Patent Publication Number: US-2023134566-A1

Title: Heat recovery apparatus and methods of increasing energy efficiency of hybrid heating systems using the apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates, in general, to hybrid heating systems, and more specifically relates to a heat recovery apparatus and methods of increasing energy efficiency of the hybrid heating systems using the apparatus. 
     BACKGROUND 
     Air conditioning systems are generally used for conditioning air within a closed space. Such air conditioning systems comes with varying capacity and power rating to heat or cool the air within the closed space based on a size of the closed space. In one example, an air conditioning system may be a heat pump for heating or cooling the closed space. In another example, a heat pump alone may not be able to meet required heating load demanded by the closed space, in such a case, a furnace is combined with the heat pump to meet the required heating load within the closed space. In such hybrid heating arrangement, the heat pump and the furnace operate together to meet the required heating load within the closed space. 
     The furnace, generally, includes a combustion chamber in which a fuel is burned to provide additional heat energy to the air within the closed space. During operation of the furnace, the combustion chamber discharges flue gas which is generally disposed to atmosphere. The flue gas also carries heat which may be recovered by a heat recovery device for different use. In a known system, the recovered heat is transferred to water, or other secondary fluid(s), which in turn transfer heat to refrigerant in a vapor compression cycle system. Hence, there remains a need to develop a system with a heat recovery apparatus to reuse the heat energy disposed to the atmosphere as waste by directly exchanging it with the refrigerant in a heat pump, thereby to improve performance of the hybrid heating system and overcome the shortcomings of the known heating system. 
     SUMMARY 
     According to one aspect of the present disclosure, an apparatus for recovering heat from flue gas discharged from a furnace of a hybrid heating system is disclosed. The apparatus includes a shell disposed in fluid communication with the furnace and configured to allow the flue gas to flow therethrough in a first direction. The shell includes an inlet port fluidly coupled with a flue gas duct of the furnace and configured to receive the flue gas. The shell includes an outlet port configured to exit the flue gas. The flue gas flows from the inlet port to the outlet port in the first direction. The apparatus further includes one or more tubes disposed in fluid contact with the shell and fluidly communicated with a heat pump which is in fluid contact with the furnace of the hybrid heating system. The one or more tubes are configured to allow a stream of refrigerant to flow therethrough in a second direction. The one or more tubes are disposed along a length of the shell and includes an inlet end and an outlet end together configured to fluidly couple with a stream of the refrigerant in the heat pump, and the refrigerant flows from the inlet end to the outlet end in the second direction. The second direction of the flow of the stream of the refrigerant is different from the first direction of the flow of the flue gas. 
     In one embodiment, the apparatus further includes a container disposed proximate a bottom end of the shell configured to collect condensate therefrom. 
     In one embodiment, the apparatus further includes an inlet conduit and an outlet conduit configured to fluidly communicate with the inlet end and the outlet end, respectively, of the one or more tubes. 
     In one embodiment, the apparatus further includes one or more valves disposed in at least one of the inlet conduit and the outlet conduit and configured to regulate access between the apparatus and the heat pump during a heating mode based on one or more operating parameters of the hybrid heating system. 
     According to another aspect of the present disclosure, a hybrid heating system including a heat pump and a furnace is disclosed. The hybrid heating system includes a heat recovery apparatus in fluid communication with the heat pump and the furnace. The heat recovery apparatus is configured to (i) recover heat from flue gas discharged from the furnace, and (ii) transfer the recovered heat to a stream of refrigerant in the heat pump. The heat recovery apparatus includes a shell disposed in fluid communication with the furnace and configured to allow the flue gas to flow therethrough in a first direction. The heat recovery apparatus further includes one or more tubes disposed in fluid communication with the shell and the heat pump. The one or more tubes are configured to allow a stream of refrigerant to flow therethrough in a second direction. The second direction of the flow of stream of the refrigerant is different from the first direction of the flow of the flue gas. The hybrid heating system further includes one or more valves configured to regulate access between the heat recovery apparatus and the stream of the refrigerant in the heat pump. The hybrid heating system further includes a control unit in communication with the one or more valves and configured to regulate access between the heat recovery apparatus and the heat pump during a heating mode based on one or more operating parameters of the hybrid heating system. The hybrid heating system further includes one or more sensing devices configured to communicate inputs indicative of the one or more operating parameters of the hybrid heating system with the control unit. 
     The heat recovery apparatus is fluidly coupled with the stream of the refrigerant in series with an evaporator of the heat pump. In one embodiment, the heat recovery apparatus is fluidly coupled with the stream of the refrigerant at a downstream end of the evaporator of the heat pump. In another embodiment, the heat recovery apparatus is fluidly coupled with the stream of the refrigerant at an upstream end of the evaporator of the heat pump. In yet another embodiment, the heat recovery apparatus is fluidly coupled with the stream of the refrigerant in parallel to the evaporator of the heat pump. Particularly, an inlet end and an outlet end of the one or more tubes of the heat recovery apparatus are fluidly coupled with the upstream end and the downstream end, respectively, of the evaporator of the heat pump. The heat recovery apparatus is configured to supply refrigerant to the stream of the refrigerant in the heat pump during a defrosting mode and a heating mode of the hybrid heating system. 
     According to yet another aspect of the present disclosure, a method of increasing energy efficiency of a hybrid heating system during a heating mode is disclosed. The hybrid heating system includes a heat pump and a furnace. The method includes contacting a heat recovery apparatus with a stream of flue gas discharged from the furnace and a stream of refrigerant in the heat pump. The heat recovery apparatus includes a shell and one or more tubes disposed in contact with the shell. The method further includes directing the flue gas to flow through the shell in a first direction and directing the stream of the refrigerant to flow through the one or more tubes in a second direction. The one or more tubes are fluidly communicated with a stream of the refrigerant in the heat pump. The second direction of the flow of the stream of the refrigerant is different from the first direction of the flow of the flue gas. The method further includes introducing, via the heat recovery apparatus, a stream of refrigerant to the stream of the refrigerant in the heat pump during the heating mode and a defrosting mode. In an embodiment, the method further includes regulating access between the heat recovery apparatus and the heat pump during the heating mode, via a control system and one or more valves, based on one or more operating parameters of the hybrid heating system. 
     The method further includes fluidly coupling the heat recovery apparatus with the stream of the refrigerant in series with an evaporator of the heat pump. In one embodiment, the method includes fluidly coupling the heat recovery apparatus with the stream of the refrigerant at a downstream end of the evaporator of the heat pump. In another embodiment, the method includes fluidly coupling the heat recovery apparatus with the stream of the refrigerant at an upstream end of the evaporator of the heat pump. In yet another embodiment, the method includes fluidly coupling the heat recovery apparatus with the stream of refrigerant in parallel to the evaporator of the heat pump. 
