Patent Publication Number: US-2023160605-A1

Title: Top fired outdoor gas heat exchanger

Description:
BACKGROUND 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure and are described below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be noted that these statements are to be read in this light, and not as admissions of prior art. 
     Heating, ventilation, and air conditioning (HVAC) systems are utilized to control environmental properties, such as temperature and humidity, for occupants of residential, commercial, and industrial environments. The HVAC systems may control the environmental properties through control of an air flow delivered to the environment. For example, an HVAC system may include several heat exchangers, such as a heat exchanger configured to place an air flow in a heat exchange relationship with a refrigerant of a vapor compression circuit (e.g., evaporator, condenser), a heat exchanger configured to place an air flow in a heat exchange relationship with combustion products (e.g., a furnace), or both. In general, the heat exchange relationship(s) may cause a change in pressures and/or temperatures of the air, the refrigerant, the combustion products, or any combination thereof. As the temperatures and/or pressures of the above-described fluids change, liquid condensate may be formed in or on the associated heat exchangers. 
     SUMMARY 
     A summary of certain embodiments disclosed herein is set forth below. It should be noted that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
     In an embodiment, a furnace for a heating, ventilation, and air conditioning (HVAC) unit includes a heat exchange tube configured to flow combustion products therethrough and place the combustion products in a heat exchange relationship with an air flow directed across the heat exchange tube. The furnace also includes a burner assembly fluidly coupled to a first port of the heat exchange tube and configured to generate the combustion products directed into the heat exchange tube via the first port, and a draft inducer blower fluidly coupled to a second port of the heat exchange tube and configured to draw the combustion products through the heat exchange tube. The burner assembly is higher in position than the draft inducer blower relative to a base of the HVAC unit. 
     In another embodiment, a furnace for a heating, ventilation, and air conditioning (HVAC) system includes a panel comprising an inlet and an outlet, and a heat exchange tube fluidly coupled to the inlet and to the outlet on a first side of the panel. The heat exchange tube is configured to direct combustion products from the inlet to the outlet and place the combustion products in a heat exchange relationship with an air flow directed across the heat exchange tube along an air flow path through the furnace. The furnace also includes a burner assembly coupled to a second side of the panel at a first position along a vertical axis, and a draft inducer blower coupled to the second side of the panel at a second position along the vertical axis. The first position is above the second position along the vertical axis. The burner assembly is configured to generate the combustion products and direct the combustion products into the heat exchange tube via the inlet, the draft inducer blower is configured to draw the combustion products through the heat exchange tube towards the outlet. 
     In another embodiment, a furnace for a heating, ventilation, and air conditioning (HVAC) system includes a heat exchange tube having a first port configured to receive combustion products and a second port configured to discharge the combustion products. The heat exchange tube is configured to direct the combustion products from the first port to the second port. The furnace also includes a burner assembly fluidly coupled to the first port, and a draft inducer blower fluidly coupled to the second port. The burner assembly is configured to generate the combustion products and direct the combustion products into the heat exchange tube via the first port, and the draft inducer blower is configured to draw the combustion products through the heat exchange tube and remove the combustion products from the heat exchange tube via the second port. The first port is above the second port relative to gravity, and the heat exchange tube is configured to discharge liquid condensate from the heat exchange tube via the second port. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
         FIG.  1    is a perspective view of a building having an embodiment of a heating, ventilation, and air conditioning (HVAC) system for environmental management that may employ one or more HVAC units, in accordance with an aspect of the present disclosure; 
         FIG.  2    is a perspective view of an embodiment of a packaged HVAC unit that may be used in the HVAC system of  FIG.  1   , in accordance with an aspect of the present disclosure; 
         FIG.  3    is a cutaway perspective view of an embodiment of a residential, split HVAC system, in accordance with an aspect of the present disclosure; 
         FIG.  4    is a schematic illustration of an embodiment of a vapor compression system that can be used in any of the systems of  FIGS.  1 - 3   , in accordance with an aspect of the present disclosure; 
         FIG.  5    is a perspective view of an embodiment of an HVAC unit, in accordance with an aspect of the present disclosure; 
         FIG.  6    is a side view of an embodiment of a furnace, in accordance with an aspect of the present disclosure; 
         FIG.  7    is a schematic side view of an embodiment of a furnace, illustrating flow of liquid condensate within the furnace, in accordance with an aspect of the present disclosure; 
         FIG.  8    is an schematic side view of an embodiment of a draft inducer of a furnace, in accordance with an aspect of the present disclosure; and 
         FIG.  9    is a front perspective view of an embodiment of a furnace, in accordance with an aspect of the present disclosure. 
         FIG.  10    is a front perspective view of an embodiment of a furnace, in accordance with an aspect of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be noted that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be noted that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be noted that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     The present disclosure is directed to a heat exchanger for heating, ventilation, and air conditioning (HVAC) systems configured to increase the temperature of an air flow directed through the HVAC system. In some embodiments, the heat exchanger (e.g., furnace) may be disposed in a packaged outdoor unit or a rooftop unit configured to both heat and cool an air flow, such as a supply air flow that is conditioned and directed to a conditioned space (e.g., a building). For example, the furnace may include a heat exchanger having tubes that is configured to receive relatively hot combustion products (e.g., flue gas) generated via a burner assembly. The furnace may also include a draft inducer (e.g., draft inducer blower) configured to circulate the combustion products through the tubes of the heat exchanger. Further, the furnace may include a blower configured to direct the supply air flow across the tubes, thereby placing the supply air flow in a heat exchange relationship with the relatively hot combustion products to heat the supply air flow. 
     In some circumstances, liquid condensate may form in or on the above-described heat exchanger. For example, during a cooling mode of the HVAC system (e.g., when the furnace is in an inoperative mode or shut-off), relatively cool supply air flow may be directed across the tubes of the heat exchanger. The relatively cool supply air flow may cause air within the tubes of the heat exchanger (e.g., ambient air) to cool, thereby causing moisture contained within the air to condense. As the air within the tubes condenses, liquid condensate may form within the tubes. However, collection of condensate within the tubes may adversely affect the heat exchanger, and therefore it may be desirable to drain the condensate from the heat exchanger. Unfortunately, traditional heat exchangers (e.g., furnaces) may be configured in a manner that does not adequately allow the condensate to drain from the heat exchanger. For example, existing designs may cause condensate to flow via gravity to the burner assembly, which may lead to degradation, operating interruptions, and/or inefficiencies in the heat exchanger. That is, traditional heat exchanger configurations typically include a burner assembly connected to heat exchange tubes at a base (e.g., bottom side, near a drain outlet) of the heat exchanger and a draft inducer connected to the heat exchange tubes near a top side of the heat exchanger. In such a configuration, the burner assembly is susceptible to potential degradation from liquid or liquid condensate that may flow toward the burner assembly via gravity. 
     It is now recognized that improved heat exchanger configurations and related features are desired to limit an amount of liquid condensate that may reach the burner assembly, thereby limiting potential degradation and inefficiencies of a furnace. In accordance with the present techniques, the heat exchanger may be configured to enable a liquid (e.g., condensate) within the heat exchange tubes to flow towards a drain outlet at a base of the heat exchanger. For example, one or more segments of the tubes may be positioned at an angle relative to horizontal to enable drainage of liquid therein via gravity. A draft inducer may be fluidly connected to the heat exchange tubes at a base of the heat exchanger and proximate to the drain outlet of the heat exchanger. A burner assembly may also be fluidly connected to the heat exchange tubes at a position above (e.g., top-fired heat exchanger) the draft inducer relative to gravity (e.g., near the top of the heat exchanger), such that liquid condensate formed within the heat exchange tubes (e.g., via condensation) will be directed away from the burner assembly and towards the drain outlet via gravity. The term “top-fired heat exchanger” used herein may refer to a general configuration in which the burner assembly is connected to a first end or port of the heat exchange tubes at a first position, the draft inducer is connected to a second end or port of the heat exchange tubes at a second position, and the first position of the burner assembly is higher than the second position of the draft inducer, relative to gravity. Such a configuration may limit an amount of liquid condensate from reaching the burner assembly, thereby increasing efficiency and reducing a likelihood of degradation to certain aspects of the furnace. 
