Patent Publication Number: US-2023160587-A1

Title: Hvac system leak detection

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 17/352,077, filed Jun. 18, 2021, by Payam Delgoshaei, entitled “HVAC SYSTEM LEAK DETECTION” which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to heating, ventilation, and air conditioning (HVAC) systems and methods of their use. In particular, the present disclosure relates to HVAC system leak detection. 
     BACKGROUND 
     Heating, ventilation, and air conditioning (HVAC) systems are used to regulate environmental conditions within an enclosed space. Air is cooled via heat transfer with refrigerant flowing through the HVAC system and returned to the enclosed space as cooled conditioned air. Leakage of the refrigerant can result in decreased system performance. 
     SUMMARY OF THE DISCLOSURE 
     The leakage of refrigerant, or loss of charge, from an HVAC system can result in decreased system performance (e.g., loss of desired cooling, increased energy consumption, etc.), eventual damage to system components, and potential risk to people and the environment. For example, the cooling capacity of an HVAC system may decrease as refrigerant leaks from the system. The leaked refrigerant may be harmful to people and the environment. Previous technology used to detect refrigerant leaks fails to provide information about the location of a leak. Because of this, extensive diagnostics must be performed by specially trained technicians to search for the location of a refrigerant leak. This can result in significant delays in locating and repairing a refrigerant leak and corresponding downtimes during which cooling cannot be provided to a space. In some cases, even small refrigerant leaks, which may go undetected and/or unlocated by previous technology, may pose a risk to people and/or the environment. For instance, certain HVAC systems use flammable refrigerant. Flammable refrigerant may leak, causing unsafe concentrations of gas to be dispersed within an occupied space. In addition to the risk of fire, unsafe concentrations of gas within the space may be harmful to the health of the space&#39;s occupants and particularly to the elderly and sick. 
     This disclosure provides technical solutions to the problems of previous technology, including those described above. For example, this disclosure recognizes that the ability to automatically and reliably detect a leak and its location within an HVAC system can decrease maintenance downtimes, increase the lifetime of the HVAC system and its components, and improve the safety of people and the environment. As described further below, if a possible leak is detected, a valve may be closed between high-pressure and low-pressure subsystems of the HVAC system (e.g., between a condenser and evaporator). The HVAC system&#39;s compressor can then be operated until a predetermined input refrigerant pressure is reached for the compressor. Operation of the compressor is then stopped, and the pressure of the low-pressure side of the HVAC system is monitored for a period of time. This disclosure recognizes that the rate of change of this monitored pressure can be used to determine a leak location of the refrigerant (e.g., whether refrigerant is leaking from the low-pressure or high-pressure subsystem of the HVAC system). 
     Embodiments of this disclosure may improve the speed and reliability with which refrigerant leaks can be detected and appropriate maintenance can be performed. As such, the system described in this disclosure may significantly decrease downtimes during which cooling cannot be provided by an HVAC system. Embodiments of this disclosure may also improve the overall safety of HVAC systems and the spaces cooled by HVAC systems. For example, refrigerant may be primarily held in a high-pressure subsystem of the HVAC system that is in an outdoor space when a refrigerant leak is detected. Since the majority of the refrigerant is held in the outdoor space, accumulation of leaking refrigerant in an indoor living space is reduced or prevented. Certain embodiments may include none, some, or all of the above technical advantages. One or more other technical advantages may be readily apparent to one skilled in the art from the figures, descriptions, and claims included herein. 