     These and other aspects and features of non-limiting embodiments of the present disclosure will become apparent to those skilled in the art upon review of the following description of specific non-limiting embodiments of the disclosure in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of embodiments of the present disclosure (including alternatives and/or variations thereof) may be obtained with reference to the detailed description of the embodiments along with the following drawings, in which: 
         FIG.  1    is a schematic block diagram of a hybrid heating system having a heat recovery apparatus, according to an embodiment of the present disclosure; 
         FIG.  2    is a schematic diagram of the heat recovery apparatus of  FIG.  1   , according to an embodiment of the present disclosure; 
         FIG.  3    is a schematic block diagram of the hybrid heating system operating in a defrosting mode, according to an embodiment of the present disclosure; 
         FIG.  4    is a schematic block diagram of a hybrid heating system having the heat recovery apparatus connected in series with an outdoor heat exchanger, according to an embodiment of the present disclosure; 
         FIG.  5    is a schematic block diagram of a hybrid heating system having the heat recovery apparatus connected in parallel with the outdoor heat exchanger, according to an embodiment of the present disclosure; and 
         FIG.  6    is a flow diagram of a method of increasing energy efficiency of the hybrid heating system of  FIG.  1   , according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to specific embodiments or features, examples of which are illustrated in the accompanying drawings. Wherever possible, corresponding or similar reference numbers will be used throughout the drawings to refer to the same or corresponding parts. Moreover, references to various elements described herein, are made collectively or individually when there may be more than one element of the same type. However, such references are merely exemplary in nature. It may be noted that any reference to elements in the singular may also be construed to relate to the plural and vice-versa without limiting the scope of the disclosure to the exact number or type of such elements unless set forth explicitly in the appended claims. 
     Referring to  FIG.  1   , a schematic block diagram of a hybrid heating system  100  is illustrated, according to an embodiment of the present disclosure. The hybrid heating system  100  includes a heat pump  102  and a furnace  104  in fluid contact with each other. The heat pump  102  is used for conditioning air, that is heating or cooling of air to a desired temperature, within a closed space  106 . In heating mode, the t furnace  104  works in tandem with the heat pump  102  to meet a desired heating load when the temperature of the air within the closed space  106  is low and the heat pump  102  alone is not able to meet the desired heating load. The heat pump  102  includes an outdoor heat exchanger  110 , which is acting as an evaporator  110 , and an indoor heat exchanger  112 , which is acting as a condenser  112 , fluidly and cyclically communicated to each other via multiple heat pump conduits  114 . The indoor heat exchanger  112  is disposed in fluid contact with the closed space  106  and the outdoor heat exchanger  110  is disposed outside the closed space  106  to exchange heat with ambient air. In an example, the outdoor heat exchanger  110  may be a liquid-to-refrigerant heat exchanger or an air-to-refrigerant heat exchanger. The outdoor heat exchanger  110  and the indoor heat exchanger  112  are fluidly coupled with the heat pump conduits  114  and configured to allow a stream of refrigerant to flow therethrough in a cyclic manner. 
     The heat pump  102  further includes a compressor  116  fluidly coupled with the heat pump conduits  114  and disposed between the outdoor heat exchanger  110  and the indoor heat exchanger  112  at a downstream end  117  of the of the outdoor heat exchanger  110 . A muffler  118  may be connected to the compressor  116  to deaden any undue noise that may be generated by the compressor  116  when the heat pump  102  is an operating mode. Further, the heat pump  102  includes a reversing valve  120  disposed in fluid communication with the outdoor heat exchanger  110 , the indoor heat exchanger  112  and the compressor  116 . The reversing valve  120  is configured to reverse flow of the stream of refrigerant between the outdoor heat exchanger  110  and the indoor heat exchanger  112  such that the heat pump  102  can be operated in a heating mode and a cooling mode. During the heating operation, the reversing valve  120  may be actuated for a shorter period to operate the heat pump  102  in the cooling mode, which is otherwise referred to as a defrosting mode. In the heating mode, the reversing valve  120  allows flow of refrigerant such that the outdoor heat exchanger  110  act as an evaporator and the indoor heat exchanger  112  acts as a condenser, whereas, in the cooling mode, the reversing valve  120  allows flow of the refrigerant such that the indoor heat exchanger  112  act as an evaporator and the outdoor heat exchanger  110  act as a condenser. The heat pump  102  may further include a charge compensator  122  fluidly coupled with the heat pump conduits  114  and disposed between the outdoor heat exchanger  110  and the reversing valve  120 . The charge compensator  122  may be configured to receive certain quantity of the refrigerant out of circulation during the heating mode and return the refrigerant back to the circulation during the cooling mode. The heat pump  102  may further include an accumulator  124  fluidly coupled with the heat pump conduits  114  and disposed between the compressor  116  and the reversing valve  120 . The accumulator  124  is configured to receive moisture content of refrigerant before the refrigerant reaches the compressor  116 . 
     The heat pump  102  further includes a first expansion valve  126  fluidly coupled with the heat pump conduits  114  and disposed between the outdoor heat exchanger  110  and the indoor heat exchanger  112  at an upstream end  127  of the of the outdoor heat exchanger  110 , and a second expansion valve  128  fluidly coupled with the heat pump conduits  114  and disposed between the outdoor heat exchanger  110  and the indoor heat exchanger  112  at a downstream end of the indoor heat exchanger  112 . The first expansion valve  126  and the second expansion valve  128  may be parallelly connected with a first flow check valve  130  and a second flow check valve  132 , respectively, to allow flow of the refrigerant when the corresponding expansion valve is not operational during the heating mode and the cooling mode. In an example, the first expansion valve  126  and the second expansion valve  128  may be unidirectional, i.e., allow flow of the refrigerant in only one direction. The heat pump  102  further includes a filter drier  134  fluidly coupled with the heat pump conduits  114  and disposed between the first expansion valve  126  and the second expansion valve  128 . The filter drier  134  is configured to remove contaminants from the refrigerant while flowing therethrough. 
     The indoor heat exchanger  112  is configured to fluidly contact with the furnace  104  which receives air from the closed space  106  or ambient air and supply warm air to the closed space  106  in addition to the heat supplied by the heat pump  102 . The furnace  104  includes a housing  136  to accommodate an air blower  138  and a combustion chamber  140 . The air blower  138  may be disposed at bottom of the housing  136  and configured to receive the air from the closed space  106  via an air inlet duct  142 . The combustion chamber  140  having a fuel burner  144  is configured to heat the air supplied by the air blower  138 . The combustion chamber  140  is in fluid communication with a gas inlet duct  146 , through which a gas fuel may be supplied to the fuel burner  144 . Further, the combustion chamber  140  is in fluid communication with a flue gas duct  148  to discharge a flue gas therethrough. The indoor heat exchanger  112  of the heat pump  102  is disposed above the combustion chamber  140  such that the heated air coming out of the combustion chamber  140  may be further heated by the stream of high temperature refrigerant flowing through the indoor heat exchanger  112 . Such heated air is supplied to the closed space  106  using an air outlet duct  149  that is in fluid communication with the indoor heat exchanger  112  to meet the desired heating load within the closed space  106 . In some embodiments, the indoor heat exchanger  112  of the heat pump  102  may be disposed below the combustion chamber  140 , or the furnace  104 , or disposed either of the sides of the furnace  104  in accordance with an air flow path of the furnace  104 . The indoor heat exchanger  112 , the combustion chamber  140  and the air blower  138  together constitute an air handler  141  of the hybrid heating system  100 . The air handler  141  is configured to condition the air, such as the process of heating or cooling the air, within the closed space  106 . 
