Patent Publication Number: US-2018031267-A1

Title: System and method for detecting flow restrictions across a coil of an outdoor heat exchanger

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from and the benefit of U.S. Provisional Application Ser. No. 62/367,304, filed Jul. 27, 2016, entitled “OD COIL FOULING SENSING METHOD,” the disclosure of which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     BACKGROUND 
     The present disclosure relates generally to heating, ventilation, and air conditioning (HVAC) systems, and more particularly to detection of coil conditions on an outdoor unit of the HVAC system. 
     A wide range of applications exist for HVAC systems. For example, residential, light commercial, commercial, and industrial systems are used to control temperatures and air quality in residences and buildings. Generally, HVAC systems may circulate a fluid, such as a refrigerant, through a closed loop between an evaporator coil where the fluid absorbs heat and a condenser where the fluid releases heat. The fluid flowing within the closed loop is generally formulated to undergo phase changes within the normal operating temperatures and pressures of the system so that quantities of heat can be exchanged by virtue of the latent heat of vaporization of the fluid. A fan may blow air over, or pull air across, the coils of the heat exchanger(s) in order to condition the air. 
     HVAC systems may include an indoor unit, an outdoor unit, or both. Certain HVAC systems having both an indoor and outdoor unit may operate in several modes, including a heat pump mode. When the HVAC system operates in the heat pump mode, a heat exchanger of the outdoor unit may act as the evaporator. When the environmental temperature is near, at, or below freezing, and while the HVAC system operates in the heat pump mode such that the heat exchanger of the outdoor unit acts as the evaporator, water may condense and/or freeze on a coil of the heat exchanger of the outdoor unit, which lowers activity and efficiency of the HVAC system. Further, the outdoor unit may be susceptible to fouled coils, regardless of the operating mode. For example, dust, pollen, grass, and/or other contaminants surrounding the outdoor unit may blow across the coils. These contaminants may block the air flow across the coils, thereby increasing a pressure drop across the coils, which reduces an efficiency of the HVAC system. Accordingly, detection and, in certain embodiments, mitigation of outdoor coil conditions (e.g., frozen coil and fouled coil) is desired. 
     SUMMARY 
     The present disclosure relates to a heating, ventilation, and air conditioning (HVAC) system. The HVAC system includes an outdoor unit having a heat exchanger. The heat exchanger includes a coil configured to route refrigerant therethrough. The HVAC system also includes a motor configured to drive a fan, a motor controller configured to regulate operation of the motor, and a global controller configured to regulate operation of global aspects of the HVAC system. The HVAC system also includes a power sensor configured to detect a power parameter relating to a power input to the motor controller, a power output from the motor controller, or a power input to the motor, wherein the global controller is configured to receive data indicative of the power parameter from the power sensor, and wherein the global controller is configured to analyze the data indicative of the power parameter to detect a frozen coil condition, a fouled coil condition, or both. 
     The present disclosure also relates to a method of detecting coil conditions of a heat exchanger of an outdoor unit of an HVAC system. The method includes detecting, via a power sensor, a power parameter of a power input to a motor controller, a power output from the motor controller, or a power input to a fan motor. The method also includes receiving, at a global controller and from the power sensor, data indicative of the power parameter. The method also includes determining, via the global controller, that the power parameter exceeds a threshold power parameter. The method also includes outputting, via the global controller and to an output device, a notification indicating a flow restriction across a coil of the heat exchanger of the outdoor unit. 
     The present disclosure also relates to an HVAC system including an outdoor unit having a heat exchanger, where the heat exchanger includes a coil configured to receive a refrigerant. The HVAC system also includes a constant RPM motor configured to drive a fan that blows air across, or pulls air across, the coil of the heat exchanger. The HVAC system also includes a control system communicatively coupled with the constant RPM motor and configured to regulate operation of the constant RPM motor. The HVAC system also includes a power sensor configured to detect a power parameter relating to a power input to the control system, a power output from the control system, or a power input to the constant RPM motor, where the control system is configured to receive data indicative of the power parameter from the power sensor, and where the control system is configured to analyze the data indicative of the power parameter to detect a flow restriction across the coil. 
