Patent Publication Number: US-2023152003-A1

Title: Integrated hydronic heating and refrigerant cooling heat exchanger

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provision Application No. 63/279,407, filed Nov. 15, 2021, the entirety of which is hereby incorporated by reference. 
    
    
     FIELD OF TECHNOLOGY 
     The disclosed technology relates generally to air handling units used in heating ventilation and air conditioning (HVAC) systems and, more particularly, to heat exchangers used in air handling units. 
     BACKGROUND 
     Air handlers, or air handling units, are commonly used as part of a heating, ventilation, and air conditioning (HVAC) system to regulate and circulate air throughout a building or ventilated space. As illustrated in  FIG.  1 A , existing air handling units  100  commonly include an enclosure  101  having air inlet  102  and an air outlet  104 . The enclosure  101  can house at least an air moving device  106  (e.g., a fan, a blower, etc.) and an evaporator coil  110  (sometimes referred to as a direct expansion (DX) coil). As illustrated in  FIG.  1 B , evaporator coils  110  typically include one or more heat exchanger assemblies  150  having a refrigerant flow path  152  through which refrigerant is circulated. The one or more heat exchanger assemblies  150  of the evaporator coil  110  can be arranged in N-coil configuration (as illustrated in  FIG.  1 B ), an A-coil configuration, a V-coil configuration, or other suitable configurations depending on the application. The evaporator coil  110  is configured to remove heat from air that is passed over the evaporator coil  110  by the air moving device  106  to provide cooled air to the ventilated space. The evaporator coil  110  cools the surrounding air by facilitating heat transfer between the surrounding air and a refrigerant that is passed around the refrigerant flow path  152 . As refrigerant is passed through the evaporator coil  110 , the refrigerant expands as it absorbs heat from the surrounding air which is cooled and directed to the ventilated space. The refrigerant is then circulated to a condenser coil  114  where the refrigerant is condensed and the absorbed heat is released to the surrounding air (generally the ambient air outside of the ventilated space). 
     Some existing air handling units  100  include a heat source such as a resistive heating element or a hydronic heat exchanger  108 . Hydronic heat exchangers  108  are configured to exchange heat between the air circulated through the air handling unit  100  and heated water that is circulated through the hydronic heat exchanger  108 . Unfortunately, existing air handling units  100  that include a hydronic heat exchanger  108  are generally quite large to accommodate the hydronic heat exchanger  108  and the evaporator coil  110 . The large size of the air handling unit  100  limits the number of applications where the air handling unit  100  can be implemented. Furthermore, because air is passed through two heat exchangers (e.g., the hydronic heat exchanger  108  and the evaporator coil  110 ), the air moving device  106  must consume a large amount of energy to overcome the large pressure drop associated with having two heat exchangers. 
     What is needed, therefore, is a heat exchanger for an air handling unit that can reduce the overall size of the air handling unit while also increasing the efficiency of the air handling unit. These and other problems are addressed by the technology disclosed herein. 
     SUMMARY 
     The disclosed technology relates generally to air handling units used in heating ventilation and air conditioning (HVAC) systems and, more particularly, to heat exchangers used in air handling units. 
     The disclosed technology can include a unitary heat exchanger structure configured to be positioned in an airflow path of an HVAC system and comprising a refrigerant heat exchanger coil and a hydronic heat exchanger coil. The refrigerant heat exchanger coil can include a refrigerant flow path configured to (i) receive a refrigerant circulated through the refrigerant heat exchanger coil and (ii) facilitate heat exchange between the refrigerant and air directed across the refrigerant heat exchanger coil. The hydronic heat exchanger coil can include a water flow path configured to (a) receive water circulated through the hydronic heat exchanger coil and (b) facilitate heat exchange between the water and air directed across the hydronic heat exchanger coil. 
     The refrigerant heat exchanger coil can include a first tube having a first cross-sectional area and the hydronic heat exchanger coil can include a second tube having a second cross-sectional area. The second cross-sectional area can be greater than the first cross-sectional area. 
     The refrigerant heat exchanger coil can be in a downstream position relative the hydronic heat exchanger coil when the unitary heat exchanger structure is installed in the HVAC system. Alternatively, the hydronic heat exchanger coil can be in a downstream position relative the refrigerant heat exchanger coil when the unitary heat exchanger structure is installed in the HVAC system. The hydronic heat exchanger coil can be configured to reheat the air passed through the refrigerant heat exchanger coil. 