     In an embodiment, an HVAC system includes a high-pressure subsystem and a low-pressure subsystem. The high-pressure subsystem includes a condenser operable to receive refrigerant and transfer heat from the refrigerant to a flow of outdoor air, thereby generating a cooled refrigerant, and a controllable valve positioned in a refrigerant conduit connecting the condenser to an evaporator. The low-pressure subsystem includes the evaporator operable to receive the cooled refrigerant and transfer heat from a flow of air to the cooled refrigerant, a compressor operable to compress the refrigerant, and a pressure sensor operable to measure a refrigerant pressure in the low-pressure subsystem. A controller is communicatively coupled to the pressure sensor, the compressor, and the controllable valve. The controller determines that refrigerant leak diagnostics should be performed for the HVAC system (e.g., based on a timer/schedule and/or a detection of the possible leak of refrigerant). After determining that the refrigerant leak diagnostics should be performed, the controllable valve is closed. The compressor then operates until a predetermined input refrigerant pressure is reached. After the predetermined input refrigerant pressure is reached, operation of the compressor is stopped (e.g., the compressor is turned off or to a speed of zero). After stopping operation of the compressor and waiting at least a predetermined wait time, the pressure in the low-pressure subsystem of the HVAC system is monitored for a period of time. A rate of change of the pressure in the low-pressure subsystem is determined for the period of time. If the rate of change is negative and a magnitude of the rate of change is greater than a threshold value, a leak location of the refrigerant is determined to be in the low-pressure subsystem of the HVAC system. However, if one or both of the rate of change is not negative and the magnitude of the rate of change is not greater than a threshold value, the leak location of the refrigerant may be in the high-pressure subsystem of the HVAC system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a diagram of an example HVAC system configured for leak detection and diagnosis; 
         FIG.  2    is a plot of an example measured low-side pressure over time for leak detection; and 
         FIG.  3    is a flowchart of an example method of operating the HVAC system of  FIG.  1    to detect a refrigerant leak and determine a system diagnosis. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure and its advantages are best understood by referring to  FIGS.  1 - 3    of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     As described above, prior to this disclosure, there was a lack of tools for reliably detecting and locating/diagnosing refrigerant leaks. The system described in this disclosure facilitates the detection of refrigerant leaks and the proactive determination of a likely diagnosis of which portion, or subsystem, of the HVAC system is leaking refrigerant. Once a location of the leak is determined, a system diagnosis including this information may be provided proactively to an occupant of the space and/or a service provider, such that appropriate maintenance can be provided more rapidly than was possible using previous leak detection technology. 
     As used in the present disclosure, a “saturated” refrigerant refers to a fluid in the liquid state that is in thermodynamic equilibrium with the vapor state of the fluid for a given pressure. A “saturated” refrigerant is said to be at the saturation temperature for a given pressure. If the temperature of a saturated liquid is increased above the saturation temperature, the saturated liquid generally begins to vaporize. A “superheated” refrigerant refers to a fluid in the vapor state that is heated to a temperature that is greater than the saturation temperature of the fluid at a given pressure. 
     HVAC System 
       FIG.  1    is a schematic diagram of an example HVAC system  100  configured to detect and locate system faults, such as a leak of refrigerant. The HVAC system  100  conditions air for delivery to a space. The space may be, for example, a room, a house, an office building, a warehouse, or the like. In some embodiments, the HVAC system  100  is a rooftop unit (RTU) that is positioned on the roof of a building, and conditioned air  122  is delivered to the interior of the building. In other embodiments, portion(s) of the HVAC system  100  may be located within the building and portion(s) outside the building. The HVAC system  100  may be configured as shown in  FIG.  1    or in any other suitable configuration. For example, the HVAC system  100  may include additional components or may omit one or more components shown in  FIG.  1   . 
     The HVAC system  100  includes a working-fluid conduit subsystem  102 , a compressor  106 , a condenser  108 , an outdoor fan  110 , a check valve  114 , an expansion device  116 , an evaporator  118 , a blower  130 , sensors  134 ,  136 ,  138 ,  140 ,  142 ,  146 , a return air filter  144 , one or more thermostats  148 , and a controller  154 . As described in greater detail below, the controller  154  of the HVAC system  100  is generally configured to determine that refrigerant leak diagnostics should be performed, for instance, based on a timer/schedule or the detection of a likely leak of refrigerant (e.g., based on a change in low-side pressure  170  or high-side pressure  190  and/or a gas leak signal  168 ). If it is determined that the refrigerant leak diagnostics should be performed, the expansion valve  116  may be closed between the high-pressure subsystem  104   a  and low-pressure subsystem  104   b  of the HVAC system  100 . The compressor  100  is then operated until a predetermined input refrigerant pressure (e.g., low-side pressure  170  measured by sensor  138 ) is reached. Operation of the compressor  106  is then stopped, and the low-side pressure  170  of the low-pressure subsystem  104   b  of the HVAC system  100  is monitored for a period of time. The rate of change  172  of this monitored low-side pressure  170  is used to determine a leak location of the refrigerant (e.g., whether refrigerant is leaking from the low-pressure subsystem  104   b  or high-pressure subsystem  104   a  of the HVAC system  100 ). Operation of the controller  154  is described in greater detail below and with respect to the method of  FIG.  3   . In some cases, the system diagnosis  182  may be automatically provided to a service provider  188  (e.g., a maintenance provider). This may facilitate proactive repairs of the HVAC system  100 , such that there is limited or no downtime during which desired heating or cooling is not available. 
     The working-fluid conduit subsystem  102  facilitates the movement of a refrigerant through a refrigeration cycle such that the refrigerant flows as illustrated by the dashed arrows in  FIG.  1   . The working-fluid conduit subsystem  102  includes conduit, tubing, and the like that facilitates the movement of refrigerant between components of the HVAC system  100 . The refrigerant may be any acceptable refrigerant including, but not limited to, fluorocarbons (e.g. chlorofluorocarbons), ammonia, non-halogenated hydrocarbons (e.g. propane), hydroflurocarbons (e.g. R- 410 A), or any other suitable type of refrigerant. In some cases, the refrigerant may be flammable or pose a risk to occupants of the space cooled by the HVAC system  100 . 