    
    
     
       DRAWINGS 
         FIG. 1  is an illustration of an embodiment of a commercial or industrial HVAC system, in accordance with the present techniques; 
         FIG. 2  is an illustration of an embodiment of a packaged unit of the HVAC system shown in  FIG. 1 , in accordance with the present techniques; 
         FIG. 3  is an illustration of an embodiment of a split system of the HVAC system shown in  FIG. 1 , in accordance with the present techniques; 
         FIG. 4  is a schematic diagram of an embodiment of a refrigeration system of the HVAC system shown in  FIG. 1 , in accordance with the present techniques; 
         FIG. 5  is a schematic diagram of an embodiment of an air flow system of an outdoor unit, including a coil condition detection sub-system, in accordance with the present techniques; and 
         FIG. 6  is a process flow diagram illustrating an embodiment of a process of detecting a coil condition of an outdoor unit of an HVAC system, in accordance with the present techniques. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is directed toward heat exchangers of a commercial, industrial, or residential heating, ventilation, air conditioning, and refrigerant system (“HVAC system”). More particularly, the present disclosure is directed toward detection of conditions of a heat exchanger of an outdoor unit in the HVAC system. 
     For example, HVAC systems in accordance with the present disclosure may include an outdoor unit having a heat exchanger that acts as a condenser when the HVAC system operates in an air conditioning mode, and an evaporator when the HVAC system operates in a heat pump mode. As air is blown or pulled by a fan across the heat exchanger over time, dust, pollen, grass, and other contaminants may gather on and foul a coil of the heat exchanger (i.e., at least partially block air flow passages across the coil). As the coil is fouled, a pressure drop across the coil of the heat exchanger increases. The fan causing the air flow over the coil may be coupled to a constant revolutions-per-minute (RPM) motor, which drives the fan. As the pressure drop increases, a power input to the fan (and, thus, a power output from [and power input to] a component powering the fan, such as a motor controller) may be increased in order to maintain the RPM of the motor, where the power increase reduces an efficiency of the HVAC system. It should be noted that, while constant RPM motors generally included different RPM settings, the constant RPM motor is configured to maintain a desired RPM at any given time. In other words, the RPM set point may be changed based on operating modes and conditions, but during a particular time period (e.g., having a particular operating mode at particular operating conditions), the constant RPM motor is configured to maintain a certain RPM. 
     In addition to the fouled coil condition described above, the coil of the heat exchanger of the outdoor system may be susceptible to a frozen coil condition when the HVAC operates in the heat pump mode (i.e., such that the heat exchanger of the outdoor unit acts as the evaporator). For example, when the heat exchanger of the outdoor system acts as the evaporator, and when an outdoor temperature proximate the outdoor unit is near, at, or below freezing, water may condense and freeze on the coil of the heat exchanger. Frozen coil conditions restrict an air flow through the coil, thereby increasing a pressure drop across the coil and decreasing an efficiency of the HVAC system (e.g., similar to the fouled coil condition described above). Frozen coil conditions may also impact refrigerant flow through the coil. 
     In accordance with present techniques, a motor controller may control operation of the constant RPM motor driving the fan. The motor controller is separate from the motor, and in some embodiments the motor controller may be incorporated with a variable speed drive (“VSD”). A power sensor may sample, detect, or otherwise monitor a power input to the VSD and corresponding motor controller (e.g., from line voltage), a power output from the motor controller (e.g., toward the constant RPM motor), or a power input to the constant RPM motor (e.g., from the motor controller). For example, the power sensor may detect a voltage, a current, a resistance, a torque, a frequency, a wattage, or some other power parameter. Further, the power sensor may be integrated with, or disposed on or proximate to, the motor controller and/or the constant RPM motor. 