     The unitary heat exchanger structure can be a microchannel heat exchanger. The refrigerant heat exchanger coil and the hydronic heat exchanger coil can be interlaced. The refrigerant heat exchanger coil can include a first flat tube having a first cross-sectional area and the hydronic heat exchanger coil can include a second flat tube having a second cross-sectional area. The second cross-sectional area can be greater than the first cross-sectional area. The hydronic heat exchanger coil can include a turbulator. 
     The heat exchanger can include a plurality of fins connecting the refrigerant heat exchanger coil and the hydronic heat exchanger coil. The plurality of fins can be configured to facilitate heat exchange between the refrigerant and air and the water and air. 
     The disclosed technology can include an air handling unit for a heating, ventilation, and air conditioning (HVAC) system. The air handling unit can include a housing, an air inlet, an air outlet, and an air moving device configured to move air from the air inlet, through the housing, and out the air outlet. The air handling unit can include a unitary heat exchanger structure disposed at last partially within the housing. 
     The unitary heat exchanger structure can include a first heat exchanger coil and a second heat exchanger coil. The first heat exchanger coil can be configured to receive refrigerant and facilitate heat exchange between the refrigerant and air directed across the first heat exchanger coil by the air moving device. The second heat exchanger coil can be configured to receive water and facilitate heat exchange between the water and air directed across the second heat exchanger coil by the air moving device. 
     The first heat exchanger coil can include a first tube having a first cross-sectional area and the second heat exchanger coil can include a second tube having a second cross-sectional area. The second cross-sectional area can be greater than the first cross-sectional area. 
     The first heat exchanger coil can be positioned in an airflow path downstream of the second heat exchanger. Alternatively, the second heat exchanger coil can be positioned in an airflow path downstream of the first heat exchanger. The second heat exchanger can be configured to reheat the air passed through the first heat exchanger coil. 
     The unitary heat exchanger structure can be a microchannel heat exchanger. The first heat exchanger coil can include a first flat tube having a first cross-sectional area and the second heat exchanger coil can include a second flat tube having a second cross-sectional area. The second cross-sectional area can be greater than the first cross-sectional area. The second heat exchanger coil can include a turbulator. The first heat exchanger coil and the second heat exchanger coil can be interlaced. 
     The disclosed technology can include an HVAC system having an air handling unit. The air handling unit can include a housing, an air inlet, an air outlet, and an air moving device configured to move air from the air inlet, through the housing, and out the air outlet. The air handling unit can include a unitary heat exchanger structure disposed at last partially within the housing. The unitary heat exchanger structure can include a first heat exchanger coil and a second heat exchanger coil. The first heat exchanger coil can be configured to receive refrigerant and facilitate heat exchange between the refrigerant and air directed across the first heat exchanger coil by the air moving device. The second heat exchanger coil can be configured to receive water and facilitate heat exchange between the water and air directed across the second heat exchanger coil by the air moving device. The HVAC system can include a tankless water heater configured to supply heated water to the second heat exchanger coil. 
     Additional features, functionalities, and applications of the disclosed technology are discussed herein in more detail. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various aspects of the presently disclosed subject matter and serve to explain the principles of the presently disclosed subject matter. The drawings are not intended to limit the scope of the presently disclosed subject matter in any manner. 
         FIG.  1 A  illustrates a schematic diagram of an existing air handling unit of an HVAC system. 
         FIG.  1 B  illustrates a perspective view of an existing evaporator coil. 
         FIG.  2    illustrates a schematic diagram of an air handling unit of an HVAC system, in accordance with the disclosed technology. 
         FIG.  3    illustrates a schematic diagram of another air handling unit of an HVAC system, in accordance with the disclosed technology. 
         FIG.  4    illustrates a schematic diagram of a side view of a heat exchanger having a water heat exchanger coil and a refrigerant heat exchanger coil, in accordance with the disclosed technology. 
         FIG.  5    illustrates a schematic diagram of a side view of another heat exchanger having a water heat exchanger coil and a refrigerant heat exchanger coil, in accordance with the disclosed technology. 
