Patent Publication Number: US-11378316-B2

Title: Diagnostic mode of operation to detect refrigerant leaks in a refrigeration circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 15/876,408, filed Jan. 22, 2018, entitled “DIAGNOSTIC MODE OF OPERATION TO DETECT REFRIGERANT LEAKS IN A REFRIGERATION CIRCUIT,” which claims priority from and the benefit of U.S. Provisional Application No. 62/593,578, entitled “DIAGNOSTIC MODE OF OPERATION TO DETECT REFRIGERANT LEAKS IN A REFRIGERATION CIRCUIT,” filed Dec. 1, 2017, which are hereby incorporated by reference in their entirety for all purposes. 
    
    
     BACKGROUND 
     The present disclosure relates generally to heating, ventilating, and air conditioning (HVAC) systems, and more particularly to refrigerant leak detection for HVAC systems. 
     A wide range of applications exist for HVAC systems. For example, residential, light commercial, commercial, and industrial HVAC systems are used to control temperatures and air quality in residences and buildings. Generally, the HVAC systems may circulate a refrigerant through a closed refrigeration circuit between an evaporator, where the refrigerant absorbs heat, and a condenser, where the refrigerant releases heat. The refrigerant flowing within the refrigeration circuit 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 refrigerant. As such, the refrigerant flowing within an HVAC system travels through multiple conduits and components of the refrigeration circuit. Inasmuch as refrigerant leaks compromise system performance or result in increased costs, it is accordingly desirable to provide detection and response systems and methods for the HVAC system to reliably detect and respond to any refrigerant leaks of the HVAC system. 
     SUMMARY 
     The present disclosure relates to a refrigeration circuit that includes a controller that is communicatively coupled to a compressor, an expansion valve, and a sensor of the refrigeration circuit. The controller may activate the compressor and actuate the expansion valve such that the compressor is active while the expansion valve is closed. The controller may also measure a pressure of a refrigerant in the refrigeration circuit using the sensor at least while the compressor is active and the expansion valve is closed. Additionally, the controller may determine whether a refrigerant leak exists based on a time difference between a first time associated with the compressor being active while the expansion valve is closed and a second time associated with the measured pressure falling below a threshold value. 
     The present disclosure also relates to a heating, ventilating, and air conditioning (HVAC) unit that includes a refrigeration circuit that has a compressor and an expansion valve. The HVAC unit also includes a sensor configured to measure a pressure of the refrigerant within the refrigeration circuit. Additionally, the HVAC unit includes a controller configured to activate the compressor with the expansion valve closed and to obtain a measure of the pressure of the refrigerant from the sensor. Additionally, the controller is configured to determine whether a refrigerant leak exists based on a time difference between a time associated with activation of the compressor and a subsequent time associated with the pressure falling below a threshold value. 
     The present disclosure further relates to a diagnostic mode method of operation of a heating, ventilating, and air conditioning (HVAC) unit having a refrigeration circuit. The diagnostic mode method includes activating a compressor and closing an expansion valve of the refrigeration circuit such that the compressor is active and the expansion valve is closed during a timeframe. The diagnostic mode method also includes measuring, during the timeframe, a pressure of the refrigerant in the refrigeration circuit using a sensor of the refrigeration circuit that is positioned downstream of the expansion valve and upstream of an evaporator of the refrigeration circuit. Additionally, the diagnostic mode method includes determining a pump-down time as a difference between a beginning of the timeframe and a time associated with reaching a threshold value for the pressure. Furthermore, the diagnostic mode method includes determining whether a refrigerant leak exists based on a comparison between the determined pump-down time and an expected pump-down down. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a packaged or commercial heating, ventilating, and air conditioning (HVAC) system for building environmental management, in accordance with embodiments described herein; 
         FIG. 2  is a perspective view of the HVAC unit of  FIG. 1 , in accordance with embodiments described herein; 
         FIG. 3  is a perspective view of a residential HVAC system, in accordance with embodiments described herein; 
         FIG. 4  is a schematic diagram of a vapor compression system that may be used in the HVAC system of  FIG. 1  and the residential HVAC system of  FIG. 3 , in accordance with embodiments described herein; 
         FIG. 5  is a schematic diagram of the HVAC unit of  FIG. 2 , in accordance with embodiments described herein; and 
         FIG. 6  is a flow diagram of a process for determining whether a refrigerant leak exists in the HVAC unit, in accordance with embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is generally directed to detecting refrigerant leaks in HVAC systems. As discussed above, an HVAC system generally includes a refrigerant flowing within a refrigeration circuit. The refrigerant flows through multiple conduits and components while undergoing phase changes to enable the HVAC system to condition an interior space of a structure. In certain embodiments, the refrigerant may include R-32, R-452B, R-134A, R-447A, R-455A, R-1234ze, R-1234yf, R-454A, R-454C, and R-454B. The present techniques enable HVAC systems to reliably detect and manage refrigerant leaks. 