     The hybrid heating system  100  further includes an apparatus  150 , which is alternatively referred to as ‘the heat recovery apparatus  150 ’, for recovering heat from the flue gas discharged from the furnace  104 . The heat recovery apparatus  150  is further configured to exchange the recovered heat energy with a stream of refrigerant in the heat pump  102  to increase energy efficiency of the hybrid heating system  100  during the heating mode. 
     Referring to  FIG.  2   , a schematic diagram of the heat recovery apparatus  150  is illustrated, according to an embodiment of the present disclosure. The heat recovery apparatus  150  includes a shell  202  disposed in fluid communication with the furnace  104 . The shell  202  may be an elongated hollow body having a length ‘L’ defined between a first end  204  and a second end  206 . In one embodiment, the length ‘L’ of the shell  202  may be one foot. In some embodiments, the length ‘L’ of the shell  202  may be more than one foot. The shell  202  is disposed in fluid communication with the furnace  104  in such a way that the shell  202  allows the flue gas to flow therethrough in a first direction ‘D 1 ’. In one embodiment, the shell  202  may be a single hollow body configured to allow the flue gas to flow therethrough in the first direction ‘D 1 ’. The shell  202  includes an inlet port  208  defined at the first end  204  and fluidly coupled with the flue gas duct  148  of the furnace  104 . The inlet port  208  is fluid tightly coupled with the flue gas duct  148  and configured to receive the flue gas from the flue gas duct  148 . The shell  202  further includes an outlet port  210  defined at the second end  206  and configured to discharge the flue gas to atmosphere. As such, the flue gas received within the shell  202  flows from the inlet port  208  to the outlet port  210  in the first direction ‘D 1 ’. The shell  202  may be made of polyvinyl chloride (PVC), metals or metal alloys such as stainless steel and aluminum. 
     The heat recovery apparatus  150  further includes one or more tubes  212  disposed in fluid contact with the shell  202 . In case of the single hollow body shell, the one or more tubes  212  may be disposed within the shell  202 . The one or more tubes  212  are fluidly communicated with the stream of the refrigerant in the heat pump  102 , as such the one or more tubes  212  allow the stream of the refrigerant to flow therethrough in a second direction ‘D 2 ’. The one or more tubes  212  may have a length equal to the length ‘L’ of the shell  202  and are disposed along the length ‘L’ of the shell  202 . The tubes  212  define an inlet end  214  at the second end  206  of the shell  202  and an outlet end  216  at the first end  204  of the shell  202 . The inlet end  214  and the outlet end  216  are together configured to fluidly couple with the stream of the refrigerant in the heat pump  102  such that the refrigerant flows from the inlet end  214  to the outlet end  216  in the second direction ‘D 2 ’. The one or more tubes  212  may be made of metals or metal alloys such as stainless steel, aluminum, or any other high thermally conductive material that has better acid corrosion properties from acidic gas flue moisture condensate. In some embodiments, the tubes  212  may be a group of micro or mini channels stacked together to allow the refrigerant to flow therethrough in the second direction ‘D 2 ’. In such a case, the shell  202  and the micro or mini channels of the tubes  212  may be alternatively stacked to make fluid contact with each other. Further, the one or more tubes  212  can be smooth tube or have enhanced heat transfer surface to improve heat transfer. 
     The second direction ‘D 2 ’ of the flow of the stream of refrigerant is different from the first direction ‘D 1 ’ of the flow of the flue gas. In one embodiment, the second direction ‘D 2 ’ of the flow of the stream of refrigerant is parallel and opposite to the first direction ‘D 1 ’ of the flow of the flue gas. Such arrangement of the flow of refrigerant and the flue gas is otherwise referred to as a counter flow arrangement. The counter flow arrangement may improve rate of heat transfer between the flue gas and the refrigerant which in turn may cause increase in heat transfer efficiency of the heat recovery apparatus  150 . In another embodiment, the second direction ‘D 2 ’ of the flow of stream of the refrigerant is perpendicular to the first direction ‘D 1 ’ of the flow of the flue gas. In such a case, fins, or baffles (not shown) may be disposed transverse to the length ‘L’ of the shell  202 . In some embodiments, the second direction ‘D 2 ’ of the flow of stream of the refrigerant is parallel and identical to the first direction ‘D 1 ’ of the flow of the flue gas. 
     The heat recovery apparatus  150  further includes a container  220  disposed proximate a bottom end of the shell  202  and configured to collect condensate therefrom. During the operation of the hybrid heating system  100 , if the flue gas contains moisture, then the moisture may condensate and collected in the container  220 . The container  220  is located at the bottom end of the heat recovery apparatus  150  such that the condensate may flow down due to gravity and received in the container  220 . In an embodiment, the container  220  may be movably and adjustably coupled to the shell  202  of the heat recovery apparatus  150  such that the container  220  may be located at the bottom end of the shell  202  based on orientation of the heat recovery apparatus  150  during implementation thereof. In one implementation, the heat recovery apparatus  150  may be vertically disposed, and in another implementation, the heat recovery apparatus  150  may be horizontally disposed. In one embodiment, the container  220  may be disposed at the first end  204  of the shell  202  when the heat recovery apparatus  150  is vertically disposed during implementation. In another embodiment, the container  220  may be disposed parallel along the length ‘L’ of the shell  202  at the bottom end thereof when the heat recovery apparatus  150  is horizontally disposed during implementation. The condensate collected in a trap section of the container  220  may further help in preventing the flue gas from escaping through the container  220 . Further, the condensate collected in the container  220  may be further treated for acid neutralization thereby the water can be further utilized for commercial use or could be disposed as waste. The heat recovery apparatus  150  including the one or more tubes  212  and the shell  202  may be fully insulated to avoid transfer of heat to the ambient thereby reduce heat loss in the heat recovery apparatus  150 . 
     The heat recovery apparatus  150  further includes an inlet conduit  222  and an outlet conduit  224  configured to fluidly communicate with the inlet end  214  and the outlet end  216 , respectively, of the one or more tubes  212 . In one embodiment, the inlet conduit  222  and the outlet conduit  224  may be a pipe made of nonflexible material such as a metal. In another embodiment, the inlet conduit  222  and the outlet conduit  224  may be a hose made of flexible material such as elastomers. The inlet conduit  222  and the outlet conduit  224  are configured to fluid tightly couple with the stream of the refrigerant in the heat pump  102 . The stream of the refrigerant in the heat pump  102  is otherwise referred to as a portion of the heat pump conduit  114  through which the refrigerant flows at a desired pressure. 
     The heat recovery apparatus  150  further includes one or more valves  230  disposed in at least one of the inlet conduit  222  and the outlet conduit  224 . The valves  230  may include one or more isolation valves, one or more check valves, and a combination thereof based on application of the hybrid heating system  100 . The one or more valves  230  are configured to regulate access between the heat recovery apparatus  150  and the heat pump  102  during the heating mode based on one or more operating parameters of the hybrid heating system  100 . The one or more valves  230  are also configured to selectively disconnect the heat recovery apparatus  150  from the heat pump  102  during the cooling mode/defrosting mode based on the one or more operating parameters of the hybrid heating system  100 . In an embodiment, the one or more valves  230  may be electronically actuated valves. 