         FIG.  6    illustrates a schematic diagram of a microchannel heat exchanger having a water heat exchanger coil and a refrigerant heat exchanger coil, in accordance with the disclosed technology. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosed technology includes heat exchangers for air handling units used in HVAC systems. In particular, the disclosed technology includes heat exchangers having an evaporator coil integrated with a hydronic heat exchanger coil. In other words, the evaporator coil and the hydronic heat exchanger coil can form a unitary heat exchanger structure. As will become apparent throughout this disclosure, by integrating the evaporator coil with the hydronic heat exchanger, the overall size of the air handling unit can be reduced, enabling the air handling unit to be implemented in a wider variety of applications. Furthermore, the overall efficiency of the air handling unit can be increased because the pressure drop through the air handling unit is reduced by having only a single integrated, or unitary, heat exchanger unit. As will be described in greater detail herein, the hydronic heat exchanger can be used in place of comparatively inefficient resistive heating elements to reheat the air passed through the evaporator coil which may be at low temperature to help remove humidity from the air supplied to the ventilated space. Further configurations and advantages of the disclosed technology will become apparent throughout this disclosure. 
     Although various aspects of the disclosed technology are explained in detail herein, it is to be understood that other aspects of the disclosed technology are contemplated. Accordingly, it is not intended that the disclosed technology is limited in its scope to the details of construction and arrangement of components expressly set forth in the following description or illustrated in the drawings. The disclosed technology can be implemented and practiced or carried out in various ways. In particular, the presently disclosed subject matter is described in the context of being a heat exchanger for an air handler of an HVAC system. The present disclosure, however, is not so limited, and can be applicable in other contexts. The present disclosure, for example, can be used in process controls (e.g., for controlling a temperature of any fluid), in residential, commercial, and/or industrial refrigeration systems (including cryogenic refrigeration), and any residential, commercial, and/or industrial HVAC system (including air conditioning systems, heat pump systems, air handling systems, etc.). Accordingly, when the present disclosure is described in the context of a heat exchanger for an air handler of an HVAC system, it will be understood that other implementations can take the place of those referred to. 
     It should also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. References to a composition containing “a” constituent is intended to include other constituents in addition to the one named. 
     Also, in describing the disclosed technology, terminology will be resorted to for the sake of clarity. It is intended that each term contemplates its broadest meaning as understood by those skilled in the art and includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
     Ranges may be expressed herein as from “about” or “approximately” or “substantially” one particular value and/or to “about” or “approximately” or “substantially” another particular value. When such a range is expressed, the disclosed technology can include from the one particular value and/or to the other particular value. Further, ranges described as being between a first value and a second value are inclusive of the first and second values. Likewise, ranges described as being from a first value and to a second value are inclusive of the first and second values. 
     It is also to be understood that the mention of one or more method steps does not preclude the presence of additional method steps or intervening method steps between those steps expressly identified. Moreover, although the term “step” can be used herein to connote different aspects of methods employed, the term should not be interpreted as implying any particular order among or between various steps herein disclosed unless and except when the order of individual steps is explicitly required. Further, the disclosed technology does not necessarily require all steps included in the methods and processes described herein. That is, the disclosed technology includes methods that omit one or more steps expressly discussed with respect to the methods described herein. 
     Herein, the use of terms such as “having,” “has,” “including,” or “includes” are open-ended and are intended to have the same meaning as terms such as “comprising” or “comprises” and not preclude the presence of other structure, material, or acts. Similarly, though the use of terms such as “can” or “may” are intended to be open-ended and to reflect that structure, material, or acts are not necessary, the failure to use such terms is not intended to reflect that structure, material, or acts are essential. To the extent that structure, material, or acts are presently considered to be essential, they are identified as such. 
     The components described hereinafter as making up various elements of the disclosed technology are intended to be illustrative and not restrictive. Many suitable components that would perform the same or similar functions as the components described herein are intended to be embraced within the scope of the disclosed technology. Such other components not described herein can include, but are not limited to, similar components that are developed after development of the presently disclosed subject matter. 
     Referring now to the drawings, in which like numerals represent like elements, the present disclosure is herein described.  FIG.  2    illustrates a schematic diagram of an air handling unit  200  of an HVAC system, in accordance with the disclosed technology. The air handling unit  200  can include an enclosure  201  having air inlet  202  and an air outlet  204 . The enclosure  201  can house at least an air moving device  206  (e.g., a fan, a blower, etc.) and a unitary heat exchanger structure  210 . As will be appreciated, the term “unitary” as used throughout this disclosure can mean forming a single or uniform entity or structure. As will be described in greater detail herein, the unitary heat exchanger structure  210  can include a refrigerant heat exchanger coil  210 A and a water heat exchanger coil  210 B (e.g., a hydronic heating coil) that are integrated together into a single, uniform heat exchanger assembly. In other words, the refrigerant heat exchanger coil  210 A and the water heat exchanger coil  210 B are united to form a single unit (i.e., the unitary heat exchanger structure  210 ). The unitary heat exchanger structure  210  (e.g., the refrigerant heat exchanger coil  210 A and the water heat exchanger coil  210 B) can be configured to exchange heat with the refrigerant and air and/or the water and air. The air moving device  206  can direct air through the air inlet  202 , across the unitary heat exchanger structure  210 , and out the air outlet  204  to provide cooled or heated air to the ventilated space. 