     More specifically, the present disclosure relates to a diagnostic mode that may be implemented by an HVAC system to determine a volume of refrigerant within the HVAC system and, thereby, determine whether a refrigerant leak exists. As set forth below, present embodiments generally involve particular control strategies in which an HVAC controller provides suitable control signals to instruct a compressor of an HVAC unit to drive refrigerant into an outdoor portion of a refrigeration circuit, such as a condenser of the refrigeration circuit, during an off-cycle of a HVAC unit. As discussed below, the disclosed HVAC unit includes at least one high-sensitivity pressure sensor that is capable of measuring pressures within the refrigeration circuit that are substantially lower than those typically encountered during active operation of the HVAC unit. Using pressure data collected from such a sensor, the HVAC controller monitors an amount of time that the compressor operates to drive the refrigerant into the outdoor portion of the HVAC unit. Specifically, when the pressure in a portion of the refrigeration circuit falls below a threshold pressure value, the refrigerant is determined to have been driven to the outdoor portion of the HVAC unit. The HVAC controller subsequently compares this amount of time to an expected amount of time to determine whether a refrigerant leak is present in the refrigeration circuit. Furthermore, the HVAC controller can determine the expected amount of time based on previous iterations of the diagnostic mode, as well as data acquired by sensors of the HVAC unit. In other words, the expected amount of time may be updated by the HVAC unit and vary between iterations of the diagnostic mode. Accordingly the presently disclosed control strategy effectively enables the HVAC controller to determine whether a refrigerant leak exists. 
     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-410A, 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  80  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. 
     As discussed above, HVAC units such as HVAC unit  12  may include a refrigerant that is used to condition air before the conditioned air is sent to a conditioned space, such as a conditioned interior space of building  10 . For instance, the refrigerant may be a refrigerant or be a mixture of one or more compounds. As discussed below, a controller may cause the HVAC unit  12  to undergo a diagnostic mode, which is a refrigerant leak testing mode used to determine whether a refrigerant leak exists. For instance, normal operation of the HVAC unit  12  may be stopped, and the refrigerant disposed within a refrigeration circuit of the HVAC unit  12  may be pumped into an outdoor portion of a refrigeration circuit. Based partly on pressure values sensed by a pressure sensor, the control panel  82  determines how long the compressor  74  is operated with the expansion valve  78  closed to drive the refrigerant into the outdoor portion of the refrigeration circuit, and compares this amount of time to an expected amount of time to determine whether a refrigerant leak is present. 
     As used herein, “pump-down time” refers to the amount of time that the compressor  74  is operated with the expansion valve  78  closed to pump the refrigerant into an outdoor portion of the vapor compression system  72 . As discussed below, pump-down times may be determined by finding the difference between a first time (associated with the expansion valve  78  being closed and the compressor  74  being activated to initiate the pump-down process) and a second time (associated with a measured pressure value falling below a predetermined pressure value). Additionally, using data collected during execution of the diagnostic mode, the control panel  82  may determine and update the expected amount of time to account for variations between subsequent executions of the diagnostic mode, such as ambient temperature variations, ambient pressure variations, and aging of the HVAC system. 
     With the foregoing discussion in mind,  FIG. 5  is a schematic diagram of an embodiment of the HVAC unit  12 . The HVAC unit  12  is capable of implementing a diagnostic mode in which processor  86  of the control panel  82  determines whether there is a refrigerant leak in the vapor compression system  72 . While the discussion of the diagnostic mode is discussed in relation to the HVAC unit  12 , it should be noted that the diagnostic mode may be implemented in HVAC systems that include other types of HVAC units. For example, the diagnostic mode may be implemented in the residential heating and cooling system  50  of  FIG. 3 . Furthermore, the diagnostic mode may be implemented by processor  86  executing instructions stored on the memory  88 . For example, the memory  88  may include a profile  100  and instructions for implementing the diagnostic mode, as discussed in detail below. 