     Referring to  FIG.  1    and  FIG.  2   , the heat recovery apparatus  150  is disposed in fluid contact with the flue gas duct  148  of the furnace  104 . Particularly, the inlet port  208  of the shell  202  is fluidly coupled with the flue gas duct  148 . The heat recovery apparatus  150  is implemented in such a way that the outlet port  210  of the shell  202  may discharge the flue gas to atmosphere. In one implementation, the heat recovery apparatus  150  may be disposed indoor. In another implementation, the heat recovery apparatus  150  may be disposed outdoor. The inlet end  214  and the outlet end  216  of the one or more tubes  212  of the heat recovery apparatus  150  are fluid tightly coupled with the downstream end  117  of the outdoor heat exchanger  110  using the inlet conduit  222  and the outlet conduit  224 , respectively. More particularly, the inlet conduit  222  is coupled to the heat pump conduit  114  at a first junction ‘J 1 ’ at the downstream end  117  of the outdoor heat exchanger  110 . Further, the outlet conduit  224  is coupled to the heat pump conduit  114  at a second junction ‘J 2 ’ at the downstream end  117  of the outdoor heat exchanger  110 , such that the heat recovery apparatus  150  is fluidly coupled with the stream of the refrigerant in series with the outdoor heat exchanger  110  of the heat pump  102 . The hybrid heating system  100  further includes the one or more valves  230  configured to regulate the access between the heat recovery apparatus  150  and the stream of the refrigerant in the heat pump  102 . In one embodiment, the hybrid heating system  100  includes a first isolation valve  230 A disposed at the first junction ‘J 1 ’ between the inlet conduit  222  and the heat pump conduit  114 , a second isolation valve  230 B disposed at the second junction ‘J 2 ’ between the outlet conduit  224  and the heat pump conduit  114 , and a third isolation valve  230 C disposed in the heat pump conduit  114  between the first isolation valve  230 A and the second isolation valve  230 B. The first isolation valve  230 A, the second isolation valve  230 B, and the third isolation valve  230 C are collectively referred to as ‘the valves  230 ’ and individually referred to as ‘the valve  230 ’ unless otherwise specifically mentioned. 
     The hybrid heating system  100  further includes a control unit  240  in communication with the valves  230  such as the first isolation valve  230 A, the second isolation valve  230 B and the third isolation valve  230 C. The control unit  240  is configured to regulate the access between the heat recovery heat recovery apparatus  150  and the heat pump  102  during the heating mode based on the one or more operating parameters of the hybrid heating system  100 . The control unit  240  may also be in electric communication with the reversing valve  120  of the heat pump  102  to actuate the reversing valve  120  and thereby to selectively operate the hybrid heating system  100  in the heating mode and the cooling mode based on the operating parameters of the hybrid heating system  100 . In some embodiments, the control unit  240  may also be in communication with the furnace  104  to control electronic controlled valves (not shown) used for controlling supply of gas fuel through the gas inlet duct  146 . 
     In some embodiments, the control unit  240  may be in communication with the compressor  116  to control a speed of a motor and an inverter drive thereof. Further, the compressor  116  may include pressure limit sensors and discharge pressure sensors in communications with the control unit  240 . If discharge pressure exceeds a desired upper limit pressure, or if suction pressure exceeds a desired lower limit pressure, the compressor  116  may be turned off. Also, if the discharge pressure is too low, then it indicates loss of heat load at the outdoor heat exchanger  110  in the heating mode or formation of frost on the outdoor heat exchanger  110 . The control unit  240  may also be in communication with the first and second expansion valves  126 ,  128  if they are electronically controlled valves. The control unit  240  may also communicate with a defrost sensor placed on the outdoor heat exchanger  110 . Further, the control unit  240  may be configured to control speed of the air blower  138 . In some embodiments, the outdoor heat exchanger  110  may be a liquid-to-refrigerant heat exchanger, in such a case the control unit  240  may control operation of pump used for supplying liquid. In some embodiments, the outdoor heat exchanger  110  may be an air-to-refrigerant heat exchanger, in such a case the control unit  240  may control operation of air fan used for controlling flow of air. 
     The control unit  240  may be an electronic controller that operates in a logical fashion to perform operations, execute control algorithms, store and retrieve data and other desired operations. The control unit  240  may include access memory, secondary storage devices, processors, and any other components for running an application. The memory and secondary storage devices may be in the form of read-only memory (ROM) or random-access memory (RAM) or integrated circuitry that is accessible by the control unit  240 . Various other circuits may be associated with the control unit  240  such as power supply circuitry, signal conditioning circuitry, driver circuitry, and other types of circuitries. Further, the control unit  240  may be a single controller or may include multiple controllers disposed to control various functions and/or features of the hybrid heating system  100 . The term “controller” is meant to be used in its broadest sense to include one or more controllers and/or microprocessors that may be associated with the hybrid heating system  100  and that may cooperate in controlling various functions and operations of the hybrid heating system  100 . The control unit  240  may rely on data relating to preset data of various components of the hybrid heating system  100  as well as the lab test data captured based on the operational experiments of the hybrid heating system  100 , which may be stored in the memory of the control unit  240 . The data may be presented in the form of tables, graphs, and/or equations. 
     The hybrid heating system  100  further includes one or more sensing devices  242  configured to communicate inputs indicative of the one or more operating parameters of the hybrid heating system  100  with the control unit  240 . In one embodiment, the sensing devices  242  include a first sensor  242 A configured to generate an input signal indicative of temperature of the air within the closed space  106 , a second sensor  242 B configured to generate an input signal indicative of temperature of the refrigerant flowing through the outdoor heat exchanger  110 , a third sensor  242 C configured to generate an input signal indicative of temperature of the refrigerant flowing through the heat recovery apparatus  150 , and a fourth sensor  242 D configured to generate an input signal indicative of temperature of the flue gas entering the heat recovery apparatus  150  through the flue gas duct  148 . The first sensor  242 A, the second sensor  242 B, the third sensor  242 C, and the fourth sensor  242 D may be collectively referred to as ‘the sensing devices  242 ’ and individually referred to as ‘the sensing device  242 ’ unless otherwise specifically mentioned. In some embodiments, the flue gas duct  148  may be provided with a sensor to generate an input signal indicative of flow of the flue gas therethrough. In some embodiments, the hybrid heating system  100  may include multiple sensors disposed on the compressor  116 , the first expansion valve  126 , the second expansion valve  128 , and the indoor heat exchanger  112  to generate input signals indicative of the pressure and the temperature of the refrigerant flowing through the heat pump conduit  114  at various stages of the cyclic operation of the heat pump  102 . 