     By including a unitary heat exchanger structure  210  that comprises a refrigerant heat exchanger coil  210 A that is integrated with the water heat exchanger coil  210 B, the air handling unit  200  can be configured to provide both heating and cooling to the ventilated space while remaining a comparatively compact design when compared to existing air handling units. Furthermore, because the refrigerant heat exchanger coil  210 A and water heat exchanger coil  210 B are integrated together, the air handling unit  200  can be configured to operate more efficiently than existing air handling units because the air moving device  206  must pass the air through only the unitary heat exchanger structure  210  rather than two separate heat exchanger coils, reducing the pressure drop through the air handling unit  200 . The air handling unit  200  can also provide efficient reheat of air supplied to the ventilated space by using the water heat exchanger coil  210 B to reheat air cooled by the refrigerant heat exchanger coil  210 A rather than inefficient resistive heating elements. Further advantages and features of the disclosed technology will become apparent throughout this disclosure. 
     The air moving device  206  can be any type of air moving device that is configured to move air through the air handling unit  200 . For example, the air moving device  206  can be a draft inducer, a fan, a blower, or any other air moving device configured to move air through the system. The unitary heat exchanger structure  210  can be any type of heat exchanger that can facilitate heat exchange between refrigerant and air and/or water and air. The unitary heat exchanger structure  210 , for example, can be an A-coil, an N-coil, a Z-coil, a slab coil, a cased coil, an uncased coil, a microchannel coil, or any other suitable type of heat exchanger for the application. Furthermore, the unitary heat exchanger structure  210  can be made of any suitable material for the application. For example, the unitary heat exchanger structure  210  can be made of aluminum, copper, titanium, stainless steel, cupronickel, carbon steel, composite materials, or other suitable materials. Although illustrated as being positioned downstream of the air moving device  206 , the unitary heat exchanger structure  210  can be positioned upstream of the air moving device  206  without departing from the scope of this disclosure. 
     The refrigerant heat exchanger coil  210 A can be in fluid communication with an outdoor coil  214 . The outdoor coil can be configured to receive from, or provide heat to, refrigerant that is passed through the outdoor coil  214 . Although described as being an outdoor coil  214 , one of skill in the art will appreciate that the outdoor coil  214  can be located in any suitable location to facilitate heat transfer between the refrigerant and air or another fluid. For example, the outdoor coil  214  can be located outside of a building, inside of a building (e.g., an attic, a garage, etc.), under the ground (e.g., a ground source heat pump), or in any other suitable location for the application. Furthermore, although described herein as exchanging heat between the refrigerant and ambient air, the outdoor coil  214  can be configured to exchange heat between the refrigerant and any suitable fluid (e.g., water, glycol, etc.). 
     The refrigerant heat exchanger coil  210 A can be configured to function as an evaporator or a condenser depending on the particular application. In other words, the refrigerant heat exchanger coil  210 A can be part of an air conditioning system and be configured to perform a cooling function. Alternatively, the refrigerant heat exchanger coil  210 A can be part of a heat pump system and be configured to perform both a cooling and a heating function. For example, if the temperature of the air within the ventilated space is greater than a predetermined high temperature, the refrigerant heat exchanger coil  210 A can be configured to function as an evaporator to absorb heat from the air passed across the unitary heat exchanger structure  210 , providing cooled air to the ventilated space. On the other hand, if the temperature of the air within the ventilated space is less than a predetermined low temperature, the refrigerant heat exchanger coil  210 A can be configured to function as a condenser and provide heat to the air passed across the unitary heat exchanger structure  210 , providing heated air to the ventilated space. The predetermined high temperature can be the same temperature as, or a greater temperature than, the predetermined low temperature. 
     The water heat exchanger coil  210 B can be in fluid communication with a water heater  212  that can be configured to heat water and circulate the heated water through the water heat exchanger coil  210 B. The water heater  212  can be any suitable type of water heater for the application. For example, the water heater  212  can be a tanked or tankless water heater and can be a gas fired, electric heating element, heat pump, or other suitable type of water heater. Furthermore, although described herein as being a water heater, the water heater  212  can be configured to heat fluids other than water. For example, the water heater can be configured to heat organic- or inorganic-based fluids such as glycol, refrigerant, hydrocarbon oils, silicones, or other suitable fluids. Thus, when described herein as being a water heater, one of skill in the art will appreciate that the water heater  212  can be configured to heat fluids other than water. 