     As will be appreciated, the heat exchanger systems disclosed herein may be used in association with any of a variety of HVAC systems, including those in residential and commercial settings. For example, the heat exchanger systems may be utilized in a rooftop unit (RTU), a dedicated outdoor air system, or a split system. Non-limiting examples of systems that may use the heat exchanger system of the present disclosure are described herein with respect to  FIGS.  1 - 4   . 
     Turning now to the drawings,  FIG.  1    illustrates a heating, ventilation, and air conditioning (HVAC) system for building environmental management that may employ one or more HVAC units. As used herein, an HVAC system includes any number of components configured to enable regulation of parameters related to climate characteristics, such as temperature, humidity, air flow, pressure, air quality, and so forth. For example, an “HVAC system” as used herein is defined as conventionally understood and as further described herein. Components or parts of an “HVAC system” may include, but are not limited to, all, some of, or individual parts such as a heat exchanger, a heater, an air flow control device, such as a fan, a sensor configured to detect a climate characteristic or operating parameter, a filter, a control device configured to regulate operation of an HVAC system component, a component configured to enable regulation of climate characteristics, or a combination thereof. An “HVAC system” is a system configured to provide such functions as heating, cooling, ventilation, dehumidification, pressurization, refrigeration, filtration, or any combination thereof. The embodiments described herein may be utilized in a variety of applications to control climate characteristics, such as residential, commercial, industrial, transportation, or other applications where climate control is desired. 
     In the illustrated embodiment, a building  10  is air conditioned by a system that includes an HVAC unit  12 . The building  10  may be a commercial structure or a residential structure. As shown, the HVAC unit  12  is disposed on the roof of the building  10 ; however, the HVAC unit  12  may be located in other equipment rooms or areas adjacent the building  10 . The HVAC unit  12  may be a single package unit containing other equipment, such as a blower, integrated air handler, and/or auxiliary heating unit. In other embodiments, the HVAC unit  12  may be part of a split HVAC system, such as the system shown in  FIG.  3   , which includes an outdoor HVAC unit  58  and an indoor HVAC unit  56 . 
     The HVAC unit  12  is an air cooled device that implements a refrigeration cycle to provide conditioned air to the building  10 . Specifically, the HVAC unit  12  may include one or more heat exchangers across which an air flow is passed to condition the air flow before the air flow is supplied to the building. In the illustrated embodiment, the HVAC unit  12  is a rooftop unit (RTU) that conditions a supply air stream, such as environmental air and/or a return air flow from the building  10 . After the HVAC unit  12  conditions the air, the air is supplied to the building  10  via ductwork  14  extending throughout the building  10  from the HVAC unit  12 . In certain embodiments, the HVAC unit  12  may be a heat pump that provides both heating and cooling to the building with one refrigeration circuit configured to operate in different modes. In other embodiments, the HVAC unit  12  may include one or more refrigeration circuits for cooling an air stream and a furnace for heating the air stream. 
     A control device  16 , one type of which may be a thermostat, may be used to designate the temperature of the conditioned air. The control device  16  also may be used to control the flow of air through the ductwork  14 . For example, the control device  16  may be used to regulate operation of one or more components of the HVAC unit  12  or other components, such as dampers and fans, within the building  10  that may control flow of air through and/or from the ductwork  14 . In some embodiments, other devices may be included in the system, such as pressure and/or temperature transducers or switches that sense the temperatures and pressures of the supply air, return air, and so forth. Moreover, the control device  16  may include computer systems that are integrated with or separate from other building control or monitoring systems, and even systems that are remote from the building  10 . 
       FIG.  2    is a perspective view of an embodiment of the HVAC unit  12 . In the illustrated embodiment, the HVAC unit  12  is a single package unit that may include one or more independent refrigeration circuits and components that are tested, charged, wired, piped, and ready for installation. The HVAC unit  12  may provide a variety of heating and/or cooling functions, such as cooling only, heating only, cooling with electric heat, cooling with dehumidification, cooling with gas heat, or cooling with a heat pump. As described above, the HVAC unit  12  may directly cool and/or heat an air stream provided to the building  10  to condition a space in the building  10 . 
     As shown in the illustrated embodiment of  FIG.  2   , a cabinet  24  encloses the HVAC unit  12  and provides structural support and protection to the internal components from environmental and other contaminants. In some embodiments, the cabinet  24  may be constructed of galvanized steel and insulated with aluminum foil faced insulation. Rails  26  may be joined to the bottom perimeter of the cabinet  24  and provide a foundation for the HVAC unit  12 . In certain embodiments, the rails  26  may provide access for a forklift and/or overhead rigging to facilitate installation and/or removal of the HVAC unit  12 . In some embodiments, the rails  26  may fit onto “curbs” on the roof to enable the HVAC unit  12  to provide air to the ductwork  14  from the bottom of the HVAC unit  12  while blocking elements such as rain from leaking into the building  10 . 
     The HVAC unit  12  includes heat exchangers  28  and  30  in fluid communication with one or more refrigeration circuits. Tubes within the heat exchangers  28  and  30  may circulate refrigerant, such as R-410A, through the heat exchangers  28  and  30 . The tubes may be of various types, such as multichannel tubes, conventional copper or aluminum tubing, and so forth. Together, the heat exchangers  28  and  30  may implement a thermal cycle in which the refrigerant undergoes phase changes and/or temperature changes as it flows through the heat exchangers  28  and  30  to produce heated and/or cooled air. For example, the heat exchanger  28  may function as a condenser where heat is released from the refrigerant to ambient air, and the heat exchanger  30  may function as an evaporator where the refrigerant absorbs heat to cool an air stream. In other embodiments, the HVAC unit  12  may operate in a heat pump mode where the roles of the heat exchangers  28  and  30  may be reversed. That is, the heat exchanger  28  may function as an evaporator and the heat exchanger  30  may function as a condenser. In further embodiments, the HVAC unit  12  may include a furnace for heating the air stream that is supplied to the building  10 . While the illustrated embodiment of  FIG.  2    shows the HVAC unit  12  having two of the heat exchangers  28  and  30 , in other embodiments, the HVAC unit  12  may include one heat exchanger or more than two heat exchangers. 
     The heat exchanger  30  is located within a compartment  31  that separates the heat exchanger  30  from the heat exchanger  28 . Fans  32  draw air from the environment through the heat exchanger  28 . Air may be heated and/or cooled as the air flows through the heat exchanger  28  before being released back to the environment surrounding the HVAC unit  12 . A blower assembly  34 , powered by a motor  36 , draws air through the heat exchanger  30  to heat or cool the air. The heated or cooled air may be directed to the building  10  by the ductwork  14 , which may be connected to the HVAC unit  12 . Before flowing through the heat exchanger  30 , the conditioned air flows through one or more filters  38  that may remove particulates and contaminants from the air. In certain embodiments, the filters  38  may be disposed on the air intake side of the heat exchanger  30  to prevent contaminants from contacting the heat exchanger  30 . 
     The HVAC unit  12  also may include other equipment for implementing the thermal cycle. Compressors  42  increase the pressure and temperature of the refrigerant before the refrigerant enters the heat exchanger  28 . The compressors  42  may be any suitable type of compressors, such as scroll compressors, rotary compressors, screw compressors, or reciprocating compressors. In some embodiments, the compressors  42  may include a pair of hermetic direct drive compressors arranged in a dual stage configuration  44 . However, in other embodiments, any number of the compressors  42  may be provided to achieve various stages of heating and/or cooling. Additional equipment and devices may be included in the HVAC unit  12 , such as a solid-core filter drier, a drain pan, a disconnect switch, an economizer, pressure switches, phase monitors, and humidity sensors, among other things. 