     The HVAC system  100  generally includes a “high side” or high-pressure subsystem  104   a  and a “low side” or low-pressure subsystem  104   b . The high-pressure subsystem  104   a  generally includes components and portions of the working-fluid conduit subsystem  102  that contain refrigerant at a relatively high pressure (e.g., after the refrigerant is pressurized, or compressed, by the compressor  106 . The low-pressure subsystem  104   b  includes components and portions of the working-fluid conduit subsystem  102  that contain refrigerant at a relatively low pressure (e.g., after the refrigerant is expanded by the expansion device  116 ). In some cases, the high-pressure subsystem  104   a  is primarily located outdoors, while the low-pressure subsystem  104   b  may be located indoors. 
     The HVAC system  100  includes a compressor  106 , a condenser  108 , and a fan  110 . In some embodiments, the compressor  106 , condenser  108 , and fan  110  are combined in an outdoor unit while at least certain other components of the HVAC system  100  may be located indoors (e.g., components of the low-pressure subsystem  104   b ). The compressor  106  is coupled to the working-fluid conduit subsystem  102  and compresses (i.e., increases the pressure of) the refrigerant. The compressor  106  may be a single-speed, variable-speed, or multiple stage compressor. A single-speed compressor is generally configured to operate at a single, predefined speed. A variable-speed compressor is generally configured to operate at different speeds to increase the pressure of the refrigerant to keep the refrigerant moving along the working-fluid conduit subsystem  102 . In the variable-speed compressor configuration, the speed of compressor  106  can be modified to adjust the cooling capacity of the HVAC system  100 . Meanwhile, in the multi-stage compressor configuration, one or more compressors can be turned on or off to adjust the cooling capacity of the HVAC system  100 . 
     The compressor  106  is in signal communication with the controller  154  using wired and/or wireless connection. The controller  154  provides commands or signals to control operation of the compressor  106  and/or receives signals from the compressor  106  corresponding to a status of the compressor  106 . For example, the controller  154  may transmit signals to adjust compressor speed and/or staging. The controller  154  may operate the compressor  106  in different modes corresponding, for example, to an operating mode indication  150  (e.g., a heating, cooling, or diagnostic mode), to load conditions (e.g., the amount of cooling or heating required by the HVAC system  100 ), to a difference between a setpoint temperature and an indoor air temperature, and the like. 
     A check valve  114  may be positioned at the outlet of the compressor  106 . The check valve prevents backflow of refrigerant into the compressor  106  when the compressor  106  is not operated (e.g., as in during at least a portion of the diagnostic operations described in this disclosure). The check valve  114  may be operated based on a pressure of refrigerant in the conduit  102  connecting the compressor  106  to the condenser  108  relative to the pressure of refrigerant in the compressor  106 . For example, if the pressure in the conduit  102  exceeds the pressure in the condenser  106 , then the check valve  114  may automatically close to prevent backflow of refrigerant into the compressor  106 . In some cases, the check valve  114  may be controlled by the controller  154 . For example, the check valve  114  may be in signal communication with the controller  154  using wired and/or wireless connection. In such cases, the controller  154  provides commands or signals to control operation of the check valve  114 . For example, the controller  154  may cause the check valve  114  to be appropriately adjusted to prevent the refrigerant from flowing into the outlet of the compressor  106  after operation of the compressor  106  is stopped for refrigerant leak diagnostics. 
     The condenser  108  is generally located downstream of the compressor  106  and is configured, when the HVAC system  100  is operating in a cooling mode, to remove heat from the refrigerant. The fan  110  is configured to move air  112  across the condenser  108 . For example, the fan  110  may be configured to blow outside air through the condenser  108  to help cool the refrigerant flowing therethrough. In the cooling mode, the compressed, cooled refrigerant flows from the condenser  108  toward the expansion device  116 . 
     The expansion device  116  is coupled to the working-fluid conduit subsystem  102  downstream of the condenser  108  and is configured to remove pressure from the refrigerant. The expansion device  116  is generally a controllable valve positioned in refrigerant conduit of the working-fluid conduit subsystem  102  that connects the condenser  108  to the evaporator  118 . In this way, the refrigerant is delivered to the evaporator  118  and receives heat from airflow  120  to produce a conditioned airflow  122  that is delivered by a duct subsystem  124  to the conditioned space. In general, the expansion device  116  may be a valve such as an expansion valve or a flow control valve (e.g., a thermostatic expansion valve) or any other suitable valve for removing pressure from the refrigerant while, optionally, providing control of the rate of flow of the refrigerant. In some cases, the expansion device  116  may include two devices, for example, a thermostatic expansion valve (TXV) with a solenoid valve located upstream of the TXV. The expansion device  116  may be in communication with the controller  154  (e.g., via wired and/or wireless communication) to receive control signals for opening and/or closing associated valves and/or provide flow measurement signals corresponding to the rate of refrigerant flow through the working-fluid conduit subsystem  102 . 