     A global controller of the HVAC system (e.g., where the global controller regulates operation of several or all the HVAC components) may receive sensor feedback from the power sensor. After analyzing the sensor feedback, the global controller or may determine that the power parameter has deviated too far from a set point of the power parameter (e.g., beyond a certain threshold), where the set point of the power parameter corresponds with the aforementioned desired RPM of the constant RPM motor during normal operating conditions in which the coil is not substantially fouled or frozen. The power parameter exceeding the threshold (e.g., where the threshold is set relative to the set point) indicates a flow restriction across the coil. Thus, if the controller determines that the power parameter has exceeded the threshold, the global controller may infer the flow restriction across the coil. 
     In accordance with the present techniques, an air flow sensor and/or a temperature sensor may be incorporated at, in, or proximate to, the heat exchanger of the outdoor unit. The air flow sensor and the temperature sensor may communicate air flow (e.g., pressure, mass flow rate, etc.) parameters and temperature data, respectively, to the motor controller global controller. Thus, the data from the air flow sensor and/or temperature sensor may be analyzed to differentiate between the fouled coil condition and the frozen coil condition, respectively. The global controller may trigger a fouled coil mitigation mode or a frozen coil mitigation mode in response to the differentiation therebetween. These and other features will be described in detail below with reference to the drawings. 
     Turning now to the drawings,  FIG. 1  illustrates a heating, ventilating, and air conditioning (HVAC) system for building environmental management that may employ one or more HVAC units. 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 . For example, the ductwork  14  may extend to various individual floors or other sections of the building  10 . 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 into “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 (for example, R- 410 A, steam, or water) 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 rooftop 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. As may be appreciated, 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 being 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, not shown) 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 (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 (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 outdoor the 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  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 (that is, separate from heat exchanger  62 ), such that air directed by the blower  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, in accordance with present techniques, an outdoor air flow restriction detection system, which determines air flow restriction across a heat exchanger of an outdoor unit by analyzing a power input to a fan motor (or power output from a controller powering the fan motor, or power input to the motor controller powering the fan [e.g., from line voltage]), may be incorporated in any of the systems illustrated in  FIGS. 1-4 . The outdoor air flow restriction detection system will be described in detail below with reference to  FIGS. 5-7 . 
       FIG. 5  is a schematic diagram of an embodiment of an air flow system  100  of an outdoor unit  58 , including a coil condition detection sub-system, in accordance with the present techniques. As described with respect to previous drawings, the outdoor unit  58  may include, as shown in the illustrated embodiment, the heat exchanger  60  and the fan  64 . The fan  64  in the illustrated embodiment blows the environmental air  96  across a coil  101  of the heat exchanger  60 . In other embodiments, the fan  64  may instead pull the environmental air  96  across the coil  101  of the heat exchanger  60  toward the fan  64 . The refrigerant conduits  54  may route refrigerant to, and from, the heat exchanger  60  of the outdoor unit  58 . As previously described, the heat exchanger  60  may act as a condenser during an air conditioning mode. The heat exchanger  60  may act as an evaporator during a heat pump mode. 
     The air flow system  100  also includes a fan motor  102 , a VSD  104  separate from the fan motor  102 , a dedicated motor controller  106  integrated with the VSD  104 , and the previously described controller  82  (e.g., global controller). The global controller  82  is configured to regulate operation of several aspects (e.g., global aspects) of the HVAC system, as will be appreciated in view of the description below, while the motor controller  106  is configured to operate to control (and power) the fan motor  102 . The global controller  82  and the motor controller  106  form at least a portion of a control system  108  of the HVAC system. 