     The HVAC unit  12  may receive power through a terminal block  46 . For example, a high voltage power source may be connected to the terminal block  46  to power the equipment. The operation of the HVAC unit  12  may be governed or regulated by a control board  48 . The control board  48  may include control circuitry connected to a thermostat, sensors, and alarms. One or more of these components may be referred to herein separately or collectively as the control device  16 . The control circuitry may be configured to control operation of the equipment, provide alarms, and monitor safety switches. Wiring  49  may connect the control board  48  and the terminal block  46  to the equipment of the HVAC unit  12 . 
       FIG.  3    illustrates a residential heating and cooling system  50 , also in accordance with present techniques. The residential heating and cooling system  50  may provide heated and cooled air to a residential structure, as well as provide outside air for ventilation and provide improved indoor air quality (IAQ) through devices such as ultraviolet lights and air filters. In the illustrated embodiment, the residential heating and cooling system  50  is a split HVAC system. In general, a residence  52  conditioned by a split HVAC system may include refrigerant conduits  54  that operatively couple the indoor unit  56  to the outdoor unit  58 . The indoor unit  56  may be positioned in a utility room, an attic, a basement, and so forth. The outdoor unit  58  is typically situated adjacent to a side of residence  52  and is covered by a shroud to protect the system components and to prevent leaves and other debris or contaminants from entering the unit. The refrigerant conduits  54  transfer refrigerant between the indoor unit  56  and the outdoor unit  58 , typically transferring primarily liquid refrigerant in one direction and primarily vaporized refrigerant in an opposite direction. 
     When the system shown in  FIG.  3    is operating as an air conditioner, a heat exchanger  60  in the outdoor unit  58  serves as a condenser for re-condensing vaporized refrigerant flowing from the indoor unit  56  to the outdoor unit  58  via one of the refrigerant conduits  54 . In these applications, a heat exchanger  62  of the indoor unit functions as an evaporator. Specifically, the heat exchanger  62  receives liquid refrigerant, which may be expanded by an expansion device, and evaporates the refrigerant before returning it to the outdoor unit  58 . 
     The outdoor unit  58  draws environmental air through the heat exchanger  60  using a fan  64  and expels the air above the outdoor unit  58 . When operating as an air conditioner, the air is heated by the heat exchanger  60  within the outdoor unit  58  and exits the unit at a temperature higher than it entered. The indoor unit  56  includes a blower or fan  66  that directs air through or across the indoor heat exchanger  62 , where the air is cooled when the system is operating in air conditioning mode. Thereafter, the air is passed through ductwork  68  that directs the air to the residence  52 . The overall system operates to maintain a desired temperature as set by a system controller. When the temperature sensed inside the residence  52  is higher than the set point on the thermostat, or the set point plus a small amount, the residential heating and cooling system  50  may become operative to refrigerate additional air for circulation through the residence  52 . When the temperature reaches the set point, or the set point minus a small amount, the residential heating and cooling system  50  may stop the refrigeration cycle temporarily. 
     The residential heating and cooling system  50  may also operate as a heat pump. When operating as a heat pump, the roles of heat exchangers  60  and  62  are reversed. That is, the heat exchanger  60  of the outdoor unit  58  will serve as an evaporator to evaporate refrigerant and thereby cool air entering the outdoor unit  58  as the air passes over the outdoor heat exchanger  60 . The indoor heat exchanger  62  will receive a stream of air blown over it and will heat the air by condensing the refrigerant. 
     In some embodiments, the indoor unit  56  may include a furnace system  70 . For example, the indoor unit  56  may include the furnace system  70  when the residential heating and cooling system  50  is not configured to operate as a heat pump. The furnace system  70  may include a burner assembly and heat exchanger, among other components, inside the indoor unit  56 . Fuel is provided to the burner assembly of the furnace system  70  where it is mixed with air and combusted to form combustion products. The combustion products may pass through tubes or piping in a heat exchanger, separate from heat exchanger  62 , such that air directed by the blower or fan  66  passes over the tubes or pipes and extracts heat from the combustion products. The heated air may then be routed from the furnace system  70  to the ductwork  68  for heating the residence  52 . 
       FIG.  4    is an embodiment of a vapor compression system  72  that can be used in any of the systems described above. The vapor compression system  72  may circulate a refrigerant through a circuit starting with a compressor  74 . The circuit may also include a condenser  76 , an expansion valve(s) or device(s)  78 , and an evaporator  80 . The vapor compression system  72  may further include a control panel  82  that has an analog to digital (A/D) converter  84 , a microprocessor  86 , a non-volatile memory  88 , and/or an interface board  90 . The control panel  82  and its components may function to regulate operation of the vapor compression system  72  based on feedback from an operator, from sensors of the vapor compression system  72  that detect operating conditions, and so forth. 
     In some embodiments, the vapor compression system  72  may use one or more of a variable speed drive (VSDs)  92 , a motor  94 , the compressor  74 , the condenser  76 , the expansion valve or device  78 , and/or the evaporator  80 . The motor  94  may drive the compressor  74  and may be powered by the variable speed drive (VSD)  92 . The VSD  92  receives alternating current (AC) power having a particular fixed line voltage and fixed line frequency from an AC power source, and provides power having a variable voltage and frequency to the motor  94 . In other embodiments, the motor  94  may be powered directly from an AC or direct current (DC) power source. The motor  94  may include any type of electric motor that can be powered by a VSD or directly from an AC or DC power source, such as a switched reluctance motor, an induction motor, an electronically commutated permanent magnet motor, or another suitable motor. 
     The compressor  74  compresses a refrigerant vapor and delivers the vapor to the condenser  76  through a discharge passage. In some embodiments, the compressor  74  may be a centrifugal compressor. The refrigerant vapor delivered by the compressor  74  to the condenser  76  may transfer heat to a fluid passing across the condenser  76 , such as ambient or environmental air  96 . The refrigerant vapor may condense to a refrigerant liquid in the condenser  76  as a result of thermal heat transfer with the environmental air  96 . The liquid refrigerant from the condenser  76  may flow through the expansion device  78  to the evaporator  80 . 
     The liquid refrigerant delivered to the evaporator  80  may absorb heat from another air stream, such as a supply air stream  98  provided to the building  10  or the residence  52 . For example, the supply air stream  98  may include ambient or environmental air, return air from a building, or a combination of the two. The liquid refrigerant in the evaporator  80  may undergo a phase change from the liquid refrigerant to a refrigerant vapor. In this manner, the evaporator  80  may reduce the temperature of the supply air stream  98  via thermal heat transfer with the refrigerant. Thereafter, the vapor refrigerant exits the evaporator  80  and returns to the compressor  74  by a suction line to complete the cycle. 
     In some embodiments, the vapor compression system  72  may further include a reheat coil in addition to the evaporator  80 . For example, the reheat coil may be positioned downstream of the evaporator relative to the supply air stream  98  and may reheat the supply air stream  98  when the supply air stream  98  is overcooled to remove humidity from the supply air stream  98  before the supply air stream  98  is directed to the building  10  or the residence  52 . 
     It should be appreciated that any of the features described herein may be incorporated with the HVAC unit  12 , the residential heating and cooling system  50 , or other HVAC systems. Additionally, while the features disclosed herein are described in the context of embodiments that directly heat and cool a supply air stream provided to a building or other load, embodiments of the present disclosure may be applicable to other HVAC systems as well. For example, the features described herein may be applied to mechanical cooling systems, free cooling systems, chiller systems, or other heat pump or refrigeration applications. 
     Further, any of the systems illustrated in  FIGS.  1 - 4    may include or operate in conjunction with a furnace in accordance with the present disclosure, such as the furnace system  70  of  FIG.  3   . For example, the furnace system  70  of  FIG.  3    may generate combustion products, sometimes referred to as flue gas or exhaust gas, and then rout the combustion products through tubes (or coils) of the furnace system  70 . During an operative mode (e.g., heating mode), a supply air flow may be forced across the tubes of the furnace system  70 , for example by a fan or blower, such that the supply air flow is heated by the combustion products in the tubes of the furnace system  70  prior to delivery of the heated air flow to a conditioned space. Similarly, during a cooling mode (e.g., when the furnace is shut-off or inoperative), ambient or other air may remain in the tubes of the furnace, and a relatively cool supply air flow may be directed across the tubes. As the air within the tubes is cooled via heat exchange with the supply air flow, liquid condensate may form inside of the tubes of the furnace system  70 . 