     The evaporator  118  is generally any heat exchanger configured to provide heat transfer between air flowing through (or across) the evaporator  118  (i.e., air  120  contacting an outer surface of one or more coils of the evaporator  118 ) and refrigerant passing through the interior of the evaporator  118 , when the HVAC system  100  is operated in the cooling mode. The evaporator  118  may include one or more circuits. 
     The evaporator  118  is fluidically connected to the compressor  106 , such that refrigerant generally flows from the evaporator  118  to the compressor  106 . A portion of the HVAC system  100  is configured to move air  120  across the evaporator  118  and out of the duct subsystem  124  as conditioned air  122 . In some embodiments, the HVAC system  100  may include a heating element (not shown for clarity and conciseness). The heating element is generally any device for heating the flow of air  120  and providing heated air  122  to the conditioned space, when the HVAC system  100  operates in a heating mode. 
     Return air  126 , which may be air returning from the building, air from outside, or some combination, is pulled into a return duct  128 . An inlet or suction side of the blower  130  pulls the return air  126 . The return air  126  may pass through an air filter  144 . The air filter  144  is generally a piece of porous material that removes particulates from the return air  126 . As described further below, sensor(s)  146  may be located on each side of the air filter  144  and configured to measure an air pressure drop  180  across the air filter  144 . The air pressure drop  180  may be used to determine when the air filter  144  is blocked by accumulated particulates and should be changed. The blower  130  discharges air  120  into a duct  132  such that air  120  crosses the evaporator  118  to produce conditioned air  122 . The blower  130  is any mechanism for providing a flow of air through the HVAC system  100 . For example, the blower  130  may be a constant-speed or variable-speed circulation blower or fan. Examples of a variable-speed blower include, but are not limited to, belt-drive blowers controlled by inverters, direct-drive blowers with electronic commuted motors (ECM), or any other suitable type of blower. 
     The blower  130  is in signal communication with the controller  154  using any suitable type of wired and/or wireless connection. The controller  154  is configured to provide commands and/or signals to the blower  130  to control its operation. For example, the controller  154  may receive an indication of the blower status  178  indicating whether the blower is operating as intended. Generally, when functioning as intended, the blower  130  provides airflow  120  across the evaporator  118 , but the blower may not provide the appropriate or expected airflow  120  when the blower  130  is not functioning as intended. The controller  154  may include the blower status  178  in the system diagnosis  182  to improve the system diagnosis  182 . For example, this may improve the speed with which a malfunctioning blower  130  can be repaired. 
     The HVAC system  100  includes one or more of the sensors  134 ,  136 ,  138 ,  140 ,  142 ,  146  illustrated in  FIG.  1   . The sensors  134 ,  136 ,  138 ,  140 ,  142 ,  146  are in wired and/or wireless signal communication with controller  154 . Signals corresponding to the properties measured by sensors  134 ,  136 ,  138 ,  140 ,  142 ,  146  are provided to the controller  154 . In some embodiments, one or more of the sensors  134 ,  136 ,  138 ,  140 ,  142 ,  146  or another sensor integrated with the HVAC system  100  may be an internet-of-things (IOT) device. For example, one or more of the sensors  134 ,  136 ,  138 ,  140 ,  142 ,  146  may communicate wirelessly with the controller  154  (e.g., via a wireless network associated with the conditioned space). In other examples, the HVAC system  100  may include other sensors (not shown for clarity and conciseness) positioned and configured to measure any other property associated with operation of the HVAC system  100  (e.g., the temperature and/or relative humidity of air at one or more locations within the conditioned space and/or outdoors). 
     Sensors  134  and  136  are positioned proximate or inside the evaporator  118  to measure properties of the refrigerant flowing therethrough. For example, sensors  134 ,  136  may measure temperatures and/or pressures of the refrigerant at different points in the evaporator  118 . The measured temperatures and/or pressures may be used by the controller  154  to determine a superheat (SH)  164 . SH  164  is the difference between the temperature of refrigerant exiting the evaporator  118  (e.g., measured by sensor  136 ) and the vaporization temperature of the refrigerant in the evaporator  118  (e.g., measured via temperature or pressure measured by sensor  134 ). For example, the first evaporator sensor  134  may be positioned and configured to measure a saturated suction temperature (SST)  162  of the refrigerant in the evaporator  118 , while the second sensor  136  may be positioned and configured to measure a superheated vapor temperature of the refrigerant in the evaporator  118 . The controller  154  may determine the SH  164  based on a difference between the SST  162  and the superheated vapor temperature. If both the SST  162  is less than a threshold value  166  and the SH  164  is less than a threshold value  166 , a refrigerant leak may be detected by the controller  154 . 