     During the heating mode, the refrigerant enters the outdoor heat exchanger  110 . The refrigerant is at low temperature compared to temperature of the ambient air such that the refrigerant that enters the outdoor heat exchanger  110  becomes vapor as the heat from the ambient air is transferred to the refrigerant. The refrigerant further enters the compressor  116  via the reversing valve  120 . The refrigerant is compressed by the compressor  116  to higher pressure such that the temperature of the refrigerant becomes greater than the temperature of the ambient air. Such high temperature refrigerant flows through the reversing valve  120  and enters the indoor heat exchanger  112  that is in communication with the closed space  106 . As the temperature of the refrigerant is greater than the temperature of the air in the closed space  106  and temperature of air flowing through the indoor heat exchanger  112 , the heat from the refrigerant is transferred to the flowing air and the air in the closed space  106 . Thus, the air within the closed space  106  is heated to achieve the desired heating load within the closed space  106 . The low temperature and low-pressure refrigerant flows through the first expansion valve  126  and the second flow check valve  132 , as the second expansion valve  128  is not operational and the first flow check valve  130  does not allow the refrigerant to flow therethrough. At the first expansion valve  126 , the warm refrigerant further drops the temperature and the pressure and becomes two-phase form of the refrigerant and enters the outdoor heat exchanger  110 . Thus, the pattern of operation continues cyclically to serve the heat load of the closed space  106 . During the heating mode, the first isolation valve  230 A and the second isolation valve  230 B are shut down to disconnect the heat recovery apparatus  150  from the heat pump  102 , whereas the third isolation valve  230 C is actuated in an open position to allow flow of the refrigerant therethrough. 
     In an embodiment, when the control unit  240  determines that the heat pump  102  is not able to meet the desired heating load within the closed space  106  after certain number of cyclic operations, the control unit  240  may actuate the furnace  104  to burn the gas fuel to provide additional heat energy within the closed space  106 . Particularly, the air within the closed space  106  is sucked by the air blower  138  and allowed to flow through the combustion chamber  140 , in which the air is further heated when flowing over the furnace  104  where combustion of fuel happens inside, and flow across the indoor heat exchanger  112 . As such, the additional heat energy is transferred to the closed space  106  apart from the heat energy provided by the heat pump  102 . The control unit  240  may initiate the operation of the furnace  104  based on the input signal indicative of the temperature of the air within the closed space  106  received from the first sensor  242 A. In an embodiment, the control unit  240  may determine the desired heating load to be established within the closed space  106  based on inputs received from a user. The desired heating load may be defined as a rate at which the air within the closed space  106  is heated to achieve a desired temperature. The control unit  240  may further determine the temperature of the air within the closed space  106 , with respect to time, based on the input signal received from the first sensor  242 A may be after every cyclic operation of the heat pump  102 . Further, the control unit  240  may compare the determined temperature value of the air with the desired heating load. If the temperature of the air within the closed space  106  for a predefined time is less than the desired heating load, then the control unit  240  may actuate the furnace  104 . When the furnace  104  also put in operation to meet the desired heating load within the closed space  106 , the control unit  240  actuates the first isolation valve  230 A and the second isolation valve  230 B to an open position, whereas the third isolation valve  230 C is kept in a closed position. As such, the heat recovery apparatus  150  establishes fluid communication with the heat pump  102  in series with the outdoor heat exchanger  110  at the downstream end  117  thereof. 
     The shell  202  of the heat recovery apparatus  150  receives the flue gas that is discharged from the furnace  104 . As the flue gas flows through the shell  202  from the inlet port  208  to the outlet port  210 , the heat from the flue gas is transferred to the refrigerant flowing through the tubes  212  from the inlet end  214  to the outlet end  216 . Thus, the heat recovery apparatus  150  is configured to recover heat from the flue gas discharged from the furnace  104  and transfer the recovered heat to the stream of the refrigerant in the heat pump  102 . With the series connection of the heat recovery apparatus  150  with the outdoor heat exchanger  110  at the downstream end  117  thereof, the refrigerant exiting the outdoor heat exchanger  110  is further heated by the heat recovery apparatus  150  before reaching to the compressor  116 . Thus, the heat recovery apparatus  150  is configured to supply refrigerant to the stream of the refrigerant in the heat pump  102  during the heating mode of the hybrid heating system  100 . 
     When the refrigerant inside the outdoor heat exchanger  110  is not able to absorb enough heat energy from outside air flowing over the outdoor heat exchanger  110  and accompanied by drop in refrigerant pressure as it is sucked by the compressor  116 , the refrigerant temperature inside the outdoor heat exchanger  110  can fall below water freezing temperature. This causes moisture from outside ambient air to freeze or frost on the outdoor heat exchanger  110 . In  FIG.  1    or  FIG.  4   , the heat recovery apparatus  150  will have the refrigerant gain energy from flue gas, which will prevent or delay the drop of refrigerant pressure and temperature inside the outdoor heat exchanger  110  and thus helps to delay frost formation of the outdoor heat exchanger  110 . 
     When the refrigerant inside the outdoor heat exchanger  110  is not able to absorb enough heat energy from the outside air flowing over the outdoor heat exchanger  110  and accompanied by drop in refrigerant pressure as it is sucked by the compressor  116 , the refrigerant temperature inside the outdoor heat exchanger  110  can fall below water freezing temperature, which causes moisture from the outside ambient air to freeze or frost on the outdoor heat exchanger  110 . The control unit  240  can sense one or more of the second sensor  242 B, the defrost sensor, and suction pressures at the inlet of the compressor  116  and thereby initiate a defrost cycle. 
     Referring to  FIG.  3   , a schematic block diagram of the hybrid heating system  100  operating in the defrosting mode is illustrated, according to an embodiment of the present disclosure. During the heating mode, the outdoor heat exchanger  110  may frost and lose capacity, which may lead to drop in the expected performance of the hybrid heating system  100 . In such a scenario, the heat pump  102  may be operated at the defrosting mode for a predetermined time to defrost the outdoor heat exchanger  110 . 
     During the defrosting mode, the control unit  240  may actuate the reversing valve  120  to direct flow of the high-pressure refrigerant from the compressor  116  to the outdoor heat exchanger  110  to act as a refrigerant condenser for short period of time. In an embodiment, the second sensor  242 B that is in communication with the control unit  240  may provide the input signal indicative of the temperature of the refrigerant in the outdoor heat exchanger  110 . The control unit  240  may determine the temperature of the refrigerant in the outdoor heat exchanger  110  and compare the determined temperature value of the outdoor heat exchanger  110  with a threshold temperature value of the outdoor heat exchanger  110  that is preset in the control unit  240 . If the determined temperature value of the outdoor heat exchanger  110  is smaller than the threshold temperature value, then the control unit  240  may actuate the reversing valve  120  to direct flow of the refrigerant from the compressor  116  to the outdoor heat exchanger  110 . As such, the high temperature refrigerant flows through the reversing valve  120  and enters the outdoor heat exchanger  110 . Simultaneously, as the temperature of the refrigerant is greater than the temperature of the ambient air and frost on the outdoor heat exchanger  110 , the heat from the refrigerant is transferred to the ambient air and frost to melt the frost (or defrost). The high temperature refrigerant that flows from the compressor  116  help to defrost the outdoor heat exchanger  110  in a first time period. The first time period may be defined as the amount of time taken by the heat pump  102  to defrost the outdoor heat exchanger  110  when only the heat pump  102  is operating in the heating mode to meet the desired heating load within the closed space  106 . 