     The air flow system  100  in the illustrated embodiment also includes a power sensor  110  configured to detect a power parameter (e.g., voltage, current, resistance, or torque, frequency, wattage) of the power output from (or power input to) the motor controller  106  or a power input to the motor  102 . In the illustrated embodiment, the power sensor  110  operates to detect a power parameter of the power input to the motor  102 . However, the power sensor  110  may, in another embodiment, be disposed at or proximate to (or within an internal circuitry of) the motor controller  106 , such that the power sensor  110  detects a power parameter of the power output from (or power input to) the motor controller  106 . In some embodiments, the power sensor  110  may detect a power input to the motor controller  106 , or more generally a power input to the VSD  104  having the motor controller  106  (e.g., where the motor controller  106  utilizes the power input to the VSD  104  to power the motor  102 ). 
     The fan motor  102  in the illustrated embodiment is a constant revolutions-per-minute (RPM) motor. Thus, the fan motor  102  maintains a desired RPM that corresponds with an operating mode of the outdoor unit  58  and operating conditions surrounding the outdoor unit  58 . In other words, the desired RPM corresponds with normal operating conditions. It should be noted that the desired RPM may change with the desired operating mode, but that for a given operating mode, a desired RPM is maintained by the constant RPM motor  102 . 
     As the fan  64  blows or pulls the environmental air  96  across the coil  101  of the heat exchanger  60  over time, contaminants (e.g., dust, pollen, grass, dirt, etc.) may foul the coil  101  of the heat exchanger  60 . As the coil  101  is fouled, a pressure drop across the coil  101  increases. 
     Further, when the HVAC system operates in a heat pump mode such that the heat exchanger  60  operates as the evaporator of the HVAC system, the coil  101  may condense water thereon if the outdoor temperature is near, at, or below freezing. As the water condenses and at least partially freezes on the coil  101 , a pressure drop across the coil  101  increases (and, in some embodiments, movement of the refrigerant through the refrigerant conduits  54  and the coil  101  is impacted). 
     As the pressure drop across the coil  101  increases due to the frozen coil condition and/or the fouled coil condition described above, the motor  102  may increase its power input in order to maintain the previously described desired RPM. In other words, as the pressure drop across the coil  101  increases, a back pressure on the fan  64  may cause the motor  102  to draw more power from the motor controller  106  in order to maintain the desired RPM corresponding with the operating mode and normal operating conditions. The power sensor  110  detects (e.g., samples) a power parameter corresponding with the power input to the constant RPM fan motor  102  (or power input to or output from the motor controller  106 ), and conveys data indicative of the power parameter to the global controller  82 . The global controller  82  may compare the data received from the power sensor  110  with a set point of the power parameter, where the set point corresponds with an amount of power needed to achieve the aforementioned desired RPM of the motor  102  during normal operating conditions. In particular, the global controller  82  may detect a flow restriction condition if the power parameter exceeds a power threshold limit. In some embodiments, the power threshold limit may be a percentage of the set point (e.g., if the threshold limit is 125% of the set point, the flow restriction condition is determined when the detected power parameter is 125% of the set point). 
     In some embodiments, analysis of the power parameter over time may be adequate to differentiate between a frozen coil condition and a fouled coil condition, both of which being encompassed by the aforementioned flow restriction condition. For example, if the power parameter changes rapidly or within a particular time period following activation of the heat pump mode of the HVAC system, the frozen coil condition may be determined by the global controller  82 . Further, if the global controller  82  is equipped with a digital clock having date stamp features, the global controller  82  may infer either the frozen coil condition or the fouled coil condition on the basis of time-related information, such as time of day, season, etc. 
     In some embodiments, additional sensor feedback may be utilized to differentiate between the frozen coil condition and the fouled coil condition. Differentiating between the two may be advantageous because mitigation techniques of the frozen coil condition may differ from mitigation techniques of the fouled coil condition, as will be appreciated in view of the description below. In the illustrated embodiment, the outdoor unit  58  may be optionally equipped with a temperature sensor  112  disposed at, in, or proximate to the heat exchanger  60 , and/or an air flow sensor  114  (or series of air flow sensors) disposed at, in, or proximate to the heat exchanger  60 . The temperature sensor  112  may be configured to detect a temperature of the coil  101  of the heat exchanger  60 , and may communicate data indicative of the temperature to the global controller  82 . The air flow sensor  114  (or series of air flow sensors) may be configured to detect an air flow parameter (e.g., pressure, mass flow rate, etc.) adjacent the coil  101 , and may communicate data indicative of the air flow parameter to the global controller  82 . However, other types of sensors may also be used to diagnose flow restriction conditions (e.g., to determine the frozen coil or fouled coil condition). For example, a refrigerant pressure transducer may be utilized, a photo-optic sensor may be utilized, and/or other sensors may be utilized to facilitate diagnosis of certain flow restriction conditions. 