     Sensor  138  is located proximate the inlet of the compressor  106  or in the portion of the working-fluid conduit  102  leading into the inlet of the compressor  106 . While in the example of  FIG.  1   , the sensor  138  is shown relatively near the inlet of the compressor  106 , this sensor  138  could be located further upstream from the inlet of the compressor  106  (e.g., nearer the outlet of the evaporator  118 ). The controller  154  uses a signal from the sensor  138  to determine the low-side pressure  170 . The low-side pressure  170  is a pressure of refrigerant in the low-pressure subsystem  104   b  of the HVAC system  100 . In some cases, the SST  162  may be determined from a pressure (e.g., low-side pressure  170 ) measured by sensor  138 . In some embodiments, the sensor  138  includes a pressure switch. A pressure switch signal  176  provided to the controller  154  by such a pressure switch may be used to determine when the low-side pressure  170  is less than a threshold value  166  for stopping operation of the compressor  106  in order to perform leak diagnostics, as described further below and with respect to  FIG.  3   . 
     Sensor  140  measures a high-side pressure  190 . The high-side pressure  190  is the pressure of the refrigerant in the high-pressure subsystem  104   a  of the HVAC system  100 . While in the example of  FIG.  1   , the sensor  140  is shown between the outlet of the compressor  106  and the inlet of the condenser  108 , this sensor  140  could be located at another position in the high-pressure subsystem  104   a  of the HVAC system  100  (e.g., proximate or downstream of the outlet of the condenser  108 ). The controller  154  uses a signal from the sensor  140  to determine the high-side pressure  190 . 
     Sensor  142  is positioned and configured to measure a discharge air temperature of airflow  122  or a temperature of air provided to the space conditioned by the HVAC system  100 . Sensor(s)  146  may be located on each side of the air filter  144  and configured to measure an air pressure drop  180  across the air filter  144 . The air pressure drop  180  may be used to determine when the air filter  144  is blocked and/or should be changed. This information may be used to improve the system diagnosis  182  determined by the controller  154  (e.g., by including an indication of a blockage of the air filter  144  in the system diagnosis  182 ). 
     Information from sensors  134 ,  136 ,  138 ,  140 ,  142  may be used to determine that a refrigerant leak is detected. For example, if one or both of the low-side pressure  170  and the high-side pressure  190  decreases below a corresponding threshold  166 , a leak may be detected. In some cases, one or more of the sensors  134 ,  136 ,  138 ,  140 ,  142  may include a leak detection device, such as a gas sensor configured to detect refrigerant gas that is emitted from the HVAC system  100 . Such a sensor may provide a gas leak signal  168  to the controller  154  to indicate that a refrigerant leak is detected and that leak diagnostics are needed to determine a location of the refrigerant leak. 
     The HVAC system  100  includes one or more thermostats  148 , for example, located within the conditioned space (e.g. a room or building). The thermostat(s)  148  are generally in signal communication with the controller  154  using any suitable type of wired and/or wireless connection. In some embodiments, one or more functions of the controller  154  may be performed by the thermostat(s)  148 . For example, the thermostat  148  may include the controller  154 . The thermostat(s)  148  may include one or more single-stage thermostats, one or more multi-stage thermostat, or any suitable type of thermostat(s). The thermostat(s)  148  are configured to allow a user to input a desired temperature or temperature setpoint for the conditioned space and/or for a designated space or zone, such as a room, in the conditioned space. The thermostat(s) generally include or are in communication with a sensor for measuring an indoor air temperature (e.g., sensor  142 ). 
     The controller  154  may use information from the thermostat  148  such as the temperature setpoint, indoor air temperature, and/or mode indication  150  for controlling the compressor  106 , the blower  130 , and the fan  110 . In some embodiments, a thermostat  148  includes a user interface and/or display for displaying information related to the operation and/or status of the HVAC system  100 . For example, the user interface may display operational, diagnostic, and/or status messages and provide a visual interface that allows at least one of an installer, a user, a support entity, and a service provider to perform actions with respect to the HVAC system  100 . For example, the user interface may provide for display of the mode indication  150 , which indicates a current operating mode of the HVAC system  100 , such as whether the HVAC system  100  is operating to provide cooling or heating or if the HVAC system  100  is temporarily operating in a diagnostic mode (e.g., with the compressor  106  turned off for a period of time). The user interface may display an alert  152 , for example, indicating a refrigerant leak is detected for the HVAC system  100 . The user interface may display a system diagnosis  182  determined by the controller  154  based on outcomes of the leak diagnostics described below and with respect to  FIG.  3   . The system diagnosis  182  generally includes an indication of a refrigerant leak location  184  and/or a failed component  186  of the HVAC system  100  that was determined to have failed. 