     In accordance with the present disclosure, a heat exchanger (e.g., a furnace) may be coupled to a heat source, such as a burner assembly (e.g., burner) that generates combustion products, to provide heat to a supply air flow directed across the heat exchanger via a supply air source (e.g., blower, fan). The heat exchanger may also be coupled to a draft inducer that directs (e.g., draws) the combustion products through one or more heat exchange tubes of the heat exchanger. The burner assembly may be fluidly connected to a first port of the heat exchange tubes at a first position proximate a top portion of the heat exchanger, and the draft inducer may be fluidly connected to a second port of the heat exchange tubes at a second position near a base portion of the heat exchanger. A drain outlet may also be located near the second end of the heat exchange tubes and may be configured to drain liquid condensate that forms within the heat exchange tubes during certain operations of the HVAC system, as described above. The first position (e.g., position of the burner assembly) may be higher relative to gravity than the second position (e.g., position of the draft inducer), thereby resulting in a top-fired heat exchanger configuration. By positioning the burner assembly at or near the top of the heat exchanger, liquid condensate formed within the heat exchange tubes may be directed away from the burner assembly at the first position and may instead be directed toward the drain outlet at the second position via the draft inducer and via gravity. In this manner, heat exchangers having the configuration discussed herein may be less susceptible to degradation, operating interruptions, and/or inefficiencies that may otherwise occur in traditional heat exchangers. 
     With this in mind,  FIG.  5    is a perspective view of an embodiment of a packaged HVAC unit  100  that may employ one or more of the heat exchangers disclosed herein. In the illustrated embodiment, the packaged HVAC unit  100  includes multiple components enclosed within an internal volume of a housing  102  of the packaged HVAC unit  100 . The packaged HVAC unit  100  may be configured to circulate air and therefore may include a return section  104  to receive an air flow, such as a return air flow from the building  10 , and a supply section  106  to output an air flow, such as a supply air flow. As an example, the packaged HVAC unit  100  may be located in an outside environment, such as on a rooftop, and may be coupled to ductwork that directs air to and/or from rooms or other areas within a building, such as the building  10  of  FIG.  1   . The ductwork may couple to the return section  104  and the supply section  106 . In this manner, the packaged HVAC unit  100  may circulate air in the building  10 . 
     In addition to circulating air, the packaged HVAC unit  100  may change the temperature of the supply air flow directed therethrough. For example, the packaged HVAC unit  100  may include a refrigerant circuit that circulates a refrigerant therethrough, where the refrigerant circuit is in thermal communication with the air flow. The refrigerant may flow through a condenser  108 , where the refrigerant may be cooled.  FIG.  5    illustrates the condenser  108  as including a fan that may direct ambient air across the condenser  108  to remove heat from the refrigerant via convection, but in other embodiments, the condenser  108  may use another means of cooling the refrigerant, such as via a coolant. After being cooled, the refrigerant may then flow through an evaporator  110 , where the refrigerant may absorb heat from the air flow (e.g., supply air flow) directed across the evaporator  110 . Thus, the refrigerant may be heated, and the air flow may be cooled at the evaporator  110 . After being heated at the evaporator  110 , the refrigerant may return to the condenser  108  where it may once again be cooled. It should be appreciated that the refrigerant circuit may include other components, such as a compressor, expansion valve, and so forth, that enable conditioning of the supply air flow via the refrigerant. 
     The packaged HVAC unit  100  may also be configured to operate in a heating mode and a cooling mode. During operation of the heating mode, air may be received by the packaged HVAC unit  100  at the return section  104  to enter an air flow path. As mentioned, air (e.g., return air) may be received from ductwork that is connected to a building. However, in other embodiments, air received by the packaged HVAC unit  100  may be ambient air, such as from an outside environment. In certain embodiments, the supply air flow directed through the packaged HVAC unit  100  may include air from the return section  104  as well as ambient air. After the air flow enters the packaged HVAC unit  100 , the air flow may pass across a filter  112 . The filter  112  may remove particles from the air flow, such as dirt or other debris. The filter  112  may be a pleated filer, an electrostatic filter, a HEPA filter, or a fiber glass filter that traps the debris when the air flow passes through the filter  112 . After being filtered, the air flow may be directed to the evaporator  110 . As discussed above, at the evaporator  110 , the air flow may be cooled by transferring heat to the refrigerant within the evaporator  110 . In addition, cooling the air flow may also remove moisture from the air flow and thus, the packaged HVAC unit  100  may also dehumidify the air flow. Once cooled, the air flow may be directed through a blower  114 , which may increase the velocity of the air flow and discharge the air flow as supply air via the supply section  106  of the packaged HVAC unit  100 . Thereafter, the supply air flow may be circulated through the ductwork. In some embodiments, the blower  114  may also operate to draw air through the return section  104  and thereby function to both draw in and expel air. 
     In some modes of operation (e.g., a heating mode), prior to exiting the packaged HVAC unit  100 , the air may be heated by a heat exchanger  116  (e.g., a furnace). By way of example, the heat exchanger  116  may be coupled to a heat source. In some embodiments, the heat exchanger  116  may be a gas heat exchanger and may be coupled to a gas burner (e.g., a burner assembly) that combusts a fuel (e.g., air-fuel mixture), such as acetylene, natural gas, propane, another gas, or any combination thereof to produce combustion products having an elevated temperature that are directed into the heat exchanger  116 . When the air flow is directed across the heat exchanger  116 , the air flow may absorb heat from the combustion products, thereby increasing the temperature of the air flow. Thereafter, the air flow may then exit the packaged HVAC unit  100  at a higher temperature compared to the air flow entering the packaged HVAC unit  100 . 
     During a cooling mode of the packaged HVAC unit  100 , the heat exchanger  116  may be inoperative (e.g., turned off). However, some of the combustion products generated during a previous heating mode may linger or remain within heat exchange tubes of the heat exchanger  116 . Additionally or alternatively, when the heat exchanger  116  is not operating, another flow of air (e.g., ambient air) may nevertheless flow or reside in the heat exchange tubes of the heat exchanger  116 . As a relatively cool air flow (e.g., supply air cooled by the evaporator  110 ) is directed across the heat exchange tubes, air within the heat exchange tubes may lose heat to the relatively cool air flow, thereby causing any moisture within the air to condense and form liquid condensate within the heat exchange tubes of the heat exchanger  116 . To mitigate collection of the condensate within the heat exchange tubes, the heat exchanger  116  of the present disclosure is configured to enable removal of the liquid condensate from the heat exchange tubes while also mitigating contact between the condensate and other components of the heat exchanger  116  (e.g., the burner assembly). In this way, degradation, inefficiency, and/or other adverse effects that may otherwise be caused by the condensate is avoided. The features and aspects of the heat exchanger  116  are discussed in further detail below. 
     To separate various components within the packaged HVAC unit  100 , the packaged HVAC unit  100  may include partitions  120  (e.g., panels, vestibule panels, dividers, separation plates, etc.). As an example, the partitions  120  may divide the internal volume defined by the housing  102  into a first volume  122 , which may contain the heat source (e.g., burner assembly) of the heat exchanger  116 , a second volume  124  (e.g., supply air section) from the supply air flow may exit the packaged HVAC unit  100 , a third volume  126  that contains the condenser  108 , and a fourth volume  128  (e.g., return air section  104 ) configured to receive air flow directed into the packaged HVAC unit  100 . Various components of the packaged HVAC unit  100  may also be oriented along a number of axes including a lateral axis  190 , a longitudinal axis  192 , and a vertical axis  194 . 