     During the diagnostic mode, refrigerant in the vapor compression system  72  is pumped into and stored within the condenser  76 . More specifically, the processor  86  sends signals to close the expansion valve  78  of the vapor compression system  72  and sends signals to the compressor  74  to begin pumping the refrigerant into the condenser  76 . For example, the expansion valve  78  may be an electronic expansion valve that can be controlled via the processor  86 , and the processor  86  may send a signal that causes the expansion valve  78  to close so that the refrigerant cannot flow from the condenser  76  to the evaporator  80 . With the expansion valve  78  closed, the compressor  74  may begin to pump the refrigerant into the condenser  76 . Moreover, it should be noted that the illustrated processor  86  may be representative of multiple processors, in certain embodiments. 
     The diagnostic mode may be executed by the control panel  82  when the HVAC unit  12  is not supplying conditioned air to a conditioned space. For instance, in an on-cycle, the HVAC unit generally supplies heated or cooled air to a conditioned space, whereas in an off-cycle, the HVAC unit does not generally supply conditioned air to the conditioned space. The disclosed diagnostic mode is executed when the HVAC unit is in an off-cycle. Additionally, the processor  86  may cause the HVAC unit  12  to enter the off-cycle, which may include deactivating fans associated with the condenser  76  and evaporator  80  and deactivating the compressor  74 , before executing the diagnostic mode. 
     Continuing with the discussion of the diagnostic mode, sensor data may be utilized by the processor  86  to determine whether a refrigerant leak is present. As illustrated, the HVAC unit  12  includes sensors  102  that are communicatively coupled to the processor  86 . The sensors  102  may collect data regarding the refrigerant in the vapor compression system  72 . For example, the data collected by the sensors  102  may be indicative of a pressure, temperature, or other characteristic of the refrigerant at various points within the refrigeration circuit. In certain embodiments, one or more of the sensors  102  may provide collect data regarding the refrigerant when the HVAC unit  12  is active, such as during an on-cycle, or when the HVAC unit  12  is in diagnostic mode or a combination thereof. 
     For instance, in the illustrated embodiment, sensor  102 A measures a pressure of the refrigerant between the expansion valve  78  and the evaporator  80 . However, rather than being a relatively high pressure sensor designed to measure pressures of the refrigerant of the HVAC unit  12  during an on-cycle, the illustrated sensor  102 A is a high-sensitivity, low pressure sensor that can accurately measure low pressures and/or negative pressures, such as pressures near or below atmospheric pressure. For example, as mentioned, during performance of the diagnostic mode, the expansion valve  78  is closed and refrigerant is pumped into the condenser  76  and allowed to accumulate therein. As the refrigerant is pumped into the condenser  76 , the pressure measured by the sensor  102 A decreases. When a substantial portion, such as 95% or more, of the refrigerant has been pumped into the condenser  76 , the pressure measured by the sensor  102 A is substantially lower in comparison to pressures typically present during normal operation of the HVAC unit  12 . For example, pressures measured by the sensor  102 A may be substantially lower than typical pressures that occur during active operation of the HVAC unit  12 . More specifically, the pressures measured by the sensor  102 A may be less than 10% of pressures experienced during active operation of the HVAC unit  12  and/or below atmospheric pressure. Indeed, in some cases, data collected by the sensor  102 A during normal, on-cycle operation may not accurately reflect a pressure within the refrigeration circuit as it would be beyond the functional pressure range of the sensor  102 A. That is, the sensor  102 A may be used specifically to detect low pressures that are present within portions of the vapor compression system  72  during the diagnostic mode, and therefore may not provide any meaningful measurements during normal, on-cycle operation of the HVAC unit  12 . 
     Based on data received from the sensor  102 A, the processor  86  determines a pressure within the refrigeration circuit between the expansion valve  78  and the evaporator  80  during diagnostic mode operation. The profile  100  stored within memory  88  may include data pertaining to a predetermined pressure value, and the processor  86  may determine whether data from the sensor  102  is lower than the predetermined pressure value. For instance, the predetermined pressure value may correspond to a minimum target pressure that should occur between the expansion valve  78  and the evaporator  80  when substantially all of the refrigerant has accumulated in the condenser  76 . Thus, when the processor  86  determines that the data from sensor  102  is indicative of a pressure that is lower than the predetermined pressure value, then the processor  86  may determine that substantially all of the refrigerant has accumulated in the condenser  76 . 