     Because the water heat exchanger coil  210 B is in fluid communication with the water heater  212 , the water heat exchanger coil  210 B can transfer heat from heated water received from the water heater  212  to air that is passed across the water heat exchanger coil  210 B. In this way, the water heat exchanger coil  212 B can facilitate hydronic heating to the ventilated space. The air handling unit  200  can be further configured to provide heat to air passed over the unitary heat exchanger structure  210  via the refrigerant heat exchanger coil  210 A. In other words, the air handling unit  200  can provide heat to a ventilated space using both the refrigerant and the heated water. For example, the air handling unit  200  can be configured to provide heated air to the ventilated space using just the water heat exchanger coil  210 B. If, however, the water heat exchanger coil  210 B is unable to sufficiently heat the air circulated to the ventilated space, the refrigerant heat exchanger coil  210 A can be utilized to provide supplementary heating, and vice-versa. 
     The water heat exchanger coil  210 B can also be configured to function as a chiller by receiving and circulating cooled water through the water heat exchanger  210 B. Therefore, although described throughout this disclosure as receiving heated water from the water heater  212 , one of skill in the art will appreciate that the water heat exchanger coil  210 B can also receive cooled water from a water source (e.g., a refrigerant system, a spring, a municipal water system, etc.) and circulate the cooled water through the water heat exchanger coil  210 B to cool the air passed over the water heat exchanger coil  210 B and provide cooled air to the ventilated space. 
       FIG.  3    illustrates a schematic diagram of another air handling unit  300  of an HVAC system, in accordance with the disclosed technology. The air handling unit  300  can include all of the same components and features as the air handling unit  200  and can additionally include a resistive heating element  320 . The resistive heating element  320  can be configured to provide heat to the air passed through the air handling unit  300 . The resistive heating element  320  can be configured to heat the air provided to the ventilated space alone or in combination with the refrigerant heat exchanger coil  210 A and/or the water heat exchanger coil  210 B. For example, if the air handling unit  300  is unable to provide heat to the ventilated space via the refrigerant heat exchanger coil  210 A or the water heat exchanger coil  210 B, the resistive heating element  320  can be activated alone to heat the air provided to the ventilated space. As will be appreciated by one of skill in the art, however, resistive heating elements  320  are generally less energy efficient to operate than a heat pump or a hydronic heating system. Therefore, the air handling unit  300  can be configured in some situations to activate the resistive heating element  320  as a supplemental heat source or as an emergency heat source. For example, if the air handling unit  300  is providing heated air via the refrigerant heat exchanger coil  210 A and/or the water heat exchanger coil  210 B, but the air handling unit  300  is unable to sufficiently heat the ventilated space, the air handling unit  300  can be configured to provide additional heat to the air supplied to the ventilated space via the resistive heating element  320 . 
       FIG.  4    illustrates a schematic diagram of a side view of a heat exchanger  400  having a refrigerant heat exchanger coil  440  and a water heat exchanger coil  430 , in accordance with the disclosed technology. The heat exchanger  400  can be representative of the unitary heat exchanger structure  210  described in relation to  FIGS.  2  and  3   . As illustrated in  FIG.  4   , the refrigerant heat exchanger coil  440  (e.g., refrigerant heat exchanger coil  210 A) and the water heat exchanger coil  430  (e.g., water heat exchanger coil  210 B) can be integrated together such that the refrigerant heat exchanger coil  440  and the water heat exchanger coil  430  comprise a single, unitary heat exchanger unit. The heat exchanger  400  is shown as being in an A-coil configuration for illustrative purposes. The heat exchanger  400 , however, can be an N-coil, a Z-coil, a slab coil, a cased coil, an uncased coil, a microchannel coil, or any other suitable type of heat exchanger for the application without departing from the scope of this disclosure. 
     The water heat exchanger coil  430  can include a water inlet  432 , a water flow path  434 , a water connection line  436 , and a water outlet  438 . The water heat exchanger coil  430  can be configured to receive heated water from a water heater (e.g., from the water heater  212 ) via the water inlet  432 , pass the heated water through the water heat exchanger coil  430  via the water flow path  434 , and then pass the water out of the water heat exchanger coil  430  via the water outlet  438 . The water can enter the water heat exchanger coil  430  at a relatively high temperature (e.g., 150° F.), release heat to the air passed over the water heat exchanger coil  430  while passing through the water flow path  434 , and then exit the water heat exchanger coil  430  at a comparatively lower temperature (e.g., 80° F.). In this way, the water heat exchanger coil  430  can facilitate heat transfer from the heated water to the air passed over the water heat exchanger coil  430  to provide heated air to the ventilated space. 