       FIG.  6    is side view of an embodiment of a furnace  200  (e.g., heat exchanger) that can be used with or in any of the systems of  FIGS.  1 - 5    or any other suitable HVAC system. For example, the furnace  200  of  FIG.  6    may correspond to the heat exchanger  116  in  FIG.  5   . The furnace  200  may be disposed within a housing  130  (e.g., support structure), such as a section of the housing  102  of  FIG.  5   , a section of an air handler, a standalone housing, or any other suitable support structure. The housing  130  may include a first side  132  (e.g., top side, panel, etc.) and a base  134  (e.g., bottom side, panel, etc.). However, in some embodiments, the furnace  200  may not include the first side  132  and/or the base  134  of the housing  130 . 
     A blower  140  (e.g., fan) may be coupled or secured to the first side  132  of the housing  130  and may be configured to generate or direct an air flow  500  along an air flow path  510  of the furnace  200 . The blower  140  may correspond to the blower  114  in  FIG.  5   . The housing  130  may also include a vestibule panel  150  (e.g., side panel, panel, etc.), which may correspond to one of the partitions  120  of  FIG.  5   . In the embodiment illustrated in  FIG.  6   , the furnace  200  includes a heat exchange section  202  coupled to the vestibule panel  150 . The heat exchange section  202  may include one or more heat exchange tubes  204 , with each heat exchange tube  204  having a first port  206  (e.g., first end, top end, upper end, inlet, upstream end, etc.) and a second port  208  (e.g., second end, bottom end, lower end, outlet, downstream end, etc.) that are each coupled to the vestibule panel  150 . The heat exchange tube  204  may extend from the first port  206  to the second port  208  in any suitable configuration, geometry, or arrangement. In the illustrated embodiment, the heat exchange tube  204  also includes a first bend  210  (e.g., top bend, upstream bend), a second bend  212  (e.g., middle bend, midstream bend), and a third bend  214  (e.g., bottom bend, downstream bend). The heat exchange tube  204  extends between each of the first port  206 , second port  208 , first bend  210 , second bend  212 , and third bend  214 . In this manner, the heat exchange tube  204  defines multiple passes (e.g., tube passes, tube segments, conduit segments, etc.) of the heat exchange tube  204  through which combustion products are directed and across which the air flow  500  is directed. More specifically, the heat exchange tube  204  defines a first pass  216  extending between the first port  206  and the first bend  210 , a second pass  218  extending between the first bend  210  and the second bend  212 , a third pass  220  extending between the second bend  212  and the third bend  214 , and a fourth pass  222  extending between the third bend  214  and the second port  208 . In some embodiments, one or more of the passes  216 ,  218 ,  220 ,  222  may extend in a direction along the lateral axis  190  (e.g., in a horizontal direction, along a horizontal axis  272 ). In other embodiments, one or more of the passes  216 ,  218 ,  220 ,  22  may extend at an angle relative to the horizontal axis  272 , as described in greater detail below. 
     The first port  206  may be coupled or secured to a first side  152  of the vestibule panel  150  proximate an inlet  160  (e.g., passage, hole, aperture, opening, channel) formed in the vestibule panel  150 , and the second port  208  may be coupled to the first side  152  of the vestibule panel  150  proximate an outlet  170  (e.g., passage, hole, aperture, opening, channel) formed in the vestibule panel  150 . The first and second ports  206 ,  208  may be coupled to the inlet  160 , and outlet  170 , respectively, via a swedging process or technique (e.g., expanding the first port  206  of the heat exchange tube  204  with the first port  206  positioned within the inlet  160  of the vestibule panel  150 ), welding, brazing, or any other mechanical fastening technique. Each of the passes  216 ,  218 ,  220 , and  222  may be configured to extend crosswise relative to a direction of the air flow  500  along the flow path  510 , as described in greater detail below. It should be understood that each of the features of the heat exchange tube  204  described above may be fluidly coupled to one another to enable flow of fluids (e.g., combustion products, liquid condensate) through the heat exchange tube  204  towards the outlet  170 , as described in greater detail below. Further, in some embodiments, the heat exchange section  202  may include one or more heat exchange tubes  204  having additional features, alternative features, fewer or more bends, fewer or more passes, and so forth, based on selected characteristics, implementations, and/or operating parameters of the furnace  200 . Further still, the heat exchange tubes  204  have different orientations (e.g., offset, aligned relative to one another) to facilitate various tube configurations that may reduce an overall size, height, and/or footprint of the furnace  200 . 
     As discussed herein, the furnace  200  may also include a burner assembly  230  (e.g., combustor, heating element, burner system) configured to ignite a mixture of fuel and oxidant (e.g., air-fuel mixture) to generate combustion products. For example, the burner assembly  230  may be fluidly connected to a fuel source  232  and may also be fluidly coupled to the inlet  160  on a second side  154  of the vestibule panel  150 . The burner assembly  230  may include one or more burners (e.g., premix burners) configured to ignite the mixture of fuel and oxidant to generate the combustion products, which are then directed through the inlet  160  and into the first port  206  of the heat exchange tube  204  via the first port  206  fluidly coupled to the inlet  160 . That is, the burner assembly  230  and the first port  206  may be in fluid communication, such that the combustion products may generally travel from the burner assembly  230 , through the inlet  160 , through the first port  206 , through the first, second, third, and fourth passes  216 ,  218 ,  220 , and  222 , and towards the second port  208  of the heat exchange tube  204 . The second port  208  of the heat exchange tube  204  may be fluidly coupled to the outlet  170 , thereby enabling the combustion products to pass through the second port  208  and into the outlet  170 . 
     From the outlet  170 , the combustion products may be removed from the system (e.g., via an exhaust conduit). To this end, the furnace  200  may also include a draft inducer  240  (e.g., draft inducer blower, draft blower, draft fan, inducer fan) fluidly coupled to the outlet  170  on the second side  154  of the vestibule panel  150 . The draft inducer  240  is configured to facilitate flow of the combustion products through the heat exchange tube  204 . That is, the draft inducer  240  may be fluidly coupled to the second port  208  via the outlet  170  and may be configured to draw the combustion products through the heat exchange tube  204 . When operation of the furnace  200  is initiated to heat the air flow  500  (e.g., upon receipt of a call for heating), the draft inducer  240  may be operated prior to operation of the burner assembly  230  (e.g., 30 seconds before, a predetermined time period before, etc.), thereby removing any air or other gaseous compounds that may be present within the heat exchange tube  204  (e.g., via a suction air flow generated by the draft inducer  240 ). The draft inducer  240  may also be coupled to an exhaust conduit (not shown) which may be configured to direct combustion gases, air, and/or other gaseous compound out of the furnace  200  (e.g., the HVAC system having the furnace  200 ), as described in greater detail below. 
     As discussed above, the burner assembly  230  may be coupled or secured to the vestibule panel  150  at the inlet  160  of the vestibule panel  150 , and the draft inducer  240  may be coupled or secured to the vestibule panel  150  at the outlet  170  of the vestibule panel  150 . The burner assembly  230  and the draft inducer  240  may be secured via fasteners, brackets, pins, screws, or any other suitable mechanical fastening technique. As illustrated, the inlet  160  is located above (e.g., vertically above) the outlet  170  relative to the base  134  of the furnace  200  (e.g., relative to gravity, relative to the vertical axis  194 , etc.). Thus, when installed and coupled to the vestibule panel  150 , the burner assembly  230  is located at a top portion  260  of the furnace  200 , and the draft inducer is located at a bottom portion  270  of the furnace  200 . That is, the burner assembly  230  is higher in position than the draft inducer  240  relative to the base  134  of the furnace  200  (e.g., relative to gravity, relative to the vertical axis  194 ). This configuration (e.g., top-fired configuration, top-burner configuration) limits, reduces, and/or prevents the potential of liquid and/or liquid condensate that may form within the heat exchanger tube  204  from flowing toward the burner assembly  230 , as described in greater detail below. 