     When the furnace  104  also put in operation to meet the desired heating load within the closed space  106 , the first isolation valve  230 A and the second isolation valve  230 B also actuated to the open position by the control unit  240  to regulate the access between the heat pump  102  and the heat recovery apparatus  150 . The connection of the heat recovery apparatus  150  at the downstream end  117  of the outdoor heat exchanger  110  in series therewith causes the high temperature refrigerant to flow through the heat recovery apparatus  150  before enters the outdoor heat exchanger  110 . When the high temperature refrigerant flows through the heat recovery apparatus  150 , the heat from the flue gas is transferred to the high-pressure refrigerant, when flue gas temperature is higher than the refrigerant temperature, which is circulated back to the outdoor heat exchanger  110 . Such super-heated refrigerant help to defrost the outdoor heat exchanger  110  in a second time period which is smaller than the first time period. The second time period may be defined as the amount of time taken by the heat pump  102  to defrost the outdoor heat exchanger  110  when the heat pump  102 , the furnace  104 , and the heat recovery apparatus  150  are operating together in the heating mode to meet the desired heating load within the closed space  106 . Thus, the addition of the heat recovery apparatus  150  in series connection with the outdoor heat exchanger  110  help the hybrid heating system  100  to reduce the defrosting time. 
     Referring to  FIG.  4   , a schematic block diagram of a hybrid heating system  300  having the heat recovery apparatus  150  connected in series with the outdoor heat exchanger  110  is illustrated, according to an embodiment of the present disclosure. Referring to  FIG.  2    and  FIG.  4   , the heat recovery apparatus  150  is disposed in fluid contact with the flue gas duct  148  of the furnace  104 . Particularly, the inlet port  208  of the shell  202  is fluidly coupled with the flue gas duct  148 . The inlet end  214  and the outlet end  216  of the one or more tubes  212  of the heat recovery apparatus  150  are fluid tightly coupled with the upstream end  127  of the outdoor heat exchanger  110  using the inlet conduit  222  and the outlet conduit  224 , respectively. More particularly, the inlet conduit  222  is coupled to the heat pump conduit  114  at a first junction ‘J 3 ’ at the upstream end  127  of the outdoor heat exchanger  110 . Further, the outlet conduit  224  is coupled to the heat pump conduit  114  at a second junction ‘J 4 ’ at the upstream end  127  of the outdoor heat exchanger  110 , such that the heat recovery apparatus  150  is fluidly coupled with the stream of the refrigerant in series with the outdoor heat exchanger  110  of the heat pump  102 . The hybrid heating system  300  further includes the one or more valves  230  configured to regulate the access between the heat recovery apparatus  150  and the stream of the refrigerant in the heat pump  102 . In one embodiment, the hybrid heating system  300  includes the first isolation valve  230 A disposed at the first junction ‘J 3 ’ between the inlet conduit  222  and the heat pump conduit  114 , the second isolation valve  230 B disposed at the second junction ‘J 4 ’ between the outlet conduit  224  and the heat pump conduit  114 , and a third isolation valve  230 D disposed in the heat pump conduit  114  between the first isolation valve  230 A and the second isolation valve  230 B. 
     The hybrid heating system  300  further includes the control unit  240  in communication with the valves  230  such as the first isolation valve  230 A, the second isolation valve  230 B and the third isolation valve  230 D. The control unit  240  is configured to regulate the access between the heat recovery apparatus  150  and the heat pump  102  during the heating mode based on the one or more operating parameters of the hybrid heating system  300 . The hybrid heating system  300  further includes the sensing devices  242  such as the first sensor  242 A, the second sensor  242 B, and the third sensor  242 C and configured to communicate inputs indicative of the one or more operating parameters of the hybrid heating system  300  with the control unit  240 . 
     During the heating mode, when the heat pump  102  is operating alone, the first isolation valve  230 A and the second isolation valve  230 B are shut down to disconnect the heat recovery apparatus  150  from the heat pump  102 , whereas the third isolation valve  230 D is actuated in an open position to allow flow of the refrigerant therethrough. When the control unit  240  determines that the heat pump  102  is not able to meet the desired heating load within the closed space  106  after certain number of cyclic operations, the control unit  240  may actuate the furnace  104  to burn the gas fuel to provide additional heat energy within the closed space  106 . The control unit  240  may initiate the operation of the furnace  104  based on the input signal indicative of the temperature of the air within the closed space  106  received from the first sensor  242 A. When the furnace  104  also put in operation to meet the desired heating load within the closed space  106 , the control unit  240  actuates the first isolation valve  230 A and the second isolation valve  230 B to an open position, whereas the third isolation valve  230 D is kept in a closed position. As such, the heat recovery apparatus  150  establishes fluid communication with the heat pump  102  in series with the outdoor heat exchanger  110  at the upstream end  127  thereof. 
     The shell  202  of the heat recovery apparatus  150  receives the flue gas that is discharged from the furnace  104 . As the flue gas flows through the shell  202  from the inlet port  208  to the outlet port  210 , the heat from the flue gas is transferred to the refrigerant flowing through the tubes  212  from the inlet end  214  to the outlet end  216 . Thus, the heat recovery apparatus  150  is configured to recover the heat from the flue gas discharged from the furnace  104  and transfer the recovered heat to the stream of the refrigerant in the heat pump  102 . With the series connection of the heat recovery apparatus  150  with the outdoor heat exchanger  110  at the upstream end  127  thereof, the refrigerant exiting the heat recovery apparatus  150  may have increased enthalpy value before entering the outdoor heat exchanger  110 . 
     Referring to  FIG.  5   , a schematic block diagram of a hybrid heating system  400  having the heat recovery apparatus  150  connected in parallel with the outdoor heat exchanger  110  is illustrated, according to an embodiment of the present disclosure. Referring to  FIG.  2    and  FIG.  5   , the heat recovery apparatus  150  is disposed in fluid contact with the flue gas duct  148  of the furnace  104 . Particularly, the inlet port  208  of the shell  202  is fluidly coupled with the flue gas duct  148 . The inlet end  214  and the outlet end  216  of the one or more tubes  212  of the heat recovery apparatus  150  are fluid tightly coupled with the heat pump  102  using the inlet conduit  222  and the outlet conduit  224 , respectively. More particularly, the inlet conduit  222  is coupled to the heat pump conduit  114  at a first junction ‘J 5 ’ between the expansion valve  126  and the filter drier  134 . An isolation valve  230 E is disposed at the first junction ‘J 5 ’ to prevent flow of the refrigerant in the inlet conduit  222  when in cooling mode. Further, the outlet conduit  224  is coupled to the heat pump conduit  114  at a second junction ‘J 6 ’ at the downstream end  117  of the outdoor heat exchanger  110 , such that the heat recovery apparatus  150  is fluidly coupled with the stream of the refrigerant in parallel with the outdoor heat exchanger  110  of the heat pump  102 . Particularly, the inlet end  214  and the outlet end  216  of the one or more tubes  212  of the heat recovery apparatus  150  are fluidly coupled with the upstream end  127  and the downstream end  117 , respectively, of the evaporator  110  of the heat pump  102 . 