     In the event of a refrigerant leak, it may be the case that the pressure determined based on the data from the sensor  102 A does not reach a value lower than the predetermined pressure value. For instance, an opening from which refrigerant can leak may form in the refrigeration circuit, and air may enter the opening, causing the pressure measured by the sensor  102 A to stay above the predetermined pressure value. As another example, the pressure may not fall below the predetermined pressure value due to a faulty expansion valve  78 . For instance, the expansion valve  78  may not sufficiently close to stop refrigerant from traversing the valve  78  and returning to an indoor portion of the vapor compression system  72 . The profile  100  may include data regarding a maximum measurement time, which is a maximum amount of time that the compressor  74  should operate with the expansion valve  78  closed before which the pressure indicated by sensor  102 A should fall below the predetermined pressure value. For instance, a time corresponding to when the processor  86  sends a signal to activate the compressor  74  to begin pumping refrigerant into the condenser  76  may be stored in the memory  88 . When a second, later time is reached without the data from the sensor  102 A having been determined to be indicative of a pressure below the predetermined pressure value, the processor  86  may determine that the maximum measurement time has been exceeded, which may be one indication that a refrigerant leak exists. 
     While the amount of time spent pumping the refrigerant into the condenser  76  may be based on pressure data solely acquired by the sensor  102 A, in other embodiments, data from the sensors  102 B and  102 C may also be used. For example, the processor  86  may determine that substantially all of the refrigerant has been pumped into the condenser  76  when data from two or more of the sensors  102  are indicative of predetermined pressure values. More specifically, the profile  100  may include predetermined pressure values, such as minimum and/or maximum pressure thresholds, that are associated with the sensors  102 B and  102 C, in addition to the sensor  102 A. Furthermore, time differences that are explained in relation to data acquired by the sensor  102 A may also be determined based on data from sensors  102 B and  102 C, and the sensors  102 B and  102 C may have values associated with those time differences stored in the memory  88  and/or determinable by the processor  86 . The time differences may be expected pump-down times. Moreover, while the illustrated embodiment includes three sensors  102 , other embodiments may include a single sensor  102 , two sensors  102 , or more than three sensors  102 . 
     However, at any point in time before the maximum amount of time has occurred, the processor  86  may process the data from the sensor  102 A to determine whether the pressure indicated by the sensor  102 A is below the predetermined pressure value. When the pressure drops below the predetermined pressure value, the processor  86  may determine a time difference between sending the signal to activate the compressor  74  and the pressure dropping below the predetermined pressure value. In other words, the time difference corresponds to an amount of time that passes between a first time associated with when the compressor  74  begins to pump the refrigerant into the condenser  76  and a second time associated with when substantially all of the refrigerant is located in the condenser  76 . 
     The processor  86  may determine whether a leak is present based on the time difference. More specifically, the processor  86  may determine whether a leak is present by comparing the time difference to an expected pump-down time, which an expected amount of time to pass between when the compressor  74  begins to pump the refrigerant into the condenser  76  and when substantially all of the refrigerant is located in the condenser  76 . The expected pump-down time is determined by the processor  86  based on previous iterations of the diagnostic mode as well as air temperature values. The expected pump-down time may also be determined based on other factors such as a rate at which the compressor  74  can pump the refrigerant into the condenser  76  and an amount of refrigerant, such as a volume of refrigerant expected to be present in the refrigeration circuit. For instance, the volume of refrigerant expected to be present in the refrigeration circuit may be a volume of refrigerant originally put into the refrigeration circuit before the HVAC unit  12  began supplying conditioned air to the building  10 . Additionally, the profile  100  may include an initial value of the expected pump-down time that can be modified by the processor  86  based on one or more air temperature values sensed by one or more sensors  104  that are communicatively coupled to the processor  86 . That is, the sensors  104  are capable of sending signals to the processor  86  regarding detected air temperatures. More specifically, sensor  104 A may collect data regarding a temperature of environmental air  96 , sensor  104 B may collect data regarding a temperature of return air  106 , and sensor  104 C may measure a temperature of air within the HVAC unit  12  near a component of the vapor compression system  72 , where “near” means within approximately 6 feet. In some embodiments, the sensor  104 C may be directly coupled to a component of the vapor compression system  72 . 