     The global controller  82  may analyze the temperature data, the air flow data, or both to determine whether the flow restriction is caused by the frozen coil condition or the fouled coil condition. For example, if the global controller  82  determines that the temperature data is lower than a threshold temperature, the global controller  82  may determine the frozen coil condition. If the global controller  82  determines that the temperature data is higher than the threshold temperature, the global controller  82  may exclude a possibility the frozen coil condition, and determine the fouled coil condition. In some embodiments, the global controller  82  may consider two threshold temperatures, namely, an upper threshold temperature and a lower threshold temperature that is less than the upper threshold temperature. If the detected temperature exceeds the upper threshold temperature, the global controller  82  may determine the fouled coil condition. If the detected temperature is less than the lower threshold temperature, the global controller  82  may determine the frozen coil condition. If the detected temperature is between the upper threshold temperature and the lower threshold temperature, the global controller  82  may consider additional sensor feedback of non-temperature related parameters to determine either the frozen coil condition or the fouled coil condition (or, alternatively, communicate a notification to an output device  120  indicating a general flow restriction condition). 
     In some embodiments, analysis of air flow data may enable the global controller  82  to differentiate between the frozen coil condition and the fouled coil condition. For example, in some embodiments, the air flow sensor  114  may correspond with a series of air flow sensors that sample air flow data at various points across the coil  101 . By analyzing data from the series of air flow sensors with respect to the locations of the corresponding sensors, the global controller  82  may determine whether the flow restriction is caused by the frozen coil condition or the fouled coil condition. 
     The global controller  82  may communicate a notification to an output device  120  (e.g., output interface on the HVAC system, a portable electronic device, a thermostat, etc.) upon determining a flow restriction condition. For example, in some embodiments, the global controller  82  may be communicatively coupled with a network  122  (e.g., the Internet), which enables communication with the output device  120  (e.g., in embodiments where the output device  120  is remote to the HVAC system). The notification may indicate generally a flow restriction condition, or the notification may indicate specifically a flow restriction condition caused by either the frozen coil condition or the fouled coil condition. 
     Additionally, in some embodiments, the global controller  82  may operate, in response to determining the specific cause (e.g., frozen coil or fouled coil) of the flow restriction, to mitigate the specific cause (e.g., frozen coil or fouled coil). For example, the global controller  82  may instruct a defrost mode in which flow of the refrigerant through the refrigerant conduit  54  and the coil  101  is reversed, thereby at least temporarily ending the heat pump mode, in response to detecting the frozen coil condition. In doing so, the global controller  82  may cause the refrigerant to thaw the frozen coil condition. Alternatively, if the global controller  82  determines the fouled coil condition, the global controller  82  may instruct a blow-out mode, in which the fan  64  is controlled to reverse flow direction (e.g., such that the environmental air  96  is pulled toward the fan  64  in the illustrated embodiment). It should be noted that, in embodiments where the fan  64  is configured to pull the environmental air  96  through the heat exchanger  60  and toward the fan  64  in normal operating modes, the blow-out mode would involve reversing the flow direction of the fan  64  such that the environmental air  96  is blown by the fan  64  toward the heat exchanger  60  in the blow-out mode. 