     As will be appreciated by one of skill in the art, the illustration of the heat exchanger  400  in  FIG.  4    is a side view of the heat exchanger  400 . In the view depicted in  FIG.  4   , the water flow path  434  can be configured to pass across the water heat exchanger coil  430  such that the water flow path  434  would loop back and forth in a direction in and out of the page. Thus, the portions of the water flow path  434  visible in  FIG.  4    are the end loops where the water flow path  434  loops back into the page to form the water flow path  434  through the water heat exchanger coil  430 . In other words, although not shown in  FIG.  4   , the water flow path  434  can be configured to coil back and forth along the water heat exchanger coil  430  such that the water flow path  434  provides a sufficient surface area for the heated water to facilitate heat exchange between the air passed over the water heat exchanger coil  430  and the heated water passed through the water heat exchanger coil  430 . Furthermore, the water connection line  436  can be configured to connect the water flow path  434  on one side of the water heat exchanger coil  430  with the water flow path on the other side of the water heat exchanger coil  430 . 
     The refrigerant heat exchanger coil  440  can include a refrigerant inlet  442 , a distributor valve  444 , one or more refrigerant flow paths  446 , and one or more refrigerant outlets  448 . The refrigerant heat exchanger coil  440  can be configured to receive refrigerant from a connected refrigerant coil (e.g., outdoor coil  214 ) via the refrigerant inlet  442 , pass the refrigerant through the refrigerant heat exchanger coil  440  via the refrigerant flow path  446 , and then pass the refrigerant out of the refrigerant heat exchanger coil  440  via the refrigerant outlet  448 . As described previously, the refrigerant heat exchanger coil  440  can be configured to act as an evaporator or a condenser depending on the application. For example, if the refrigerant heat exchanger coil  440  is configured to act as an evaporator, the refrigerant heat exchanger coil  440  can be configured to absorb heat from the air passed over the refrigerant heat exchanger coil  440  to provide cooled air to the ventilated space. On the other hand, if the refrigerant heat exchanger coil  440  is configured to act as a condenser, the refrigerant heat exchanger coil  440  can be configured to release heat to the air passed over the refrigerant heat exchanger coil  440  to provide heated air to the ventilated space. As will be appreciated by one of skill in the art, whether the refrigerant heat exchanger coil  440  functions as an evaporator or a condenser can depend on the particular application and the particular mode that the air handling unit  200  is currently operating in. 
     As will be appreciated by one of skill in the art, the illustration of the heat exchanger  400  in  FIG.  4    is a side view of the heat exchanger  400 . In the view depicted in  FIG.  4   , the refrigerant flow path  446  can be configured to pass across the refrigerant heat exchanger coil  440  such that the refrigerant flow path  446  would loop back and forth in a direction in and out of the page. Thus, the portions of the refrigerant flow path  446  visible in  FIG.  4    are the end loops where the refrigerant flow path  446  loops back into the page to form the refrigerant flow path  446  through the refrigerant heat exchanger coil  440 . In other words, although not shown in  FIG.  4   , the refrigerant flow path  446  can be configured to coil back and forth along the refrigerant heat exchanger coil  440  such that the refrigerant flow path  446  provides a sufficient surface area for the refrigerant to facilitate heat exchange between the air passed over the refrigerant heat exchanger coil  440  and the refrigerant passed through the refrigerant heat exchanger coil  440 . Furthermore, the distributor valve  444  can be configured to distribute refrigerant received through the refrigerant inlet to the one or more refrigerant flow paths  446 . The refrigerant heat exchanger coil  440  is depicted as having four refrigerant flow paths  446  and four refrigerant outlets  448 , however, one of skill in the art will appreciate that the refrigerant heat exchanger coil  440  can include more or fewer refrigerant flow paths  446  and more or fewer refrigerant outlets  448  depending on the application. 