     It is presently recognized that air temperature within the HVAC unit  12  and outside of the HVAC unit  12  can affect properties of the refrigerant, which can cause differences in the amount of time required to pump substantially all of the refrigerant into the condenser  76 . For instance, as described by Gay-Lussac&#39;s Law (reproduced below), for a fixed volume, such as a volume within the refrigeration circuit of the vapor compression system  72 , an increase or decrease in temperature causes a corresponding increase or decrease in pressure, respectively. Additionally, changes in pressure can affect the amount of time it takes to pump the refrigerant into the condenser  76 . The processor  86  may receive the data from sensors  104  to determine various temperatures such as an environmental air temperature, a return air temperature, an air temperature near the refrigeration system. The processor  86  may determine an expected pump-down time based at least partially on one or more of the determined temperature values. For example, the profile  100  may include a default pump-down time that is applicable when the environmental air  96 , return air  106 , or air near the vapor compression system  72  is a specific temperature. Accordingly, the processor  86  may determine an expected pump-down time for temperatures other than the specific temperature associated with default pump-down time based on sensed temperature values. For instance, a pressure relative to a default pressure value stored on the memory  88  may be determined based on sensed temperatures values. A look-up table also included on the memory  88  may describe a pump-down times or deviations from a default pump-down time associated with the various detected temperatures and/or pressures determined based on the sensed values. Accordingly, the processor  86  may determine an expected amount pump-down time based on one or more temperature values determined based on the data collected from the sensors  104 . 
     
       
         
           
             
               
                 
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     Moreover, the expected pump-down time may be determined based on the previous iterations of the diagnostic mode. For instance, previous times required to pump the refrigerant into the condenser  76  may have been determined in the manner described above and stored on the memory  88 . Other data associated with previous executions of the diagnostic mode may also be stored in the memory  88 , such as temperatures indicated by the sensors  104 . The processor may then determine an expected pump-down time based on previously recorded pump-down times that occurred at the same or similar temperatures. For instance, one or more iterations of the diagnostic mode may be performed at the time of installation of the HVAC unit  12 . The pump-down times and temperatures indicated by the sensors  104  during execution of the diagnostic mode may be stored on the memory  88 , and the processor  86  may determine expected pump-down times based on the pump-down times and temperatures recorded in the memory  88  from the initial iteration(s) of the diagnostic mode. 
     The processor  86  may determine a leak is present when a measured pump-down time is less than the expected pump-down time. For instance, when a leak is present, less refrigerant is present in the refrigeration circuit. Due to the reduced amount of refrigerant, it would take less time to pump the refrigerant into the condenser  76 . In some embodiments, the processor may determine that a leak exists solely based on whether a determined pump-down time is less than the expected pump-down time. However, in other embodiments, the processor  86  may determine a refrigerant leak exists based on whether the determined pump-down time is a predetermined amount shorter than the expected pump-down time. For instance, the processor  86  may determine that a refrigerant leak has occurred when a determined pump-down time is less than a certain percentage of the expected pump-down time, such as 80%, 85%, 90%, 95%, or any other suitable percentage less than 100%. Additionally, as mentioned, the processor  86  may also determine that a leak of refrigerant exists when the pressure between the expansion valve  78  and the compressor  74  on the evaporator  80  side of the refrigeration circuit is unable to fall below the predetermined pressure value after operating the compressor  74  for a maximum measurement time. For instance, in the event an opening forms in the refrigeration circuit and air enters the opening, the measured pressure may not fall below the predetermined pressure value. However, in such a case, the maximum measurement time may be met or exceeded, and, in response, the processor  86  may determine that a refrigerant leak exists. 