     As described in greater detail below, the controller  154  is configured to determine a system diagnosis  182  for a detected refrigerant leak. The system diagnosis  182  may be presented on the display of the thermostat  148  and/or to a service provider  188  to ensure maintenance is performed rapidly and accurately with little or no downtime during which cooling is not available. The controller includes a processor  156 , memory  158 , and input/output (I/O) interface  160 . The processor  156  includes one or more processors operably coupled to the memory  158 . The processor  156  is any electronic circuitry including, but not limited to, state machines, one or more central processing unit (CPU) chips, logic units, cores (e.g. a multi-core processor), field-programmable gate array (FPGAs), application specific integrated circuits (ASICs), or digital signal processors (DSPs) that communicatively couples to memory  158  and controls the operation of HVAC system  100 . The processor  156  may be a programmable logic device, a microcontroller, a microprocessor, or any suitable combination of the preceding. The processor  156  is communicatively coupled to and in signal communication with the memory  158 . The one or more processors are configured to process data and may be implemented in hardware or software. For example, the processor  156  may be 8-bit, 16-bit, 32-bit, 64-bit or of any other suitable architecture. The processor  156  may include an arithmetic logic unit (ALU) for performing arithmetic and logic operations, processor registers that supply operands to the ALU and store the results of ALU operations, and a control unit that fetches instructions from memory  158  and executes them by directing the coordinated operations of the ALU, registers, and other components. The processor  156  may include other hardware and software that operates to process information, control the HVAC system  100 , and perform any of the functions described herein (e.g., with respect to  FIGS.  2  and  3   ). The processor  156  is not limited to a single processing device and may encompass multiple processing devices. Similarly, the controller  154  is not limited to a single controller but may encompass multiple controllers. 
     The memory  158  includes one or more disks, tape drives, or solid-state drives, and may be used as an over-flow data storage device, to store programs when such programs are selected for execution, and to store instructions and data that are read during program execution. The memory  158  may be volatile or non-volatile and may include ROM, RAM, ternary content-addressable memory (TCAM), dynamic random-access memory (DRAM), and static random-access memory (SRAM). The memory  158  is operable (e.g., or configured) to store information used by the controller  154  and/or any other logic and/or instructions for performing the function described in this disclosure. 
     The I/O interface  160  is configured to communicate data and signals with other devices. For example, the I/O interface  160  may be configured to communicate electrical signals with components of the HVAC system  100  including the compressor  106 , fan  110 , expansion device  116 , sensors  134 ,  136 ,  138 ,  140 ,  142 ,  146 , blower  130 , and thermostat(s)  148 . The I/O interface  160  may be configured to communicate with other devices and systems, such as the service provider  188 . The I/O interface  160  may provide and/or receive, for example, compressor speed signals blower speed signals, temperature signals, relative humidity signals, thermostat calls, temperature setpoints, environmental conditions, and an operating mode status for the HVAC system  100  and send electrical signals to the components of the HVAC system  100 . The I/O interface  160  may include ports or terminals for establishing signal communications between the controller  154  and other devices. The I/O interface  160  may be configured to enable wired and/or wireless communications. 
     In an example operation of HVAC system  100 , the HVAC system  100  starts up to operate in the cooling mode. For example, in response to the indoor temperature increasing above a temperature setpoint while the HVAC system  100  is set to operate in the cooling mode (e.g., based on mode indication  150 ), the controller  154  may cause the compressor  106 , fan  110  and blower  130  to operate. During operation of the HVAC system  100 , the controller  154  (e.g., the processor  156  of the controller  154 ) determines refrigerant leak diagnostics should be performed. For instance, the controller  154  may determine that it has been at least a threshold time since the last leak diagnostics was run (e.g., based on a schedule and/or timer of the controller  154 ). As another example, a possible leak of refrigerant may be detected from the HVAC system  100  that indicates refrigerant leak diagnostics should be performed. As described above, the possible refrigerant leak may be detected based on information from one or more of the sensors  134 ,  136 ,  138 ,  140 ,  142 . For instance, a drop in the low-side pressure  170  below a corresponding threshold  166 , a drop of the high-side pressure  190  below a corresponding threshold  166 , and/or receipt of a gas leak signal  168  may be used to determine the possible refrigerant leak. In some cases, after determining that the possible leak of refrigerant is detected, the controller  154  may cause display of a mode indication  150  on the thermostat  148  that indicates that the HVAC system  100  is operating in a diagnostic mode. During operation in the diagnostic mode, cooling or heating may not be available for a period of time. Providing this information to occupants of the space via the thermostat  148  may improve usability of the HVAC system  100 , such that occupants do not think another issue is causing the brief inability to perform cooling or heating. 