     The hybrid heating system  400  further includes an expansion valve  402  disposed on the inlet conduit  222 . The expansion valve  402  may function identical to the first expansion valve  126  as such the outdoor heat exchanger  110  and the heat recovery apparatus  150  are associated with individual expansion valves such as the first expansion valve  126  and the expansion valve  402 . The expansion valve  402  may be designed in such a way to allow flow of the refrigerant only when the refrigerant in the heat recovery apparatus  150  is sufficiently superheated. In an example, the expansion valve  402  may be unidirectional or bidirectional, i.e., allow flow of the refrigerant in one direction or both directions. The hybrid heating system  400  further includes a back pressure valve  404  disposed on the outlet conduit  224 . The back pressure valve  404  is configured to maintain an evaporation pressure in the heat recovery apparatus  150  that is higher than the evaporation pressure at the downstream end  117  of the outdoor heat exchanger  110 . Thereby, backflow of refrigerant from the outdoor heat exchanger  110  to the heat recovery apparatus  150  is prevented during the heating mode. In an embodiment, the back pressure valve  404  may be replaced with a flow check valve, in such a case, the outdoor heat exchanger  110  and the heat recovery apparatus  150  may operate at same pressure. In one embodiment, the back pressure valve  404  may be a spring loaded nonreturn valve. During the cooling mode, the back pressure valve  404  and the expansion valve  402  may isolate the heat recovery apparatus  150  from the heat pump  102  thereby prevent entry of refrigerant into the heat recovery apparatus  150 . Particularly, the control unit  240  may actuate the reversing valve  120  to reverse the flow of the refrigerant. Such that the high-pressure refrigerant discharged from the compressor  116  flow through the reversing valve  120  and reaches to the outdoor heat exchanger  110 . The back pressure valve  404  may be designed in such a way to restrict flow of the refrigerant to the heat recovery apparatus  150  due to the high pressure of the refrigerant exiting the compressor  116  during the cooling mode. 
     Referring to  FIG.  6   , a flow diagram of a method  500  of increasing energy efficiency of the hybrid heating system  100  is illustrated, according to an embodiment of the present disclosure. The method  500  is described with reference to the hybrid heating system  100  illustrated in  FIG.  1   ,  FIG.  2   , and  FIG.  3   , although the method  500  is equally implemented in the hybrid heating systems  300  and  400 . The method  500  may also be described in the general context of computer executable instructions which may be located in both, local and remote computer storage media, including memory storage devices of the control unit  240 . The order in which the method  500  is described is not intended to be construed as a limitation, and any number of the described method steps can be combined in any order to implement the method  500 . Additionally, individual steps may be deleted from the method  500  without departing from the spirit and scope of the present disclosure. 
     At step  502 , the method  500  includes contacting the heat recovery apparatus  150  with the stream of the flue gas discharged from the furnace  104  and the stream of the refrigerant in the heat pump  102 . The heat recovery apparatus  150  includes the shell  202  and the one or more tubes  212  disposed in contact with the shell  202 . The heat recovery apparatus  150  is disposed in fluid contact with the flue gas duct  148  of the furnace  104 . Particularly, the inlet port  208  of the shell  202  is fluidly coupled with the flue gas duct  148  and the outlet port  210  of the shell  202  is positioned in such a way to discharge the flue gas to the atmosphere. The inlet end  214  and the outlet end  216  of the one or more tubes  212  of the heat recovery apparatus  150  are fluid tightly coupled with the downstream end  117  of the outdoor heat exchanger  110  using the inlet conduit  222  and the outlet conduit  224 , respectively. 
     At step  504 , the method  500  includes directing the flue gas to flow through the shell  202  in the first direction ‘D 1 ’. The shell  202  is disposed in fluid communication with the furnace  104  in such a way that the shell  202  allows the flue gas to flow therethrough in the first direction ‘D 1 ’. The inlet port  208  defined at the first end  204  of the shell  202  is fluidly coupled with the flue gas duct  148  of the furnace  104 . The inlet port  208  is fluid tightly coupled with the flue gas duct  148  to receive the flue gas from the flue gas duct  148 . The outlet port  210  is defined at the second end  206  to discharge the flue gas to atmosphere. As such, the flue gas received within the shell  202  flows from the inlet port  208  to the outlet port  210  in the first direction ‘D 1 ’. 
     At step  506 , the method  500  includes directing the stream of the refrigerant to flow through the one or more tubes  212  in the second direction ‘D 2 ’. The one or more tubes  212  are fluidly communicated with the stream of the refrigerant in the heat pump  102  in such a way to allow the stream of the refrigerant to flow therethrough in the second direction ‘D 2 ’. Particularly, the inlet conduit  222  is coupled to the heat pump conduit  114  at the first junction ‘J 1 ’ at the downstream end  117  of the outdoor heat exchanger  110 . The outlet conduit  224  is coupled to the heat pump conduit  114  at the second junction ‘J 2 ’ at the downstream end  117  of the outdoor heat exchanger  110 , such that the heat recovery apparatus  150  is fluidly coupled with the stream of the refrigerant in series with the outdoor heat exchanger  110  of the heat pump  102 . The inlet end  214  and the outlet end  216  are together configured to fluidly couple with the stream of the refrigerant in the heat pump  102  via the inlet conduit  222  and the outlet conduit  224 , respectively, such that the refrigerant flows from the inlet end  214  to the outlet end  216  in the second direction ‘D 2 ’ during the heating mode. 
     The second direction ‘D 2 ’ of the flow of the stream of the refrigerant is different from the first direction ‘D 1 ’ of the flow of the flue gas. In one embodiment, the second direction ‘D 2 ’ of the flow of the stream of the refrigerant is parallel and opposite to the first direction ‘D 1 ’ of the flow of the flue gas. Such counter flow arrangement improves rate of heat transfer between the flue gas and the refrigerant which in turn causes increase in heat transfer efficiency of the heat recovery apparatus  150 . In another embodiment, the second direction ‘D 2 ’ of the flow of the stream of the refrigerant is perpendicular to the first direction ‘D 1 ’ of the flow of the flue gas. In such a case, fins may be disposed perpendicular to the length ‘L’ of the shell  202 . 
     At step  508 , the method  500  includes introducing the stream of the refrigerant to the stream of the refrigerant in the heat pump  102  during the heating mode via the heat recovery apparatus  150 . In the defrosting mode, the heat recovery apparatus  150  may exit superheated refrigerant that may be introduced to the heat pump  102 . During the heating mode, the pattern of operation continues cyclically until the desired heating load is achieved within the closed space  106 . When only the heat pump  102  is in operation during the heating mode, the first isolation valve  230 A and the second isolation valve  230 B are shut down to disconnect the heat recovery apparatus  150  from the heat pump  102 , whereas the third isolation valve  230 C is actuated in the open position to allow flow of the refrigerant therethrough. When the furnace also put in operation, the shell  202  of the heat recovery apparatus  150  receives the flue gas that is discharged from the furnace  104 . As the flue gas flows through the shell  202  from the inlet port  208  to the outlet port  210 , the heat from the flue gas is transferred to the refrigerant flowing through the tubes  212  from the inlet end  214  to the outlet end  216 . Thus, the heat recovery apparatus  150  is configured to recover heat from the flue gas discharged from the furnace  104  and transfer the recovered heat to the stream of the refrigerant in the heat pump  102 . With the series connection of the heat recovery apparatus  150  with the outdoor heat exchanger  110  at the downstream end  117  thereof, the refrigerant exiting the outdoor heat exchanger  110  is further heated by the heat recovery apparatus  150  before reaching the compressor  116 . Thus, the heat recovery apparatus  150  is configured to supply the refrigerant to the stream of the refrigerant in the heat pump  102  during the heating mode of the hybrid heating system  100 . 
