the solar collector  56  and the heat exchanger  60  when the water within the tank  112 - 1  reaches a predetermined temperature. For example, the controller  58  can receive an input  68  indicative of a temperature of the water within the tank  112 - 1 . When the input  68  reaches a predetermined limit, the controller  58  deactivates the circulating pump  54  and a valve  74 B. 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). 
     The controller  58  is described by way of example as operating based on a temperature limit (e.g., sensed from an input  66 ) and a temperature limit (e.g., sensed from an input  68 ). However, as discussed in  FIG. 3 , the controller  58  may also operate as a differential controller in which the controller  58  activates the circulating pump  54  and a valve  74 B, when the 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  and a valve  74 B, when the  66 ,  68  are indicative of at least approximately 8 degrees Fahrenheit (F) and can deactivate the pump  54  and a valve  74 B, when the temperature differential is less than approximately 8 degrees Fahrenheit (F). Similarly, the controller  58  of the heat recovery unit  16  ( FIG. 1 ) may be configured to operate as a differential controller in which the controller  58  only activates the circulating pump  54  and a valve  74 A, when the inputs  69 / 68  are indicative of at least a predetermined value. The controller  58  can also operate to deactivate the circulating pump  54  and a valve  74 B, upon the input  66  exceeding a 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  17  for lowering the pressure within the flow loop  17  in the event that the input  66  exceeds the temperature limit. In an open configuration of the relief valve, the second heat transferring medium is drained from the flow loop  17  in gas or liquid form to lower the pressure therein. 
     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 Resol module. 
     Preferably, only one common pump is needed to circulate the fluid through all of the heat recovery units and, preferably, the solar collector. The controller  58  is configured to send signals to direct the only one common pump to circulate the fluid, which becomes heated with, in effect, free heat available from the solar collection unit during daylight hours and from the at least one refrigeration unit during hours of operation of the at least one refrigeration unit so as to maintain a temperature in the common piping higher that would otherwise arise if there was just one of the solar collection unit and the refrigeration unit but not both. The higher temperature of the fluid allows the free heat to heat the fluid to a desired temperature quicker to meet demand than would otherwise be the case if the fluid temperature were at a lower temperature. 
     The controller  58  may be configured to receive a heat demand signal indicative of a demand for heating the potable water and a heat demand satisfaction signal indicative of satisfying the demand. The controller is configured to send a command signal to the only one pump to circulate the fluid to satisfy the demand if the demand is not yet met based on receipt of the heat demand signal. The controller  58  is configured to send a command signal to the only one pump to cease the fluid circulation once the demand for heating the potable water has been met based on receipt of the heat demand satisfaction signal. 
     Further piping may be provided between the tank and the solar collector unit to bypass the at least one heat exchanger to establish fluid communication directly between the tank and the solar collector unit via the further piping. 
     When heat is unavailable from either the heat recovery unit  16  or the solar collection unit  18 , the system  110  utilizes a conventional heating element  120  to heat the water in the tank  112 - 2 . Heating element  120  may be an electrically powered element, a gas-burning element, an oil-burning element, and combinations thereof. 
     The hybrid hot water heat system  110  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  110  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. 
     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. 1 , 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 . 
     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 - 1 . 
     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 . 
     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 . 
     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. 
     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.