     The water heat exchanger coil  430  can be installed in an airflow path upstream of the refrigerant heat exchanger coil  440 . As will be appreciated by one of skill in the art, if the water heat exchanger coil  430  is installed upstream of the refrigerant heat exchanger coil  440 , the air handling unit  200  can be configured to operate the water heat exchanger coil  430  only when the refrigerant heat exchanger coil  440  is not in a cooling mode to avoid unnecessarily heating the air before the air is directed across the refrigerant heat exchanger coil  440 , reducing the efficiency of the air handling unit  200 . On the other hand, if the refrigerant heat exchanger coil  440  is in a cooling mode, the water heat exchanger coil  430  can be configured to receive and circulate cooled water to provide further cooling to the air passed over the heat exchanger  400 . For example, if the refrigerant heat exchanger coil  440  is unable to sufficiently cool the air supplied to the ventilated space, cooled water can be circulated through the water heat exchanger  430  to provide supplemental cooling. Similarly, if the refrigerant heat exchanger coil  440  is in a heating mode, the water heat exchanger coil  430  and the refrigerant heat exchanger coil  440  can operate simultaneously if both heat sources are needed to sufficiently heat the ventilated space. If only the water heat exchanger coil  430  is needed to sufficiently heat the ventilated space, the refrigerant heat exchanger coil  440  can be in a standby mode and the refrigerant will not be circulated through the refrigerant heat exchanger coil  440  to avoid unnecessarily cooling the air heated by the water heat exchanger coil  430 , reducing the efficiency of the air handling unit  200 . As another example, if the water heat exchanger  430  is operating in a cooling mode, the refrigerant heat exchanger coil  440  can be operated in a heating mode to provide reheating to the air circulated to the ventilated space when the refrigerant heat exchanger coil  440  is being operated at a low temperature to help reduce the humidity of the air circulated to the ventilated space. 
     The water flow path  434  can be a tube, hose, pipe, or other similar flow path for the water to flow through while the refrigerant flow path  446  can also be a tube, hose, pipe, or other similar flow path for the refrigerant to flow through. The water flow path  434  and the refrigerant flow path  446  can both be formed to have a cross-sectional shape forming a circle, a square, a rectangle, an oval, or any other suitable shape. In some applications, the water flow path  434  and the refrigerant flow path  446  can have a relatively flat cross-sectional shape. Furthermore, the water flow path  434  and the refrigerant flow path  446  can both have the same cross-sectional area or comprise different cross-section areas. For example, as will be appreciated by one of skill in the art, because the refrigerant can be compressed while circulated through the refrigerant heat exchanger coil  440 , the water flow path  434  can have a cross-sectional area that is greater than the refrigerant flow path  446 . 
     The heat exchanger  400  can further include one or more fins (not shown for clarity of illustration) that can be attached to, and in thermal communication with, the water flow path  434  and the refrigerant flow path  446 . As will be appreciated, the fins can be configured to help direct the air across the water flow path  434  and the refrigerant flow path  446  and help to facilitate heat transfer between the refrigerant and air and/or the water and air. The fins can be connected to both the water flow path  434  and the refrigerant flow path  446 . Alternatively, or in addition, a first set of fins can be connected to just the water flow path  434  and a second set of fins can be connected to just the refrigerant flow path  446  to help reduce the amount of heat that is transferred between the water flow path  434  and the refrigerant flow path  446 . 
       FIG.  5    illustrates a schematic diagram of a side view of another heat exchanger  500  having a water heat exchanger coil  530  and a refrigerant heat exchanger coil  540 , in accordance with the disclosed technology. The heat exchanger  500  can include all of the same features described in relation to the heat exchanger  400 . Namely, the heat exchanger  500  can include a water heat exchanger coil  530  having a water inlet  532 , a water flow path  534 , a water connection line  536 , and a water outlet  538  as well as a refrigerant heat exchanger coil  540  having a refrigerant inlet  542 , a distributor valve  544 , one or more refrigerant flow paths  546 , and one or more refrigerant outlets  548 . Furthermore, the water heat exchanger coil  530  and the refrigerant heat exchanger coil  540  can each be configured to perform the same functions previously described herein. 
     The heat exchanger  500 , however, differs from the heat exchanger  400  in that the water heat exchanger coil  530  is positioned in an airflow path downstream of the refrigerant heat exchanger coil  540 . By positioning the water heat exchanger coil  530  in an airflow path downstream of the refrigerant heat exchanger coil  540 , the water heat exchanger coil  530  can be configured to perform a reheat function to the air that is passed over the refrigerant heat exchanger coil  540 . As will be appreciated by one of skill in the art, existing air handling units often include a resistive heating element positioned downstream of the evaporator coil to reheat the cooled air when the evaporator coil is being operated at a low temperature to remove humidity from the air circulated to the ventilated space. By reheating the air, the resistive heating element helps provide air at a comfortable temperature to the occupants of the ventilated space. Unfortunately, these resistive heating elements consume energy in an inefficient manner and can be expensive to operate. Thus, by positioning the water heat exchanger coil  530  in the air flow path downstream of the refrigerant heat exchanger coil  540 , the air handling unit can provide a reheat function without needing to include expensive and inefficient resistive heating elements. 