     After determining that the refrigerant leak diagnostics should be performed, the controller  154  causes the expansion valve  116  to close. For example, valve instructions  174  may be provided to close the expansion valve  116  (e.g., to close a solenoid valve of the expansion valve  116 ). After the expansion valve  116  is closed, refrigerant cannot flow from the high-pressure subsystem  104   a  into the low-pressure subsystem  104   b . The controller  154  then causes the compressor  106  to operate until a predetermined low-side pressure  170  is reached on the inlet side of the compressor  106 . For example, the compressor  106  may operate until the low-side pressure  170  measured by sensor  138  decreases to at least a threshold value  166 . After the predetermined low-side pressure  170  is reached, operation of the compressor  106  is stopped. If needed, the controller  154  may also adjust the check valve  114  after operation of the compressor  106  is stopped, such that the refrigerant from the high-pressure subsystem  104   a  cannot backflow through the compressor  106  into the low-pressure subsystem  104   b . For example, the check valve  114  may be closed. In some cases, the check valve  114  is a one-way valve which prevents backflow of refrigerant into the compressor  106  without action from the controller  154 . At this point, a majority of the refrigerant may be present in the high-pressure subsystem  104   a , which is all or mostly outdoors, such that any leak of refrigerant from the low-pressure subsystem  104   b  will present a decreased risk to occupants of the indoor space. 
     After a predetermined wait time (e.g., a threshold  166  wait time, or the waiting time  204  of  FIG.  2   , described below), the controller  154  monitors the low-side pressure  170  for some time (e.g., the monitoring time  206  of  FIG.  2   ) and determines a rate of change  172  of the low-side pressure  170  over time. This rate of change  172  is used to determine whether the detected leak is occurring in the high-pressure subsystem  104   a  or the low-pressure subsystem  104   b . For example, if the rate of change  172  is negative and a magnitude of the rate of change  172  is greater than a threshold value  166 , the controller  154  may determine that the leak location  184  of the refrigerant is in the low-pressure subsystem  104   b  of the HVAC system  100 . Alternatively, if one or both of the rate of change  172  is not negative or the magnitude of the rate of change  172  is not greater than the threshold value  166 , the controller  154  may determine that the leak location  184  of the refrigerant may be in the high-pressure subsystem  104   a . The controller  154  then determines the system diagnosis  182  that includes the determined leak location  184 . The system diagnosis  182  may be displayed via the thermostat  148  and/or provided to the service provider  188 , as described above. 
     To further illustrate the refrigerant leak diagnostics described above,  FIG.  2    shows an example plot  200  of the low-side pressure  170  as a function of time beginning from when the compressor  106  is turned off. After the compressor is turned off, the controller  154  waits for a waiting time  204  to allow the low-side pressure  170  to equilibrate. The low-side pressure  170  may increase initially during the waiting time  204  because the temperature of the refrigerant may increase after the compressor  106  is turned off. The controller  154  then monitors the low-side pressure  170  for a monitoring time  206 . During this monitoring time  206 , the controller  154  may determine the rate of change  172  of the low-side pressure  170 . The rate of change  172  may be determined, for example, as the change in pressure  208  over the monitoring time  206 . 
     Returning to  FIG.  1   , in some cases, the controller  154  may also check the functioning of other components of the HVAC system  100  and include such findings in the system diagnosis  182 . Information about functions of other system components may improve the determination of appropriate maintenance actions to take to ensure continued reliable operation of the HVAC system  100 . For example, the controller  154  may determine the blower status  178  from information provided by the blower  130  (if this information is available). If the blower status  178  indicates that the blower  130  is not operating as intended, the blower  130  may be included as a failed component  186  in the system diagnosis  182 . As another example, the controller  154  may detect a blockage of the air filter  144  if the air pressure drop  180  across the air filter  144  is greater than a threshold value  166 . If such a blockage is detected, the air filter  144  may be indicated as a failed component  186  in the system diagnosis (e.g., because of the need to clean or replace the air filter  144 ). 