     The method  500  further includes regulating the access between the heat recovery apparatus  150  and the heat pump  102  during the heating mode via the control unit  240  and the one or more valves  230  based on the one or more operating parameters of the hybrid heating system  100 . When the control unit  240  determines that the heat pump  102  is not able to meet the desired heating load within the closed space  106  after certain number of cyclic operations, the control unit  240  actuates the furnace  104  to burn the gas fuel to provide additional heat energy within the closed space  106 . As such, the additional heat energy is transferred to the closed space  106  apart from the heat energy provided by the heat pump  102 . When the furnace  104  also put in operation to meet the desired heating load within the closed space  106 , the control unit  240  actuates the first isolation valve  230 A and the second isolation valve  230 B to the open position, whereas the third isolation valve  230 C is kept in the closed position. The control unit  240  in communication with the first isolation valve  230 A, the second isolation valve  230 B and the third isolation valve  230 C regulates the access between the heat recovery apparatus  150  and the heat pump  102  during the heating mode based on the one or more operating parameters of the hybrid heating system  100 . As such, the heat recovery apparatus  150  establishes fluid communication with the heat pump  102  in series with the outdoor heat exchanger  110  at the downstream end  117  thereof to supply the refrigerant that is in two-phase to the stream of the refrigerant in the heat pump  102  during the heating mode of the hybrid heating system  100 . 
     In one embodiment, the method  500  includes fluidly coupling the heat recovery apparatus  150  with the stream of the refrigerant at the downstream end  117  of the evaporator  110  of the heat pump  102  as explained with reference to the hybrid heating system  100  shown in  FIG.  1   . In another embodiment, the method  500  includes fluidly coupling the heat recovery apparatus  150  with the stream of the refrigerant at the upstream end  127  of the evaporator  110  of the heat pump  102  as explained with reference to the hybrid heating system  300  shown in  FIG.  4   . In yet another embodiment, the method  500  includes fluidly coupling the heat recovery apparatus  150  with the stream of the refrigerant in parallel to the evaporator  110  of the heat pump  102  as explained with reference to the hybrid heating system  400  shown in  FIG.  5   . 
     INDUSTRIAL APPLICABILITY 
     The present disclosure relates to the hybrid heating systems  100 ,  300 , and  400  having the heat pump  102  and the furnace  104  fluidly connected to each other. The hybrid heating systems  100 ,  300 , and  400  include the heat recovery apparatus  150  for recovering heat from the flue gas discharged from the furnace  104  and transferring the recovered heat to the stream of the refrigerant in the heat pump  102 . In the hybrid heating systems  100 ,  300 , and  400 , the furnace  104  will start burning the gas fuel when the outside air temperature falls too low and the heat pump  102  alone may not meet the desired heating load within the closed space  106 . The furnace  104  will run at the same time as the heat pump  102  to provide the remaining balance of the desired heating load. During the heating mode and the defrosting mode, the heat recovery apparatus  150  is also operated along with the heat pump  102  to recover the heat escaping from flue gas and exchange the heat energy with the stream of the refrigerant in the heat pump  102 . 
     The heat recovery apparatus  150  of the present disclosure may be implemented with a hybrid heating system having a high efficiency furnace. The high efficiency furnace may be defined as the furnace where 90-95% of the gas fuel is burned to heat the air. The flue gas that is generally exhausted to the atmosphere from the high efficiency furnace has temperature of about 100° F.-120° F. The heat recovery apparatus  150  having one foot length may recover heat for about 130 W when used with the high efficiency furnace. In an example, volume of air flowing through the high efficiency furnace may be considered as 10 CFM for calculation purpose. Further, the heat recovery apparatus  150  may be implemented with a hybrid heating system having a mid-efficiency furnace. The mid-efficiency furnace may be defined as the furnace where 80-90% of the gas fuel is burned to heat the air. The flue gas that is generally exhausted to the atmosphere from the mid-efficiency furnace has temperature of about 275° F. The heat recovery apparatus  150  having one foot length may recover heat for about 650 W when used with the mid-efficiency furnace. In an example, volume of air flowing through the mid-efficiency furnace may be considered as 10 CFM for calculation purpose. If the length ‘L’ of the heat recovery apparatus  150  is increased to more than one foot, then additional heat energy may be recovered by the heat recovery apparatus  150 . 
     The pressure drop calculated to discharge the flue gas through the one-foot shell  202  of the heat recovery apparatus  150  is about 0.05 inch of water column, hence the power consumed by an induced draft fan may be small compared to the heat energy recovered. The induced draft fan may be located upstream or downstream of the flue gas duct  148  to discharge the flue gas from the combustion chamber  140  to the atmosphere. 
     In the hybrid heating system  100 , during the cooling mode, the first isolation valve  230 A and the second isolation valve  230 B may put the heat recovery apparatus  150  in operation with the heat pump  102 , such that the heat recovery apparatus  150  may act as a reservoir to store condensed refrigerant. In some embodiments, the first isolation valve  230 A, the second isolation valve  230 B, or both may be opened at the same time while the third isolation valve  230 C is in open condition. In the hybrid heating systems  100  and  300 , during the cooling mode, the first isolation valve  230 A and the second isolation valve  230 B may isolate the heat recovery apparatus  150  from the heat pump  102  thereby flow of the refrigerant to the heat recovery apparatus  150  is prevented and thus eliminate refrigerant flow pressure loss. 
     In the hybrid heating system  300 , when the outside air temperature falls too low, that means during low load condition on the outdoor heat exchanger  110 , the refrigerant pressure and temperature inside the outdoor heat exchanger  110  may drop as the refrigerant is sucked back to the compressor  116 , which causes water freezing on the outdoor heat exchanger  110 . Particularly, when the refrigerant inside the outdoor heat exchanger  110  is not able to absorb enough heat energy from the outside air flowing over the outdoor heat exchanger  110  (i.e., low load condition) and accompanied by drop in refrigerant pressure as it is sucked by the compressor  116 , the refrigerant temperature inside the outdoor heat exchanger  110  may fall below water freezing temperature, which causes moisture from outside ambient air to freeze or frost on the outdoor heat exchanger  110 . Therefore, the heat pump  102  is operated in the defrosting mode as explained in  FIG.  3    to defrost the outdoor heat exchanger  110 . In the hybrid heating systems  100  and  300 , the heat recovery apparatus  150  will have the energy-gain as the heat recovery apparatus  150  exchange energy with the flue gas. The flue gas load on the heat recovery apparatus  150  may prevent or delay the drop of refrigerant pressure and temperature inside the outdoor heat exchanger  110  and thus help to delay the freezing of the outdoor heat exchanger  110 . 
     While aspects of the present disclosure have been particularly shown and described with reference to the embodiments above, it will be understood by those skilled in the art that various additional embodiments may be contemplated by the modification of the disclosed methods without departing from the spirit and scope of what is disclosed. Such embodiments should be understood to fall within the scope of the present disclosure as determined based upon the claims and any equivalents thereof.