       FIG.  6    illustrates a schematic diagram of a microchannel heat exchanger  600  having a water heat exchanger coil  630  and a refrigerant heat exchanger coil  640 , in accordance with the disclosed technology. As will be appreciated by one of skill in the art, the term “microchannel heat exchanger” as used herein can refer to variations of heat exchangers having fluid channels of relatively small cross-sectional areas. For example and not limitation, a microchannel heat exchanger can have fluid channels (e.g., tubes) having a hydraulic diameter of less than one millimeter. Furthermore, the term “microchannel heat exchanger” can refer to a variation of fin and tube heat exchanger but is not necessarily limited to just variations of fin and tube heat exchangers. 
     As illustrated in  FIG.  6   , the microchannel heat exchanger  600 , similar to the heat exchanger  400  and the heat exchanger  500 , can be configured to facilitate heat exchange between air and water via the water heat exchanger coil  630  and between refrigerant and air via the refrigerant heat exchanger coil  640 . The water heat exchanger coil  630  can include a water inlet  632  to receive heated water from a water heater (e.g., the water heater  212 ), a water flow path  634 , and a water outlet  638  to permit the heated water to exit the water heat exchanger coil  630  and return to the water heater  212 . Similarly, the refrigerant heat exchanger coil  640  can include a refrigerant inlet  642  to receive refrigerant from a connected heat exchanger coil (e.g., the outdoor coil  214 ), a refrigerant flow path  644 , and a refrigerant outlet  648  to permit the refrigerant to exit the refrigerant heat exchanger coil  640  and return to the outdoor coil  614 . As will be appreciated, the water heat exchanger coil  630  and the refrigerant heat exchanger coil  640  can each be configured to pass multiple times through the microchannel heat exchanger  600  to facilitate heat transfer between the water and air and the refrigerant and air. 
     The microchannel heat exchanger  600  can include fins  652  that can help to direct the air around the water heat exchanger coil  630  and the refrigerant heat exchanger coil  640  to facilitate heat exchange between the water and air and the refrigerant and air. The fins  652  can be offset to further help facilitate heat transfer. Furthermore, although shown as having the water heat exchanger coil  630  and the refrigerant heat exchanger coil  640  beside each other, the fins  652  can be positioned between the water heat exchanger coil  630  and the refrigerant heat exchanger coil  640  to separate the water heat exchanger coil  630  from the refrigerant heat exchanger coil  640  a predetermined distance. 
     The water flow path  634  and the refrigerant flow path  644  can both be a tube, hose, pipe, or other similar flow path having a generally flat cross-sectional area. In this way, the microchannel heat exchanger  600  can be configured to be in a compact arrangement. The cross-sectional area of the water flow path  634  can be greater than the cross-sectional area of the refrigerant flow path  644 . The water flow path  634  and the refrigerant flow path  644  can also be interlaced with each other to further reduce the amount of space the microchannel heat exchanger  600  requires (e.g., the water flow path  634  and the refrigerant flow path  644  are crossed or overlapped at least one position along the length of the respective flow paths). As will be appreciated by one of skill in the art, by interlacing the water flow path  634  and the refrigerant flow path  644 , the two independent fluid flow paths can utilize a shared finned surface area. Furthermore, the water flow path  634  and/or the refrigerant flow path  644  can include a turbulator (not shown) to cause turbulence to the flow of the water and/or refrigerant circulated through the water flow path  634  and/or the refrigerant flow path  644 . In this way, the microchannel heat exchanger  600  can be configured to better facilitate heat exchange between the water and air and the refrigerant and air. 
     While the present disclosure has been described in connection with a plurality of exemplary aspects, as illustrated in the various figures and discussed above, it is understood that other similar aspects can be used, or modifications and additions can be made to the described subject matter for performing the same function of the present disclosure without deviating therefrom. In this disclosure, methods and compositions were described according to aspects of the presently disclosed subject matter. But other equivalent methods or compositions to these described aspects are also contemplated by the teachings herein. Therefore, the present disclosure should not be limited to any single aspect, but rather construed in breadth and scope in accordance with the appended claims.