       FIG. 6  is a process flow diagram illustrating an embodiment of a process  150  of detecting a coil condition of a heat exchanger of an outdoor unit of an HVAC system, in accordance with the present techniques. The process  150  includes detecting (block  152 ) a power parameter of a power input to a fan motor of the HVAC system, or of a power output from a motor controller toward the fan motor (or, in some embodiments, a power input to the motor controller [e.g., from line voltage]). For example, line voltage may power a motor controller, the motor controller may power the motor, and a power sensor at, on, or proximate to the motor controller or the motor may detect the various power inputs/outputs. 
     The process  150  also includes comparing (block  154 ) the power parameter with a threshold. For example, a global controller may receive the power parameter detected by the power sensor, and compare the power parameter against the threshold power parameter. If the detected power parameter does not exceed the threshold (block  156 ), no flow restriction condition is detected and the process begins again at the power parameter detection step (block  152 ). If the power parameter exceeds the threshold (block  158 ), a flow restriction condition may be detected. 
     As previously described, the global controller may send (block  160 ) a notification to an output device indicating the general flow restriction condition (e.g., in response to determining the flow restriction condition). As previously described, in some embodiments, the global controller may determine that the flow restriction condition is either a frozen coil condition or a fouled coil condition without additional feedback. For example, the global controller may infer the fouled coil condition or the frozen coil condition by analyzing time/date related information (e.g., seasonal information, time of day information, etc.). In such embodiments, the global controller may send the notification indicating the specific type of flow restriction condition (i.e., fouled coil condition or frozen coil condition). 
     In other embodiments, additional sensor feedback may be considered to determine either the fouled coil condition or the frozen coil condition. For example, the illustrated process  150  also includes detecting (block  162 ) a temperature at, in, of, or proximate to the coil of the outdoor heat exchanger. The temperature then may be compared, by the global controller, with a threshold temperature. If the detected temperature is lower than the threshold temperature (block  166 ), the global controller determines the frozen coil condition and sends (block  168 ) a notification to the output device indicating the frozen coil condition. In the illustrated embodiment, the global controller also activates (block  170 ) a defrost mode (e.g., by discontinuing a heat pump mode and/or reversing flow of the refrigerant through the HVAC system). 
     If the detected temperature is not lower than the threshold temperature (block  172 ), the global controller may exclude the possibility of the frozen coil condition and, instead, determine the fouled coil condition. Thus, the global controller may send (block  174 ) a notification to the output device indicating the fouled coil condition. In the illustrated embodiment, the global controller also activates (block  176 ) a blow-out mode (e.g., by instructing the fan to reverse direction of an air flow therethrough. 
     It should be noted that, in another embodiment, the detected temperature may be compared with two separate temperature thresholds. For example, if the detected temperature is higher than an upper temperature threshold, the global controller may determine the fouled coil condition. If the detected temperature is under a lower temperature threshold, the global controller may determine the frozen coil condition. If the detected temperature is between the upper temperature threshold and the lower temperature threshold, the global controller may utilize other sensor feedback (e.g., air flow sensor feedback, as previously described) to determine a source of the flow restriction, or the global controller may only communicate to the output device the notification indicating the general flow restriction. 
     One or more of the disclosed embodiments, alone or in combination, may enable detection of flow restrictions across a coil of a heat exchanger in an outdoor unit of an HVAC system via analysis of changes in a power input to a fan motor blowing/pulling air over the coil, or via analysis of changes in a power output from (or power input to) a controller powering the fan motor. In some embodiments, fouled coil conditions and/or frozen coil conditions may be determined only through analysis of power parameters relating to the aforementioned power input to the fan motor or power output from (or input to) the controller. Presently disclosed techniques reduce processing power for determining flow restrictions across the coil, and improve user accessibility to flow restriction information. 
     While only certain features and embodiments of the present disclosure have been illustrated and described, many modifications and changes may occur to those skilled in the art (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (e.g., temperatures, pressures, etc.), mounting arrangements, use of materials, colors, orientations, etc.) 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 (i.e., those unrelated to the presently contemplated best mode of carrying out an embodiment, or those unrelated to enabling the claimed embodiments). It should be appreciated 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.