     As previously described, operation of the furnace  200  may cause condensate to form within the heat exchange tube  204  as the air flow  500  travels across the heat exchange tube  204  along the flow path  510 , such as during a cooling mode of operation when the furnace  200  is not operating to heat the air flow  500 . As the liquid condensate forms within the heat exchange tube  204 , the liquid condensate may be directed away from the burner assembly  230  and towards the outlet  170 , such as via force of gravity. In some embodiments, each of the passes  216 ,  218 ,  220 , and  220  may generally extend along the lateral axis  190  and may be disposed at an angle relative to a horizontal axis  272  (e.g., a horizontal direction), such that condensate formed within the heat exchange tube  204  may directed away from the top portion  260  of the furnace  200  and towards the bottom portion  270  of the furnace  200  via gravity. The liquid condensate may flow through one or more of the passes  216 ,  218 ,  220 , and  222  and along one or more of the bends  210 ,  212 ,  214  towards the second port  208  of the heat exchange tube  204  that is in fluid communication with the outlet  170 . Liquid condensate that reaches the outlet  170  may then be discharged from the furnace  200  via a drain (e.g., drain outlet), a conduit, or any suitable discharge flow path fluidly coupled to the outlet  170 . In some embodiments, a gasket  180  (e.g., paper gasket) may be positioned between the outlet  170  of the vestibule panel  150  and the draft inducer  240 . The gasket  180  may surround the second port  208  of the heat exchange tube  204 , may have an opening formed therein that is aligned with the second port  208 , and may extend from the outlet  170  (e.g., in a horizontal direction along the horizontal axis  272 ) away from the vestibule panel  150 . The gasket  180  may be configured to facilitate drainage of the liquid condensate by providing clearance for the liquid condensate to drain out of the heat exchange tube  204  before reaching the draft inducer  240 . That is, the gasket  180  may be composed of a porous material, thereby enabling liquid condensate to drain through the gasket  180  and out of the furnace  200  before reaching the draft inducer  240 . It should be noted that various aspects of the furnace  200  may be manufactured, configured, and/or arranged to block or reduce an undesirable impact of the liquid condensate on the furnace  200  that may otherwise be caused by contact with the liquid condensate. By way of example, components of the heat exchange section  202 , such as the heat exchange tube  204 , the inlet  160 , the outlet  170 , the gasket  180 , and the vestibule panel  150  may be made of stainless steel, chromium, and/or other suitable (e.g., corrosion resistant) material to reduce undesirable effects of the liquid condensate on the structural integrity and/or performance of the components. 
     The furnace  200  may also include a controller  250  configured to control operation of the burner assembly  230  and the draft inducer  240 , such as based on an operating mode of the furnace  200 . The controller  250  may be coupled to the vestibule panel  150  via welding, fasteners, screws, or other suitable technique. During operation, the controller  250  may receive a signal indicative of a call for operation in the cooling mode, and in response, the controller  250  may operate to shut-off or power down the burner assembly  230  and the draft inducer  240  such that combustion products are no longer generated and circulated through the heat exchange tubes  204 . At a different time, the controller may receive a signal indicative of a call for operation in the heating mode, and in response, the controller  250  may operate to activate or power on the draft inducer  240  and the burner assembly  230  (e.g., sequentially, power on the draft inducer  240  prior to powering on the burner assembly  230 , etc.) such that combustion products may be generated and circulated through the heat exchange tube  204  to enable heating of the air flow  500  directed across the heat exchange tube  204  along the air flow path  510 . 
     In some circumstances, the controller  250  may be operate to activate the draft inducer  240  without activating the burner assembly  230 . For example, a presence of liquid condensate within the heat exchange tube  204  may be detected via one or more sensors  274  (e.g., a liquid sensor, humidity sensor, condensate sensor, etc. fluidly coupled to and/or disposed within the heat exchanger tube  204  and communicatively coupled to the controller  250 ). Based on detection of the presence of liquid condensate, the draft inducer  240  may be activated to draw an air flow through the heat exchange tube  204  to motivate the liquid condensate towards the second port  208  and away from the burner assembly  230 . To facilitate control of the components of the furnace  200 , the controller  250  may include a memory  252  with instructions stored thereon for controlling operation the furnace  200  and components of the furnace  200 , and processing circuitry  254  configured to execute such instructions. For example, the processing circuitry  254  may include one or more application specific integrated circuits (ASICs), one or more field programmable gate arrays (FPGAs), one or more general purpose processors, or any combination thereof. Additionally, the memory  252  may include a non-transitory computer-readable medium that may include volatile memory, such as random-access memory (RAM), and/or non-volatile memory, such as read-only memory (ROM), optical drives, hard disc drives, optical drives, solid-state drives or any other suitable non-transitory computer-readable medium storing instructions that, when executed by the processing circuitry  254 , may control operation of the furnace  200 . Although  FIG.  6    illustrates the controller  250  as being coupled to the vestibule panel  150 , in some embodiments, the controller  250  may be disposed elsewhere, such as remotely relative to the furnace  200 . 
       FIG.  7    is a schematic side view of an embodiment of the furnace  200 , illustrating various flow directions (e.g., flow paths) of liquid (e.g., liquid condensate) that may form within the heat exchange tube  204 , such as in the manners described above. As illustrated, each of the passes  216 ,  218 ,  220 ,  222  extends at least partially in a direction of the lateral axis  190  and generally crosswise to the air flow path  510  (e.g., crosswise to the vertical axis  194 ) through which the air flow  500  is directed across the heat exchange tube  204 . For example, one or more of the passes  216 ,  218 ,  220 , and  222  (e.g., passes  216 ,  222 ) of the heat exchange tube  204  may extend a length  620  (e.g., width, distance) from the vestibule panel  150 . However, some of the passes  216 ,  218 ,  220 , and  222  (e.g., passes  218 ,  220 ) may extend a length less than the length  620 . In some embodiments, the length  620  may be greater than a width  630  of the air flow path  510 , such that the air flow  500  directed along the air flow path  510  may contact each of the passes  216 ,  218 ,  220 ,  222  as the air flow  500  flows through the air flow path  510 . When air (e.g., ambient air) within the heat exchange tube  204  is cooled via a relatively cool supply air flow (e.g., air flow  500 ) directed along the air flow path  510  across the heat exchange tube  204 , such as during non-operation of the furnace  200 , moisture within the air inside the heat exchange tube  204  may condense, thereby forming liquid condensate within the heat exchange tube  204 . As mentioned above, one or more of the passes  216 ,  218 ,  220 , and  222  may be disposed at an angle relative to a horizontal axis  272  such that liquid condensate formed within the passes  216 ,  218 ,  220 , and  222  may be directed via gravity, through the heat exchange tube  204  towards the outlet  170  and the gasket  180 . Additionally, in some embodiments, one or more of the passes  216 ,  218 ,  220 , and  222  may extend in a direction along the lateral axis  190  (e.g., a horizontal direction, along the horizontal axis  272 ). 
     For example, the first port  206  may be secured to the inlet  160  (e.g., passage, hole, aperture, opening, channel) at a first position  300  (e.g., first location along the vertical axis  194 ). The first pass  216  may extend from the first port  206  to the first bend  210  at a first angle  400  (e.g., downward angle) relative to the horizontal axis  272 , such that liquid condensate formed within the first port  206  and/or the first pass  216  may be directed along a first flow path  600  of the heat exchange tube  204  towards the first bend  210  via gravity. That is, the first bend  210  may be disposed at a second position  302  (e.g., second location along the vertical axis  194 ), which is lower relative to gravity than the first position  300  of the first port  206 . Thus, condensate formed within the first port  206  and/or the first pass  216  may travel from the first position  300  to the second position  302  along the first flow path  600  via gravity. Upon reaching the first bend  210 , liquid condensate may fall (e.g., via gravity) along a second flow path  602  of the heat exchange tube  204  towards the second pass  218 . The second pass  218  may be fluidly coupled to the first bend  210  at a third position  304  (e.g., third location along the vertical axis  194 ). As shown in the illustrated embodiment, the third position  304  is lower than the second position  302  relative to gravity such that condensate traveling through the first bend  210  falls along the second flow path  602  and into the second pass  218 . 