     Example Methods of HVAC System Prognostics and Diagnostics 
       FIG.  3    illustrates a method  300  of automatically locating and diagnosing a possible refrigerant leak in the HVAC system  100  of  FIG.  1   . The method  300  may be implemented using the processor  156 , memory  158 , and I/O interface  160  of the controller  154  of  FIG.  1   . The method  300  may begin at step  302  where the controller  154  determines whether refrigerant leak diagnostics should be performed. For example, the controller  154  may determine whether it has been greater than a threshold time since the last leak diagnostic was performed For example, refrigerant leak diagnostics may be performed monthly, seasonally, or the like. As another example, refrigerant leak diagnostics may be performed when a possible refrigerant leak is detected. For example, the possible refrigerant leak may be detected based on information from one or more of the sensors  134 ,  136 ,  138 ,  140 ,  142 . For instance, a drop in the low-side pressure  170  below a corresponding threshold  166 , a drop of the high-side pressure  190  below a corresponding threshold  166 , and/or receipt of a gas leak signal  168  may be used to determine the possible refrigerant leak. If refrigerant leak diagnostics should not be performed, the controller  154  returns to start. Otherwise, if refrigerant leak diagnostics should be performed, the controller  154  proceeds to step  304 . 
     At step  304 , the controller  154  causes display of a mode indication  150  on the thermostat  148  that indicates that the HVAC system  100  is operating in a diagnostic mode. During operation in the diagnostic mode, cooling or heating may not be available for a period of time. Providing this information to occupants of the space via the thermostat  148  may improve usability of the HVAC system  100 , such that occupants do not think another issue is causing the brief inability to perform cooling or heating. 
     At step  306 , the controller  154  causes the expansion valve  116  to close. For example, valve instructions  174  may be provided to close the expansion valve  116  (e.g., to close a solenoid valve of the expansion valve  116 ). At step  308 , the controller  154  causes the compressor  106  to operate until a threshold low-side pressure  170  is achieved at the inlet of the compressor  106 . For example, the compressor  106  may operate until the low-side pressure  170  measured by sensor  138  meets a threshold value  166 . 
     At step  310 , the controller  154  causes the compressor  106  to stop operating and waits for a predefined amount of time (e.g., for the waiting time  204  of  FIG.  2   ). If needed, the controller  154  may also adjust the check valve  114  after operation of the compressor  106  is stopped, such that the refrigerant from the high-pressure subsystem  104   a  cannot backflow through the compressor  106  into the low-pressure subsystem  104   b.    
     At step  312 , the controller  154  monitors the low-side pressure  170  (e.g., as illustrated in  FIG.  2   ) for at least a period of time (e.g., the monitoring time  206  of  FIG.  2   ). At step  314 , the controller  154  determines the rate of change  172  of the low-side pressure over the period of time. For example, referring to  FIG.  2   , the rate of change  172  may be determined as the change in pressure  208  over the monitoring time  206 . 
     At step  316 , the controller  154  determines if the rate of change  172  is negative and if a magnitude of the rate of change  172  is greater than a threshold value  166 . If this is the case, the controller  154  proceeds to step  318  and determines that the refrigerant leak is in the low-pressure subsystem  104   b  of the HVAC system  100 . Otherwise, if one or both of the rate of change  172  is not negative or the magnitude of the rate of change  172  is not greater than the threshold value  166 , the controller  154  proceeds to step  320  and determines that the leak location  184  of the refrigerant may be in the high-pressure subsystem  104   a.    
     At step  322 , the controller  154  determines if there are any other failed components  186 . For example, the controller  154  may determine the blower status  178  from information provided by the blower  130 . If the blower status  178  indicates the blower  130  is not operating as intended, the blower  130  may be included as a failed component  186  in the system diagnosis  182 . Similarly, a blocked air filter  144  may be included as a failed component  186 , as described with respect to  FIG.  1    above. 
     At step  324 , the controller  154  determines the system diagnosis  182  which includes the leak location  184  from step  318  or  320  and any failed components  186  from step  322 . At step  326 , the system diagnosis  182  and/or any related alert(s)  152  (e.g., indicating a refrigerant leak) are displayed on the thermostat  148  and/or provided to the service provider  188 , as described above. 
     Modifications, additions, or omissions may be made to method  300  depicted in  FIG.  3   . Method  300  may include more, fewer, or other steps. For example, steps may be performed in parallel or in any suitable order. While at times discussed as controller  154 , HVAC system  100 , or components thereof performing the steps, any suitable HVAC system or components of the HVAC system may perform one or more steps of the method  300 . 
     While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
     In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein. 
     To aid the Patent Office, and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants note that they do not intend any of the appended claims to invoke 35 U.S.C. § 112(f) as it exists on the date of filing hereof unless the words “means for” or “step for” are explicitly used in the particular claim.