     Utilizing the sensors  102 A and  102 B, the processor  86  may also determine whether the expansion valve  78  is defective and does not closed sufficiently to stop the flow of refrigerant into the evaporator  80 . That is, the processor  86  may determine whether refrigerant continues to pass from the condenser  76  to the evaporator  80  while the diagnostic mode is performed. More specifically, the processor  86  may determine pressure values based on data sensed by the sensors  102 A,  102 B, and/or  102 C to make such a determination. For example, pressure values associated with the sensor  102 A may initially decrease after the compressor  74  begins to pump the refrigerant into the condenser  76 , while pressure values associated with the sensor  102 B may increase. However, as pressure begins to build on the condenser side of the faulty expansion valve  78 , a small amount refrigerant may begin to traverse the valve  78 , preventing the pressure threshold from being reached. As such, in certain embodiments, when the maximum measurement time is reached without the pressure threshold being reached, the processor  86  may identify whether the issue is a refrigerant leak (e.g., indicated when at least one of the measured pressure values of either sensor  102 A or  102 B is substantially constant) or a faulty expansion valve (e.g., indicated when the measured pressure value of sensor  102 A increases while the measured pressure value of the sensor  102 B decreases by a corresponding amount). 
     Similarly, the processor  86  may determine whether certain types of leaks are present. For example, the processor  86  may determine whether refrigerant is leaking out of the refrigeration circuit or whether air is entering the refrigeration circuit during diagnostic mode operation. For instance, when the maximum measurement time has been met or exceeded without the pressure having fallen below the predetermined pressure value and pressure equilibria are not indicated by the sensors  102 A and  102 B, the processor  86  may determine that air is entering the refrigeration circuit. As another example, when a measured pump-down time is less than an expected pump-down time, the processor  86  may determine that the refrigerant is leaking out of the refrigeration circuit. Additionally, the sensor  102 C may be used instead of the sensor  102 B or in combination with the sensors  102 A and  102 B. 
     When the processor  86  determines that a refrigerant leak exists, the processor  86  may take several actions. For example, the processor  86  may send a signal to activate an alarm system in the building  10  and/or otherwise indicate that a refrigerant leak is present. For instance, the processor  86  may be send signals to a transceiver  108  that can transmit data to and receive data from a network  110 . Via the network  110 , the processor  86  may send signals to cause notifications and/or alarms on other electronic devices  112 , such as computers, tablets, and phones. For example, the electronic devices  112  may receive emails, phone calls, SMS messages, or other forms of notifications based on signals sent from the processor  86 . The notifications may also include the type of leak, such as whether a leak corresponds to air entering the refrigeration circuit or refrigerant exiting the refrigeration circuit. 
     Additionally, in the event a leak is determined to be present, the processor  86  may prevent the HVAC unit  12  from resuming normal operation. That is, the processor  86  may prevent the HVAC unit  12  from entering an on-cycle. For example, the processor  86  may send a signal indicative of a lock-down mode in which the HVAC unit  12  ceases all operations. In any case, when a leak is detected, the processor  86  may ensure that the expansion valve  78  remains closed and that the refrigerant remains in the condenser  76 . For example, data from the sensors  102 B,  102 C may be used to monitor the refrigerant. 
     When the processor  86  determines that a leak does not exist, the processor  86  may allow the HVAC unit  12  to resume normal operation. In other words, the processor  86  may allow the HVAC unit  12  to enter an on-cycle. For instance, the processor  86  may send signals that cause the diagnostic mode to end, open the expansion valve  78 , and/or otherwise cause the HVAC unit  12  to resume normal operation. In other words, the signals sent by the processor  86  may allow the HVAC unit  12  to resume supplying conditioned air to the building  10 . 
     Keeping the discussion of  FIG. 5  in mind,  FIG. 6  is a flow diagram of an embodiment of a process  150  for determining whether a refrigerant leak is present in an HVAC unit or, more specifically, the vapor compression system  72 . The process  150  may be performed by the processor  86  by executing instructions stored on the memory  88 , or other suitable processing circuitry, in accordance with the present disclosure. 
     At block  152 , the processor  86  sends a signal to close the expansion valve  78 . As described above, the expansion valve  78  may be an electronic expansion valve that is communicatively coupled to the processor  86  and capable of being opened and closed based on signals from the processor  86 . By closing the expansion valve  78 , the refrigerant of the vapor compression system  72  is not able to flow from the condenser  76  to the evaporator  80 . 
     At block  154 , the processor  86  sends a signal, such as a control signal, at a first time to activate the compressor  74  to pump refrigerant into the condenser  76 . Additionally, the first time may be recorded in the memory  88 . It should be noted that the first time may not necessarily be the same time that the compressor  74  begins to operate with the expansion valve  78  closed. That is, the first time may be a time associated with the activation of the compressor  74  and/or the closure of the expansion valve  78 . For instance, the first time may be a time that occurs before or after the compressor  74  is activated and/or the expansion valve  78  is closed. At block  156 , the processor  86  receives, at a second time, data from sensors  102 . As described above, the data from sensors  102  may be indicative of pressures of the refrigerant at various portions of the vapor compression system  72 . The data received by the processor  86  includes data from sensor  102 A. As discussed above, the sensor  102 A can be a high-resolution, low pressure sensor that is generally located between the expansion valve  78  and the compressor  74 . Similar to the first time, the second time may not necessarily be the time that the data is received by the processor  86 . For instance, the second time may be a time that is associated with the processor  86  receiving the data. 
     At block  158 , the processor  86  determines a time difference between the first time and the second time. In other words, the processor  86  determines a pump-down time, which is the amount of time that passes before all or substantially all of the refrigerant is stored in the condenser  76 . At block  160 , the processor  86  determines whether the time difference is greater than a maximum measurement time. The maximum measurement time is an amount of time that may be stored on the memory  88 . As mentioned above, the maximum measurement time being exceeded may be indicative of a refrigerant leak. Accordingly, when the processor  86  determines that the time difference is greater than the maximum measurement time, at block  162 , the processor  86  sends a signal to prevent the expansion valve  78  from reopening. In other words, the processor  86  may prevent the HVAC unit  12  from exiting the diagnostic mode and/or providing conditioned air to the building  10 . For instance, the signal may cause the HVAC unit  12  to implement the lock-down mode described above. However, if the processor  86  determines that the time difference is not greater than the maximum measurement time, the processor  86  may proceed to block  164 . 
     At block  164 , the processor  86  determines a pressure of the refrigerant based on the data from the sensors  102 . For instance, the processor  86  may determine a pressure between the expansion valve  78  and the evaporator  80  based on data collected via the sensor  102 A. At block  166 , the processor  86  determines whether the pressure determined at block  158  is lower than a predetermined pressure value. As previously described, the predetermined pressure value may be stored on the profile  100  of the memory  88 , and the predetermined pressure value may correspond to a pressure value that occurs when substantially all of the refrigerant is located within the condenser  76 . If the processor  86  determines that the pressure is not lower than the predetermined pressure value, the processor  86  may continue to receive data from the sensors  102 , as indicated at block  156 . 
     However, if the processor  86  determines that the pressure is lower than the predetermined pressure value, at block  168 , the processor  86  determines an expected pump-down time. As described above, the processor  86  may determine the expected pump-down time based on previous iterations of the diagnostic mode and data from the sensors  104 . At block  170 , the processor  86  determines whether the time difference is substantially lower than an expected pump-down time. If the processor  86  determines that the time difference is substantially lower than the expected pump-down time, the processor  86  may send a signal to prevent the expansion valve  78  from reopening, as indicated by block  162 . However, if the processor  86  determines that the time difference is not less than the expected pump-down time, at block  168 , the processor  86  may send a signal that allows the HVAC unit  12  to resume active operation. Determining whether the time difference is substantially lower than the expected pump-down time allows for a tolerance, such as one percent, so that the processor  86  does not proceed to block  162  in response to time differences that are approximately equal to the expected pump-down time, where “approximately equal” means differing from the expected pump-down time but within the tolerance. Additionally, when a time difference between the first and second times is slightly lower, such as within one percent, of the expected pump-down time, the profile  100  may be updated so that an expected pump-down time in a future iteration of the process  150  may be adjusted. By updating the profile  100  in this manner, the processor  86  may account for drift or small cumulative changes over time as the refrigerant charge changes over time and/or the refrigeration circuit ages. 
     In other embodiments, the process  150  may include other steps. For example, before performing steps indicated by blocks  152  and  154 , the processor  86  may determine that the HVAC unit  12  is not operating in a manner indicative of providing conditioned air to the building  10 . That is, the processor  86  may determine whether the HVAC unit  12  is in an on-cycle or an off-cycle. The processor  86  may also send a signal to ensure that normal operation of the HVAC unit  12  is not being completed. In other words, the processor  86  may send a signal to exit an on-cycle. As another example, the process  150  may also include the processor  86  receiving data from the sensors  104 . 
     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 without materially departing from the novel teachings and advantages of the subject matter recited in the claims. For instance, the modifications and changes may include variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters such as temperatures or pressures, mounting arrangements, use of materials, colors, orientations, and the like. 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 present 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 present disclosure 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.