     The second pass  218  may extend from the first bend  210  to the second bend  212  at a second angle  402  (e.g., downward angle) relative to the horizontal axis  272 , such that liquid condensate within the second pass  218  may be directed along a third flow path  604  of the heat exchange tube  204  towards the second bend  212  via gravity. That is, the second bend  212  may be disposed at a fourth position  306  (e.g., fourth location along the vertical axis  194 ), which is lower relative to gravity than the third position  304 . Thus, condensate reaching the second pass  218  may travel from the third position  304  to the fourth position  306  along the third flow path  604  via gravity. Upon reaching the second bend  212 , liquid condensate may fall via gravity along a fourth flow path  606  of the heat exchange tube  204  towards the third pass  220 . The third pass  220  may be fluidly coupled to the second bend  212  at a fifth position  308  (e.g., fifth location along the vertical axis  194 ). As shown in the illustrated embodiment, the fifth position  308  is lower than the fourth position  306  relative to gravity such that condensate traveling through the second bend  212  falls along the fourth flow path  606  and into the third pass  220 . 
     The third pass  220  may extend from the second bend  212  to the third bend  214  at a third angle  404  (e.g., downward angle) relative to the horizontal axis  272 , such that liquid condensate within the third pass  220  may be directed along a fifth flow path  608  of the heat exchange tube  204  towards the third bend  214  via gravity. That is, the third bend  214  may be disposed at a sixth position  310  (e.g., sixth location along the vertical axis  194 ) which is lower relative to gravity than the fifth position  308 . Thus, condensate reaching the third pass  220  may travel from the fifth position  308  to the sixth position  310  along the fifth flow path  608  via gravity. Upon reaching the third bend  214 , liquid condensate may fall via gravity along a sixth flow path  610  of the heat exchange tube  204  towards the fourth pass  222 . The fourth pass  222  may be fluidly coupled to the third bend  214  at a seventh position  312  (e.g., seventh location along the vertical axis  194 ). As shown in the illustrated embodiment, the seventh position  312  is lower than the sixth position  310  relative to gravity such that condensate traveling through the third bend  214  falls along the sixth flow path  610  and into the fourth pass  222 . 
     The fourth pass  222  may extend from the third bend  214  to the second port  208  at a fourth angle  406  (e.g., downward angle) relative to the horizontal axis  272 , such that liquid condensate within the fourth pass  222  may be directed along a seventh flow path  612  of the heat exchange tube  204  towards the second port  208  via gravity. That is, the second port  208  may be disposed at an eighth position  314  (e.g., eight location along the vertical axis  194 ) which is lower relative to gravity than the seventh position  312 . Thus, condensate reaching the fourth pass  222  may travel from the seventh position  312  to the eighth position  314  along the seventh flow path  612  via gravity. As discussed above, the embodiments included herein should not be considered limiting and other embodiments of the furnace  200  may include fewer or more passes, bends and heat exchange tubes as desired based on various design considerations of the furnace  200 . In the manner described above, the furnace  200  including the features described herein enables drainage and removal of liquid condensate from the furnace while also directing the liquid condensate away from the burner assembly  230 , thereby avoiding undesirable contact between liquid condensate and the burner assembly  230  and increasing efficiency and longevity of the burner assembly  230 . 
     It should be noted that in some embodiments, one or more of the passes  216 ,  218 ,  220 , and  222  may not extend at an angle relative to the horizontal axis  272  and instead may generally extend in a direction along the lateral axis  190  (e.g., in a horizontal direction along the horizontal axis  272  as illustrated in  FIG.  6   ). That is, each heat exchange tube  204  may include one or more passes  216 ,  218 ,  220 ,  222  that extend at an angle relative to the horizontal axis  272  across the flow path  510 , one or more passes  216 ,  218 ,  220 ,  222  that extend along the horizontal axis  272  (e.g., in a horizontal direction) across the flow path  510 , or any combination thereof. 
       FIG.  8    is a schematic side view of an embodiment of a portion of the furnace  200 , illustrating the draft inducer  240  and the gasket  180  configured to facilitate removal of liquid condensate from the furnace  200 . The gasket  180  may be disposed on the second side  154  of the vestibule panel  150  between the draft inducer  240  and the outlet  170  (e.g., passage, channel, hole, aperture, opening). As described above, liquid condensate that reaches the fourth pass  222  may travel along the seventh flow path  612  of the heat exchange tube  204  towards the second port  208 , the outlet  170 , and the gasket  180 . The gasket  180  may be disposed around (e.g., circumferentially around) the outlet  170  and around the port  208  and may extend to a drain outlet  282 . The gasket  180  may provide a channel, flow path, or other guide extending from the port  208 , through the outlet  170 , and to the drain outlet  282  such that liquid condensate directed along the seventh flow path  612  may flow from the outlet  170  and pass through or along the gasket  180  to be discharged from the furnace  200 . In some embodiments, the gasket  180  may be composed of a porous material, thereby enabling liquid condensate to pass through the gasket  180  and towards the drain outlet  282  to be discharged from the furnace  200 . 
     During an operative mode (e.g., heating mode), the draft inducer  240  may be configured to discharge combustion products circulated through the heat exchange tube  204  via an exhaust outlet  280  (e.g., outlet port, discharge port), which may be fluidly coupled to the draft inducer  240 , such as via a panel (e.g., side panel) of the packaged HVAC unit  100  of  FIG.  5   . In some embodiments, the exhaust outlet  280  may be fluidly coupled to a conduit  290  (e.g., vertical exhaust, exhaust conduit) configured to receive combustion products from the draft inducer  240  and direct flow of the combustion products in a direction  700  (e.g., vertical direction), as described in greater detail below, to discharge the combustion products from the furnace  200  and/or the packaged HVAC unit  100 . 
       FIG.  9    is a front perspective view of an embodiment of the furnace  200 , illustrating multiple heat exchange tubes  204  arranged along the longitudinal axis  192 . As illustrated, each of the heat exchange tubes  204  includes the first port  206 , which is fluidly coupled to the burner assembly  230  via respective inlets  160  (e.g., passage, channel, opening, aperture, hole) of the vestibule panel  150 , and may also include the second port  208 , which is fluidly coupled to the draft inducer  240  via respective outlets  170  (e.g., passage, channel, opening, aperture, hole) of the vestibule panel  150 . As noted above, the burner assembly  230  may be coupled to the vestibule panel  150  above the draft inducer  240  relative to gravity (e.g., along the vertical axis  194 ). That is, the burner assembly  230  may be positioned above the draft inducer  240  such that the inlets  160  of the vestibule panel  150  are positioned above the outlets  170  of the vestibule panel  150  along the vertical axis  194 . Further, in some embodiments, a respective inlet  160  and the corresponding outlet  170  (e.g., inlet and outlet fluidly coupled together via a heat exchange tube  204 ) are also aligned along the vertical axis  194  such that the first port  206  and the second port  208  of each respective heat exchange tube  204  are aligned with one another along the vertical axis  194 . For example, the burner assembly  230  may be coupled to the vestibule panel  150  such that a respective inlet  160  (e.g., a first inlet) is positioned a distance  800  from the base  134  of the housing  130  and a distance  808  from a side  136  of the housing  130 , and the draft inducer  240  may be coupled to the vestibule panel  150  such that a respective outlet  170  (e.g., a first outlet fluidly coupled to the first inlet  160  via a heat exchange tube  204 ) is positioned a distance  802  from the base  134  of the housing  130  and a distance  810  from the side  136  of the housing  130 . The distance  800  may be greater than the distance  802 , and the distance  808  may be approximately equal to the distance  810 . Thus, each inlet  160  may be positioned within the vestibule panel  150  at a position above the corresponding outlet  170  relative to gravity such that the first port  206  of a respective heat exchange tube  204  is aligned with the corresponding second port  208  of the respective heat exchange tube  204  along the vertical axis. 
     As discussed above, the furnace  200  may be part of an outdoor or rooftop HVAC unit. In some embodiments, the burner assembly  230  may also be positioned within a threshold distance  806  from the first side  132  (e.g., top side) of the housing  130 , thereby providing a desired clearance between the burner assembly  230  and the first side  132 . For example, during a heating mode, the burner assembly  230  may be operated to generate combustion gases to heat an air flow. By positioning the burner assembly  230  near the first side  132  (e.g., within a threshold distance  806  from the first side  132 ), heat generated from the operation of the burner assembly  230  may melt snow accumulated on the first side  132  of the housing such that the snow may be directed away from the burner assembly  230  via gravity, thereby reducing undesirable effects on the structural integrity and/or performance of the components of the burner assembly  230  that may otherwise be caused by contact with water or other liquid. 
     In some embodiments, the exhaust outlet  280  of the draft inducer  240  may be fluidly coupled to the conduit  290 , which may extend in the direction  700 , such as along the vertical axis  194 . As shown in the illustrated embodiment, the conduit  280  may extend in the direction  700  to a position above the first side  132  of the housing  130  (e.g., along the vertical axis  194 ). Directing the combustion products along the exhaust conduit  280  in the direction  700  may also facilitate reducing undesirable effects on the structural integrity and/or performance of the components of the furnace  200 . For example, heat from the combustion products discharged via the conduit  290  may also be used to melt snow or other environmental conditions which may accumulate on the first side  132  of the housing  130  and may have an undesirable impact on the performance of the furnace  200  and/or may cause the furnace  200  to bear an undesired weight. 
       FIG.  10    is front perspective view of an embodiment of the furnace  200 , illustrating multiple heat exchange tubes  204  arranged along the longitudinal axis  192 . As described above with respect to  FIG.  9   , the respective inlets  160  may be positioned above the respective outlets  170  relative to gravity, and thus the first port  206  of each heat exchange tube  204  may also be positioned above the respective second port  208  relative to gravity. In some embodiments, the heat exchange tubes  204  may also be arranged such that the first port  206  is offset (e.g., horizontally offset) from the corresponding second port  208  of the heat exchange tube  204  along the longitudinal axis  192 . For example, the burner assembly  230  may be coupled to the vestibule panel  150  such that a respective inlet  160  is positioned a distance  820  from the side  136  of the housing  130 , and the draft inducer  240  may be coupled to the vestibule panel  150  such that the corresponding outlet  170  (e.g., outlet fluidly coupled to the respective inlet via the heat exchange tube  204 ) is positioned a distance  822  from the side  136  of the housing  130 . The distance  820  is greater than the distance  822 , such that the respective inlet  160  and the corresponding outlet  170  are offset (e.g., horizontally offset) from one another along the longitudinal axis  192  by a distance  824 . Accordingly, when installed, a respective heat exchange tube  204  may include a first port  206  that couples to the inlet  160  at the distance  820  from the side  136  of the housing  130 , and a second port  208  that couples to the outlet  170  at the distance  822  from the side  136  of the housing  130 , and thus, the first port  206  and the corresponding second port  208  of the respective heat exchange tube  204  may also be offset from one another along the longitudinal axis  192  by the distance  824 . In some embodiments, the distance  820  may be less than the distance  822 . 
     In some embodiments, the respective inlets  160  and the corresponding outlets  170  may be offset from one another by the distance  824 , and the burner assembly  230  and the draft inducer  240  may nevertheless be aligned with one another along the vertical axis  194 . For example, the first port  206  of each heat exchange tube  204  may be fluidly coupled to a respective inlet  160 , the second port  208  may be fluidly coupled to a respective outlet  170 , and the heat exchange tubes  204  may each have a geometry or configuration that enables the first ports  206  and the corresponding second ports  208  to be offset from one another by the distance  824 . By arranging the inlets  160 , the outlets  170 , and the heat exchange tubes  204  in different orientations (e.g., inlet  160  and outlet  170  aligned with one another along the vertical axis  194 , inlet  160  and outlet  170  offset from one another relative to the longitudinal axis  194 , first and second ports  206 ,  208  aligned with one another along the vertical axis  194 , first and second ports  206 ,  208  offset from one another relative to the longitudinal axis  192 ), an overall size, height, and/or footprint of the furnace  200  may be reduced, thereby reducing costs associated with manufacture, operation, and/or maintenance of the furnace  200 . For example, in the illustrated embodiment, the inlets  160  and corresponding outlets  170  are offset with one another relative to the longitudinal axis  192  (e.g., not aligned with one another along the vertical axis  194 ), which reduces an overall height occupied by the furnace  200 . 
     As described above with respect to  FIG.  7   , each of the heat exchange tubes  204  may include two or more passes (e.g., passes  216 ,  218 ,  220 ,  222  of  FIG.  7   ) and two or more bends (e.g., bends  210 ,  212 ,  214 ). In some embodiments, one or more of the bends  210 ,  212 ,  214  may generally extend from one pass to another pass along the longitudinal axis  192  and may be disposed at an angle relative to the horizontal axis  272  (e.g., a horizontal direction) such that liquid condensate formed within the heat exchange tube  204  may be directed away from the burner assembly  230  via gravity. For example, the bend  212  may generally extend along the longitudinal axis  192  from the second pass  218  to the third pass  220  and may be disposed at an angle  408  relative to the horizontal axis  272  such that the bend  212  extends cross-wise to a direction of the airflow  500  (e.g., downward direction). In other embodiments, one or more of the bends  210 ,  212 ,  214  may extend in a direction along the vertical axis  194 . It should be noted that the heat exchange tubes  204  may each have a geometry or configuration that includes one or more bends that extend from one pass to another pass in a direction along the vertical axis  194 , one or more bends that extend from one pass to another pass in a direction along the longitudinal axis  192  at an angle relative to the horizontal axis  272 , one or more passes that extend in a direction along the lateral axis  190  (e.g., horizontal direction), one or more passes that extend in a direction along the lateral axis  190  at an angle relative to the horizontal axis  272 , or any combination thereof. 
     As set forth above, the furnace of the present disclosure may provide one or more technical effects useful in the operation of HVAC systems, such as packaged HVAC units, configured to operate in a cooling mode and in a heating mode. For example, the furnace may be disposed within an air flow path of the HVAC system to enable the furnace to heat an air flow during operation of the furnace in the heating mode. During operation of the HVAC system in the cooling mode, relatively cool air may be directed across heat exchange tubes of the furnace, and air (e.g., ambient air) residing within the heat exchange tubes thereby be cooled. As a result, moisture within the air may condense and form liquid condensate within the heat exchange tubes. The top-fired burner assembly configuration disclosed herein enables discharge of the liquid condensate from the furnace while also mitigating contact between the liquid condensate and the burner assembly, thereby reducing adverse impacts on components of the HVAC system that may otherwise be caused by the liquid condensate. That is, the presently disclosed techniques may reduce a likelihood of wear and degradation to the HVAC system and its components that may be caused by water contact during operation of the HVAC system. The technical effects and technical problems in the specification are examples and are not limiting. It should be noted that the embodiments described in the specification may have other technical effects and can solve other technical problems. 
     The techniques presented and claimed herein are referenced and applied to material objects and concrete examples of a practical nature that demonstrably improve the present technical field and, as such, are not abstract, intangible or purely theoretical. Further, if any claims appended to the end of this specification contain one or more elements designated as “means for [perform]ing [a function] . . . ” or “step for [perform]ing [a function] . . . ”, it is intended that such elements are to be interpreted under 35 U.S.C. 112(f). However, for any claims containing elements designated in any other manner, it is intended that such elements are not to be interpreted under 35 U.S.C. 112(f). 
     While only certain features and embodiments of the disclosure have been illustrated and described, many modifications and changes may occur to those skilled in the art, such as variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, including temperatures and pressures, mounting arrangements, use of materials, colors, orientations, and so forth without materially departing from the novel teachings and advantages of the subject matter recited in the claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the disclosure. Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described, such as those unrelated to the presently contemplated best mode of carrying out the disclosure, or those unrelated to enabling the claimed disclosure. It should be noted that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation.