Patent Publication Number: US-11644206-B2

Title: HVAC system prognostics and diagnostics based on temperature rise or drop

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/907,915 filed Jun. 22, 2020, by Payam Delgoshaei et al., and entitled “HVAC SYSTEM PROGNOSTICS AND DIAGNOSTICS BASED ON TEMPERATURE RISE OR DROP,” 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 prognostics and diagnostics based on temperature rise or drop. 
     BACKGROUND 
     Heating, ventilation, and air conditioning (HVAC) systems are used to regulate environmental conditions within an enclosed space. In a cooling mode, air is cooled via heat transfer with refrigerant flowing through the HVAC system and returned to the enclosed space as cooled conditioned air. In a heating mode, air is heated via heat transfer with a heating element and returned to the enclosed space as heated conditioned air. 
     SUMMARY OF THE DISCLOSURE 
     In an embodiment, a heating, ventilation, and air conditioning (HVAC) system includes a heating element configured, when the HVAC system is operating in a heating mode, to heat a flow of air provided to a space. The HVAC system includes a discharge air temperature sensor positioned and configured to measure a discharge air temperature of the flow of air provided to the space. The HVAC system includes a return air temperature sensor positioned and configured to measure a return air temperature of air received from the space (e.g., by a duct of the HVAC system). A controller of the HVAC system determines that the HVAC system has been operating in the heating mode for at least a predefined amount of time. The controller receives the discharge air temperature measured by the discharge air temperature sensor and the return air temperature measured by the return air temperature sensor. A temperature rise value is determined based on the discharge air temperature and the return air temperature. In response to determining that the temperature rise value is less than a predefined minimum threshold value, the controller determines that a first fault of the HVAC system is detected and provides a first alert indicating detection of the first fault. In response to determining that the temperature rise value is greater than a predefined maximum threshold value, the controller determines that a second fault of the HVAC system is detected and provides a second alert indicating detection of the second fault. 
     In some cases, a fault of an HVAC system may result in inadequate and/or inefficient heating or cooling. Using previous technology, such faults are typically only identified after an occupant of a space conditioned by the HVAC system experiences discomfort and contacts a technician to service the HVAC system. As such, any brief or intermittent system operation issues or departure from designed operating parameters may go undetected using previous technology, such that necessary maintenance is not performed in a timely manner. This may result in increased damage to components of the HVAC system and increased downtimes for repair during which heating or cooling is not available to the conditioned space 
     This disclosure not only encompasses the recognition of the problems of previous technology, including those described above, but provides technical solutions to these problems. As described further below, a controller of an HVAC system may be configured to determine temperature rise and/or temperature drop values using measured discharge air temperatures and return (or indoor) air temperatures to evaluate the performance and health of an HVAC system. For example, a temperature rise value may be determined for an HVAC system operating in a heating mode as a difference between the discharge air temperature and the return (or indoor) air temperature, and a temperature drop value may be determined as a difference between the return (or indoor) air temperature and the discharge air temperature for an HVAC system operating in a cooling mode. If the temperature rise and/or drop values are outside a predefined range (i.e., less than a predefined minimum value or greater than a predefined maximum value), the controller may detect that a fault has occurred. The controller may provide an alert indicating detection of the fault and a likely type of the fault (e.g., an indication of a likely cause of the fault, such as a component of the HVAC system that likely has failed). 
     If the temperature rise value for an HVAC system operating in a heating mode is less than a predefined minimum value (e.g., about 25° F. for a furnace that is under-firing and/or experiencing a high air flow rate, e.g., about 10° F. for a heat pump that is under-charged and/or experiencing a high air flow rate) or is greater than a predefined maximum value (e.g., about 75° F. for a furnace that is over-firing and/or experiencing airflow restrictions, e.g., about 50° F. for a heat pump that is over-charged, has a faulty outdoor expansion device, and/or is experiencing airflow restrictions), the controller may determine that the HVAC system is experiencing a fault. As another example, if the temperature drop for an HVAC system operating in a cooling mode is less than a predefined minimum value (e.g. 15° F. for loss of charge or a high air flow rate) or the discharge air temperature is less than a threshold value (e.g. 40° F. for an airflow restriction), the controller may determine that the HVAC system is experiencing a fault. As such, the system described in this disclosure may improve the technology used to efficiently operate HVAC systems. The controller described in this disclosure may particularly be implemented in an HVAC system for the practical application of detecting system faults proactively (e.g., before occupants experience significant discomfort) and alerting an occupant and/or maintenance provider of the fault, such that corrective actions may be taken with little or no impact to the occupants (i.e., with little or no downtime during which heating or cooling is not available). 
     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. 
    
    
     
       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 fault detection; 
         FIG.  2    is a flowchart of an example method of detecting a fault for the HVAC system of  FIG.  1    operating in a heating mode; 
         FIG.  3    is a flowchart of an example method of detecting a fault for the HVAC system of  FIG.  1    operating in a cooling mode; and 
         FIG.  4    is a diagram of an example controller of the HVAC system illustrated in  FIG.  1   . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure and its advantages are best understood by referring to  FIGS.  1  through  4    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 HVAC system faults. The system described in this disclosure particularly facilitates the proactive detection of possible HVAC system faults, such that corrective action may be taken before the underlying source of the fault is exacerbated. For example, a loss of charge in the HVAC system may be detected before a component of the HVAC system is damaged. For instance, proactive detection of a fault may help in preventing the unwanted freezing of an evaporator coil of the HVAC system. The controller of the HVAC system described in this disclosure may further identify one or more likely causes of a detected fault, such that corrective action can be appropriately focused for timely correction.  FIG.  1    illustrates an example HVAC system configured for the proactive detection and identification of system faults.  FIGS.  2  and  3    illustrate methods for detecting faults and alerting appropriate parties of the faults for heating mode operation and cooling mode operation, respectively.  FIG.  4    illustrates the controller of the HVAC system in greater detail. 
     HVAC System 
       FIG.  1    is a schematic diagram of an example HVAC system  100  configured to proactively detect system faults. 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 , at least one condensing unit  104 , an expansion device  114 , an evaporator  116 , a heating element  118 , a blower  130 , sensors  134   a,b , one or more thermostats  136 , and a controller  144 . The controller  144  of the HVAC system  100  is generally configured to detect possible faults of the HVAC system  100  using measurements of discharge air temperature  146 , return air temperature  148 , and/or indoor air temperature  140  and provide alerts  142  associated with the detected faults. In some cases, an alert  142  may be automatically provided to a third party  156  (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 working fluid (e.g., a refrigerant) through a refrigeration cycle such that the working fluid flows as illustrated by the dashed arrows in  FIG.  1   . The working fluid may be any acceptable working fluid including, but not limited to, fluorocarbons (e.g. chlorofluorocarbons), ammonia, non-halogenated hydrocarbons (e.g. propane), hydrofluorocarbons (e.g. R-410A), or any other suitable type of refrigerant. 
     The condensing unit  104  includes a compressor  106 , a condenser  108 , and a fan  110 . In some embodiments, the condensing unit  104  is an outdoor unit while other components of the HVAC system  100  may be located indoors. The compressor  106  is coupled to the working-fluid conduit subsystem  102  and compresses (i.e., increases the pressure of) the working fluid. The compressor  106  of condensing unit  104  may be a single-speed, variable-speed, or multiple stage compressor. A variable-speed compressor is generally configured to operate at different speeds to increase the pressure of the working fluid to keep the working fluid 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  144  using wired and/or wireless connection. The controller  144  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  144  may transmit signals to adjust compressor speed. The controller  144  may operate the compressor  106  in different modes corresponding, for example, to a user requested mode (e.g., a heating or cooling 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  138  and an indoor air temperature  140 , and the like. 
     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 working fluid. 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 working fluid flowing therethrough. In the cooling mode, the compressed, cooled working fluid flows from the condenser  108  toward the expansion device  114 . 
     The expansion device  114  is coupled to the working-fluid conduit subsystem  102  downstream of the condenser  108  and is configured to remove pressure from the working fluid. In this way, the working fluid is delivered to the evaporator  116  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  114  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 working fluid while, optionally, providing control of the rate of flow of the working fluid. The expansion device  114  may be in communication with the controller  144  (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 working fluid through the working-fluid conduit subsystem  102 . 
     The evaporator  116  is generally any heat exchanger configured to provide heat transfer between air flowing through (or across) the evaporator  116  (i.e., air contacting an outer surface of one or more coils of the evaporator  116 ) and working fluid passing through the interior of the evaporator  116 , when the HVAC system  100  is operated in the cooling mode. The evaporator  116  may include one or more circuits. The evaporator  116  is fluidically connected to the compressor  106 , such that working fluid generally flows from the evaporator  116  to the condensing unit  104 . A portion of the HVAC system  100  is configured to move air  120  across the evaporator  116  and out of the duct subsystem  124  as conditioned air  122 . 
     The heating element  118  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. For example, the heating element  118  may be an electrical heater (e.g., comprising one or more resistive elements) or a component of a furnace of the HVAC system  100 . In some embodiments, the HVAC system  100  is configured to operate as a heat pump. Generally, when the HVAC system is operating as a heat pump in a heating mode, the flow of refrigerant is reversed, such that the condenser  108  acts an evaporator and the evaporator  116  acts as a condenser to heat the flow of air  120  passing therethrough. If the HVAC system  100  is configured to operate as a heat pump, the HVAC system  100  may include a reversing valve to reverse the flow of working fluid through the HVAC system  100  during operation in the heating mode and an outdoor expansion device for expanding the working fluid provided to the condenser  108 , which acts an evaporator in the heating mode. When the HVAC system  100  is configured to operate as a heat pump, the heating element  118  may provide supplemental and/or backup heating to the flow of air  120 . The heating element  118  may be in communication with the controller  144  (e.g., via wired and/or wireless communication) to receive control signals for activating the heating element  118  to heat the flow of air  120 , when the HVAC system  100  is operated in a heating mode. Generally, when the HVAC system  100  is operated in a heating mode, the heating element  118  and blower  130  are turned on such that the flow of air  120  is provided across and heated by the heating element  118 . When the HVAC system  100  is operated in a cooling mode, the heating element  118  is generally turned off (i.e., such that the flow of air  120  is not heated). 
     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 blower  130  discharges air  120  into a duct  132  such that air  120  crosses the evaporator  116  and/or heating element  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  144  using any suitable type of wired and/or wireless connection. The controller  144  is configured to provide commands and/or signals to the blower  130  to control its operation. 
     The HVAC system  100  includes sensors  134   a,b  in signal communication with controller  144 . Sensor  134   a  is positioned and configured to measure a discharge air temperature  146  (e.g., a temperature of airflow  122 ). Sensor  134   b  is positioned and configured to measure a return air temperature  148  (e.g., of airflow  126 ). Signals corresponding to the properties measured by sensors  134   a,b  may be provided to the controller  144 . 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). In some embodiments, one or more of the sensors  134   a,b  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   a,b  may communicate wirelessly with the controller  144  (e.g., via a wireless network associated with the conditioned space). 
     The HVAC system  100  includes one or more thermostats  136 , for example, located within the conditioned space (e.g. a room or building). The thermostat(s)  136  are generally in signal communication with the controller  144  using any suitable type of wired and/or wireless connection. In some embodiments, one or more functions of the controller  144  may be performed by the thermostat(s)  136 . For example, the thermostat  136  may include the controller  144 . The thermostat(s)  136  may include one or more single-stage thermostats, one or more multi-stage thermostat, or any suitable type of thermostat(s). The thermostat(s)  136  are configured to allow a user to input a desired temperature or temperature setpoint  138  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  140 . The indoor air temperature  140  may be a temperature of air in the conditioned space and/or in a designated space or zone of the conditioned space, such as a room in which the thermostat or an indoor air sensor associated with the thermostat  136  is installed. 
     The controller  144  may use information from the thermostat  136  such as the temperature setpoint  138  and indoor air temperature  140  for controlling the compressor  106 , the blower  130 , and the fan  110  (e.g., for operation in a cooling mode) and/or of the heating element  118  and blower  130  (e.g., for operation in a heating mode). In some embodiments, a thermostat  136  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 an alert  142  associated with any proactively detected fault of the HVAC system  100  along any other messages related to the status and/or operation of the HVAC system  100 . 
     As described in greater detail below, the controller  144  is configured to monitor the discharge air temperature  146  measured by the discharge air temperature sensor  134   a  and the return air temperature  148  measured by the return air temperature sensor  134   b . The controller  144  is described in greater detail below with respect to  FIG.  4   . To detect a possible fault during operation in a heating mode, the controller  144  uses the monitored discharge air temperature  146  and return air temperature  148  to determine a temperature rise value  150 . The temperature rise value  150  may be determined as the difference between the discharge air temperature  146  and return air temperature  148 . As described in greater detail with respect to  FIG.  2    below, if the temperature rise value  150  is outside a range established by predefined threshold values  154 , a fault may be detected. For instance, if the temperature rise value  150  is less than a predefined minimum threshold of thresholds  154 , the controller  144  may determine that a furnace is under-firing, the HVAC system  100  is under-charged with working fluid (e.g., when the HVAC system  100  is configured to operate as a heat pump), and/or a rate of airflow  120  across the heating element  118  is too high and a corresponding alert  142  may be presented on the interface of the thermostat  136  and/or provided to the third party  156 . If the temperature rise value  150  is greater than a predefined maximum value threshold of thresholds  154 , the controller  144  may determine that a furnace is over-firing, the HVAC system  100  is over-charged with working fluid (e.g., when the HVAC system  100  is configured to operate as a heat pump), and/or a rate of airflow  120  across the heating element  118  is too low and a corresponding alert  142  may be presented on the interface of the thermostat  136  and/or provided to the third party  156 . The values of the minimum and maximum thresholds  154  may be different based on the type of heating element  118  employed by the HVAC system  100 . The proactive provision of alerts  142  may facilitate timely correction of any detected fault(s). The detection of faults associated with operating the HVAC system  100  in a heating mode is described in greater detail below with respect to  FIG.  2   . 
     To detect a possible fault during operation in a cooling mode, the controller  144  uses the monitored discharge air temperature  146  and return air temperature  148  to determine a temperature drop value  152 . The temperature drop value  152  may be determined as the difference between the return air temperature  148  and the discharge air temperature  146 . As described in greater detail with respect to  FIG.  3    below, if the temperature drop value  152  is less than a predefined minimum threshold of thresholds  154 , the controller  144  may determine that there is a loss of charge in the HVAC system  100  and/or an excessively high flow rate of airflow  120  across the evaporator  116  and a corresponding alert  142  may be presented on the interface of the thermostat  136  and/or provided to the third party  156 . If the discharge air temperature  146  is less than a predefined threshold of thresholds  154 , the controller  144  may determine that the rate of airflow  120  is insufficient and a corresponding alert  142  may be presented on the interface of the thermostat  136  and/or provided to the third party  156 . The detection of faults associated with operating the HVAC system  100  in a cooling mode is described in greater detail below with respect to  FIG.  3   . 
     In some embodiments, an indoor air temperature  140  measured by the thermostat  136  is used in place of the return air temperature  148 . For example, in some cases, an HVAC system  100  may not include a return air temperature sensor  134   b  or the return air temperature sensor  134   b  may malfunction (e.g., a return air temperature  148  may not be within a predefined range of values, as described below with respect to steps  204 ,  304  of  FIGS.  2  and  3   ). In such cases, the indoor air temperature  140  may be used in place of the return air temperature  148  in order to determine the temperature rise value  150  and/or temperature drop value  152 . 
     As described above, in certain embodiments, connections between various components of the HVAC system  100  are wired. For example, conventional cable and contacts may be used to couple the controller  144  to the various components of the HVAC system  100 , including, the compressor  106 , the fan  110 , the expansion device  114 , heating element  118 , sensors  134   a,b , blower  130 , and thermostat(s)  136 . In some embodiments, a wireless connection is employed to provide at least some of the connections between components of the HVAC system  100 . In some embodiments, a data bus couples various components of the HVAC system  100  together such that data is communicated therebetween. In a typical embodiment, the data bus may include, for example, any combination of hardware, software embedded in a computer readable medium, or encoded logic incorporated in hardware or otherwise stored (e.g., firmware) to couple components of HVAC system  100  to each other. As an example and not by way of limitation, the data bus may include an Accelerated Graphics Port (AGP) or other graphics bus, a Controller Area Network (CAN) bus, a front-side bus (FSB), a HYPERTRANSPORT (HT) interconnect, an INFINIBAND interconnect, a low-pin-count (LPC) bus, a memory bus, a Micro Channel Architecture (MCA) bus, a Peripheral Component Interconnect (PCI) bus, a PCI-Express (PCI-X) bus, a serial advanced technology attachment (SATA) bus, a Video Electronics Standards Association local (VLB) bus, or any other suitable bus or a combination of two or more of these. In various embodiments, the data bus may include any number, type, or configuration of data buses, where appropriate. In certain embodiments, one or more data buses (which may each include an address bus and a data bus) may couple the controller  144  to other components of the HVAC system  100 . 
     In an example operation of HVAC system  100 , the HVAC system  100  starts up to operate in a heating mode. For example, in response to the indoor temperature  140  falling below the temperature setpoint  138  while the HVAC system  100  is set to operate in a heating mode, the controller  144  may cause the heating element  118  and the blower  130  to turn on to start up the HVAC system  100  in the heating mode. During operation of the HVAC system  100  in the heating mode, the controller  144  receives measurements of the discharge air temperature  146  and the return air temperature  148 . These temperatures  146 ,  148  are used to determine a temperature rise value  150 . The temperature rise value  150  may be determined as: 
               Temperature   ⁢           ⁢   rise   ⁢           ⁢   value     =     DAT   -     RAT   ⁡     (     or   ⁢           ⁢   IAT     )               
where DAT is the discharge air temperature  146 , RAT is the return air temperature  148 , and IAT is the indoor air temperature  140 .
 
     Before making a determination of whether the temperature rise value  150  is indicative of a fault, the controller  144  may first determine that the HVAC system  100  has been operating in the heating mode for a minimum period of time (e.g., for at least ten minutes). This may ensure that the discharge air temperature  146  and return air temperature  148  are reliable indicators of the performance of the HVAC system  100  in the heating mode. In some embodiments, the controller  144  may also or alternatively determine, as a prerequisite to detecting any system fault, that the discharge air temperature  146  and/or the return air temperature  148  have been stable (i.e., has not changed greater than a threshold amount) for a predetermined amount of time (e.g., for at least ten minutes). 
     If the temperature rise value  150  is less than a minimum threshold value of the thresholds  154  (e.g., of about 25° F. for a furnace heating element  118  or of about 10° F. for when the HVAC system operates as a heat pump), then the HVAC system  100  may be experiencing a high rate of airflow  120 , a furnace heating element  118  may be under-firing, and/or the HVAC system  100  when operating as a heat pump in a heating mode may be under-charged with working fluid. In some embodiments in which the heating element  118  is a component of a furnace of the HVAC system  100 , the minimum threshold value of thresholds  154  is determined based on one or both of the firing rate of the furnace and the rate of the flow of air  120  across the furnace heating element  118 . For example, the minimum threshold value  154  for the temperature rise value  150  may be increased at increased firing rates and/or decreased rates of the flow of air  120 . In embodiments where heating is provided by operating the HVAC system  100  as a heat pump in the heating mode and the compressor  106  is a variable speed compressor, the minimum threshold value  154  may be based on the speed of the compressor. Prior to reporting a detected fault, the controller  144  may determine whether the above conditions have been satisfied for a threshold number of times (e.g., of three or more times). If the temperature rise value  150  is found to be less than the threshold value  154  for the threshold number of times, an alert  142  may be presented on the interface of the thermostat  136  and/or provided to the third party  156 . The alert  142  may indicate the suspected high rate of airflow  120 , under-firing of the furnace heating element  118 , and/or under-charging of the HVAC system  100  operating as a heat pump in the heating mode associated with the temperature rise value  150  being less than the threshold value  154 . 
     If the temperature rise value  150  is greater than a maximum threshold value of thresholds  154  (e.g., of about 75° F. for a furnace heating element  118  or of about 50° F. when the HVAC system  100  is configured to operate as a heat pump in the heating mode), then the HVAC system  100  may be experiencing a low rate of airflow  120 , a furnace heating element  118  may be over-firing, and/or an HVAC system  100  operating as a heat pump in the heating mode may be over-charged with working fluid. In some embodiments in which the heating element  118  is a component of a furnace of the HVAC system  100 , the maximum threshold value of thresholds  154  is determined based on one or both of the firing rate of the furnace and the rate of the flow of air  120  across the furnace heating element  118 . For example, the maximum threshold value for the temperature rise value  150  may be increased at increased firing rates and/or decreased rates of the flow of air  120 . In embodiments where heating is provided by operating the HVAC system  100  as a heat pump in the heating mode and the compressor  106  is a variable speed or multi-stage compressor, the maximum threshold value  154  may be based on the speed of the compressor. Prior to reporting a detected fault, the controller  144  may determine whether the above conditions have been satisfied for a threshold number of times (e.g., of three or more times). If the temperature rise value  150  is found to be greater than the threshold value  154  for the threshold number of times, an alert  142  may be presented on the interface of the thermostat  136  and/or provided to the third party  156 . The alert  142  may indicate the suspected low rate of airflow  120 , over-firing of the furnace heating element  118 , and/or over-charging of the HVAC system  100  operating as a heat pump in the heating mode associated with the temperature rise value  150  being greater than the threshold value  154 . 
     In another example operation of the HVAC system  100 , the HVAC system  100  starts up to operate in a cooling mode. For example, in response to the indoor temperature  140  exceeding the temperature setpoint  138  while the HVAC system  100  is set to operate in a cooling mode, the controller  144  may cause the compressor  106 , fan  110 , and blower  130  to turn on to start up the HVAC system  100  in the cooling mode. During operation of the HVAC system  100  in the cooling mode, the controller  144  receives measurements of the discharge air temperature  146  and the return air temperature  148 . These temperatures  146 ,  148  are used to determine a temperature drop value  152 . The temperature drop value  152  may be determined as: 
               Temperature   ⁢           ⁢   drop   ⁢           ⁢   value     =       RAT   ⁡     (     or   ⁢           ⁢   IAT     )       -   DAT           
where DAT is the discharge air temperature  146 , RAT is the return air temperature  148 , and IAT is the indoor air temperature  140 .
 
     Before making a determination of whether the temperature drop value  152  is indicative of a fault, the controller  144  may first determine that the HVAC system  100  has been operating in the cooling mode for a minimum period of time (e.g., for at least ten minutes). This may ensure that the discharge air temperature  146  and return air temperature  148  are reliable indicators of the performance of the HVAC system  100  in the cooling mode. In some embodiments, the controller  144  may also or alternatively determine, as a prerequisite to detecting any system fault, that the discharge air temperature  146  and/or the return air temperature  148  has been stable (i.e., has not changed greater than a threshold amount) for a predetermined amount of time (e.g., for at least ten minutes). 
     If the temperature drop value  152  is less than a minimum threshold value of the thresholds  154  (e.g., of about 15° F.), then the evaporator  116  may be experiencing a high rate of airflow  120 , the expansion device  114  may be stuck in an open position, and/or the HVAC system  100  may be under-charged with working fluid. In some embodiments (e.g., in which the compressor  106  is a variable speed or multi-stage compressor), the minimum threshold value of thresholds  154  is determined based on the speed of the compressor  106 . Prior to reporting a detected fault, the controller  144  may determine whether the above conditions have been satisfied for a threshold number of times (e.g., of three or more times). If the temperature drop value  152  is found to be less than the threshold value  154  for the threshold number of times, an alert  142  may be presented on the interface of the thermostat  136  and/or provided to the third party  156 . The alert  142  may indicate the suspected high rate of airflow  120 , the stuck expansion device  114 , and/or the under-charging of the HVAC system  100  associated with the temperature drop value  150  being less than the threshold value  154 . 
     If the discharge air temperature  146  is less than a predefined threshold value of thresholds  154  (e.g., of about 40° F.), then the evaporator  116  may be experiencing a low rate of airflow  120  and/or the expansion device  114  may be stuck in a closed position. Prior to reporting a detected fault, the controller  144  may determine whether the above conditions have been satisfied for a threshold number of times (e.g., of three or more times). If the discharge air temperature  146  is found to be less than the threshold value  154  for the threshold number of times, an alert  142  may be presented on the interface of the thermostat  136  and/or provided to the third party  156 . The alert  142  may indicate the suspected low rate of airflow  120  and/or the stuck expansion device  114  associated with the discharge air temperature  146  being less than the threshold value  154 . 
     Example Methods of HVAC System Prognostics and Diagnostics 
       FIGS.  2  and  3    illustrate methods  200  and  300  of detecting and providing appropriate alerts  142  for faults of the HVAC system when the HVAC system  100  is operating in the heating mode and cooling mode, respectively. The controller  144  is configured to execute method  200  of  FIG.  2    when the HVAC system  100  is operating in the heating mode (i.e., when there is a call for heating, such as when the indoor air temperature  140  is less than the setpoint temperature  138 ) and to execute method  300  of  FIG.  3    when the HVAC system  100  is operating in the cooling mode (i.e., when there is a call for cooling, such as when the indoor air temperature  140  is greater than the setpoint temperature  138 ). 
     Method  200  may begin at step  202  where the controller  144  receives measurements of the discharge air temperature  146  and return air temperature  148 . As described above, in some cases, the controller  144  may use the indoor air temperature  140  in place of the return air temperature  148  (e.g., when a measure or return air temperature  148  is not available or is determined to be unreliable). 
     At step  204 , the controller  144  determines whether received temperatures  146 ,  148  are within a predefined range of values (e.g., a range established by thresholds  154  of  FIGS.  1  and  4   ). For example, the controller  144  may determine whether the discharge air temperature  146  and the return air temperature  148  are within a corresponding predefined range of temperatures (e.g., between about 30° F. and about 140° F.). Generally, if the temperatures  146 ,  148  are not within the predefined range, the corresponding temperature sensors  134   a ,  134   b  are likely malfunctioning. If the return air temperature  148  is not within the predefined range of values or the return air temperature  148  is otherwise unavailable (e.g., because the return air temperature sensor  134   b  is not operating or is not present in the HVAC system  100 ), the controller  144  may use the indoor air temperature  140  in its place. While the subsequent steps of method  200  are described with respect to using the return air temperature  148  to determine temperature rise values  150 , it should be understood that the return air temperature  148  may be substituted with the indoor air temperature  140 . If the temperatures  146 ,  148  are within the predefined range of values, the controller  144  proceeds to step  206 . Otherwise, the controller  144  may restart the method  200 . 
     At step  206 , the controller  144  may determine whether the discharge air temperature  146  and return air temperature  148  have been stable for a predefined period of time. For example, the controller  144  may determine whether the temperatures  146 ,  148  have changed by greater than a threshold amount  154  (e.g., of about 1° F.) within a predefined time period (e.g., of about ten minutes). If the temperatures  146 ,  148  have changed by greater than the threshold amount  154  within the predefined time period, the temperatures  146 ,  148  are determined to not be stable, and the controller  144  returns to the start of method  200 . Otherwise, if the controller  144  determines that the temperatures  146 ,  148  have not changed by greater than the threshold amount  154  within the predefined time period, the controller  144  determines the temperatures  146 ,  148  are stable and proceeds to step  208 . 
     At step  208 , the controller  144  determines whether the HVAC system  100  has been operating in the heating mode for at least a predefined amount of time (e.g., of about ten minutes). If the HVAC system  100  has been operating in the heating mode for at least the predefined amount of time, the controller  144  proceeds to step  210 . Otherwise, if the HVAC system  100  has not been operating in the heating mode for at least the predefined amount of time, the controller  144  returns to the start of method  200 . 
     At step  210 , the controller  144  determines the temperature rise value  150 . The temperature rise value  150  generally corresponds to a measure of the extent to which the space is being heated by the HVAC system  100  during operation in the heating mode. The temperature rise value  150  is determined based on the discharge air temperature  146  and the return air temperature  148  (or the indoor air temperature  140  in some cases, as described above with respect to step  204 ). As an example, the temperature rise value  150  may be determined as the difference between the discharge air temperature  146  and the return air temperature  148  (or the difference between the discharge air temperature  146  and the indoor air temperature  140 ). 
     At step  212 , the controller determines whether the temperature rise value  150  is less than a predefined minimum threshold value  154 . The predefined minimum threshold value  154  may be specific to the type of heating element  118  employed by the HVAC system  100 . For example, the predefined minimum threshold value  154  may be about 25° F. for a furnace heating element  118 , and the predefined minimum threshold value  154  may be about 10° F. for when the HVAC system  100  is operating as a heat pump in the heating mode (i.e., with the flow of refrigerant reversed such the condenser  108  acts an evaporator and the evaporator  116  acts a condenser). If the temperature rise value  150  is less than the predefined minimum threshold value  154 , then the controller  144  proceeds to step  214 . Otherwise, if the temperature rise value  150  is not less than the predefined minimum threshold value  154 , then the controller  144  proceeds to step  220 . 
     At step  214  (i.e., for cases where the temperature rise value  150  is less than the predefined minimum threshold value  154  at step  212 ), the controller  144  may increase a count of detected low temperature rise values  150 . At step  216 , the controller  144  determines whether this count of detected low temperature rise values  150  is greater than a threshold (e.g., greater three). If the count is greater than the threshold, the controller  144  proceeds to step  218  and provides an alert  142  of the detected low temperature rise value  150 . The alert  142  of the low temperature rise value  150  may include an indication of one or more faults associated with the low temperature rise value  150 . For example, the alert  142  may include an indication of a possible high rate of the airflow  120  across the heating element  118  (e.g., an indication that the rate of airflow  120  is greater than a threshold value  154 ). If the heating element  118  is a furnace, the alert  142  may indicate the possible under-firing of the furnace. If the HVAC system  100  is operating as a heat pump in the heating mode, the alert  142  may include an indication of a possible under-charging of working fluid in the HVAC system  100  (e.g., a loss of working fluid from one or more components of the HVAC system  100 ). If, at step  216 , the count is not greater than the threshold, the controller  144  may return to the start of method  200 . By requiring the count to be greater than a threshold value, false positive fault detections may be eliminated or reduced in some cases. 
     At step  220  (i.e., for cases where the temperature rise value  150  is not less than the predefined minimum threshold value  154  at step  212 ), the controller  144  determines whether the temperature rise value  150  is greater than a predefined maximum threshold value  154 . The predefined maximum threshold value  154  may be specific to the type of heating element  118  employed by the HVAC system  100 . For example, the predefined maximum threshold value  154  may be about 75° F. for a furnace heating element  118 , and the predefined maximum threshold value  154  may be about 50° F. for when the HVAC system  100  is operating as a heat pump in the heating mode. If the temperature rise value  150  is greater than the predefined maximum threshold value  154 , then the controller  144  proceeds to step  222 . Otherwise, if the temperature rise value  150  is not greater than the predefined maximum threshold value  154 , then the controller  144  may return to the start of method  200  to continue checking for possible system faults. 
     At step  222 , the controller  144  may increase a count of detected high temperature rise values  150 . At step  224 , the controller  144  determines whether this count of detected high temperature rise values  150  is greater than a threshold (e.g., greater than three). If the count is greater than the threshold, the controller  144  proceeds to step  226  and provides an alert  142  of the detected high temperature rise value  150 . The alert  142  of the high temperature rise value  150  may include an indication of one or more faults associated with the high temperature rise value  150 . For example, the alert  142  may include an indication of a possible low rate of the airflow  120  across the heating element  118  (e.g., an indication that the rate of airflow  120  is lower than a threshold value  154 ). If the heating element  118  is component of a furnace, the alert  142  may indicate the possible over-firing of the furnace. If the HVAC system  100  is operating as a heat pump in the heating mode, the alert  142  may include an indication of a possible over-charging of working fluid in the HVAC system  100  and/or an indication that an outdoor expansion device (e.g., similar to expansion device  114  of  FIG.  1   ) is stuck in a closed position. If, at step  224 , the count is not greater than the threshold, the controller  144  may return to the start of method  200 . As described above with respect to step  216 , requiring, in some embodiments, the count to be greater than a threshold value my reduce or eliminate false positive fault detections. 
     Modifications, additions, or omissions may be made to method  200  depicted in  FIG.  2   . Method  200  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  144 , 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  200 . 
     As described above, the controller  144  generally executes the method  300  of  FIG.  3    when the HVAC system is operating in a cooling mode. Method  300  may begin at step  302  where the controller  144  receives measurements of the discharge air temperature  146  and return air temperature  148 . As described above, in some cases, the controller  144  may use the indoor air temperature  140  in place of the return air temperature  148  (e.g., when a measure or return air temperature  148  is not available or is determined to be unreliable). 
     At step  304 , the controller  144  determines whether the received temperatures  146 ,  148  are within a predefined range of values (e.g., a range established by thresholds  154  of  FIGS.  1  and  4   ). For example, the controller  144  may determine whether the discharge air temperature  146  and the return air temperature  148  are within a corresponding predefined range of temperatures (e.g., between about 30° F. and about 140° F.). Generally, if the temperatures  146 ,  148  are not within the predefined range, the corresponding temperature sensors  134   a ,  134   b  are likely malfunctioning. If the return air temperature  148  is not within the predefined range of values or the return air temperature  148  is otherwise unavailable (e.g., because the return air temperature sensor  134   b  is not operating or is not present in the HVAC system  100 ), the controller  144  may use the indoor air temperature  140  in its place. While the subsequent steps of method  300  are described with respect to using the return air temperature  148  to determine temperature drop values  152 , it should be understood that the return air temperature  148  may be substituted with the indoor air temperature  140 . If the temperatures  146 ,  148  are within the predefined range of values, the controller  144  proceeds to step  306 . Otherwise, the controller  144  may restart the method  300 . 
     At step  306 , the controller  144  may determine whether the discharge air temperature  146  and return air temperature  148  have been stable for a predefined period of time. For example, the controller  144  may determine whether the temperatures  146 ,  148  have changed by greater than a threshold amount  154  (e.g., of about 1° F.) within a predefined time period (e.g., of about ten minutes). If the temperatures  146 ,  148  have changed by greater than the threshold amount  154  within the predefined time period, the temperatures  146 ,  148  are determined to not be stable, and the controller  144  returns to the start of method  300 . Otherwise, if the controller  144  determines that the temperatures  146 ,  148  have not changed by greater than the threshold amount  154  within the predefined time period, the controller  144  determines the temperatures  146 ,  148  are stable and proceeds to step  308 . 
     At step  308 , the controller  144  determines whether the HVAC system  100  has been operating in the cooling mode for at least a predefined amount of time (e.g., of about ten minutes). If the HVAC system  100  has been operating in the cooling mode for at least the predefined amount of time, the controller  144  proceeds to step  310 . Otherwise, if the HVAC system  100  has not been operating in the cooling mode for at least the predefined amount of time, the controller  144  returns to the start of method  300 . 
     At step  310 , the controller  144  determines the temperature drop value  152 . The temperature drop value  152  generally corresponds to a measure of the extent to which the space is being cooled by the HVAC system  100  during operation in the cooling mode. The temperature drop value  152  is determined based on the discharge air temperature  146  and the return air temperature  148  (or the indoor air temperature  140  in some cases, as described above with respect to step  304 ). As an example, the temperature drop value  152  may be determined as the difference between the return air temperature  148  and the discharge air temperature  146  (or the difference between the indoor air temperature  140  and the discharge air temperature  146 ). 
     At step  312 , the controller determines whether the temperature drop value  152  is less than a predefined minimum threshold value  154 . For example, the predefined minimum threshold value  154  may be about 15° F. In some embodiments (e.g., in which the compressor  106  is a variable speed or multi-stage compressor), the minimum threshold value  154  is determined based on the speed of the compressor  106 . If the temperature drop value  152  is less than the predefined minimum threshold value  154 , then the controller  144  proceeds to step  314 . Otherwise, if the temperature drop value  152  is not less than the predefined minimum threshold value  154 , then the controller  144  proceeds to step  320 . 
     At step  314  (i.e., for cases where the temperature drop value  152  is less than the predefined minimum threshold value  154  at step  312 ), the controller  144  may increase a count of detected low temperature drop values  152 . At step  316 , the controller  144  determines whether this count of detected low temperature drop values  152  is greater than a threshold (e.g., greater than three). If the count is greater than the threshold, the controller  144  proceeds to step  318  and provides an alert  142  of the detected low temperature drop value  152 . The alert  142  of the low temperature drop value  152  may include an indication of one or more faults associated with the low temperature drop value  152 . For example, the alert  142  may include an indication of a possible high rate of the airflow  120  across the evaporator  116  (e.g., an indication that the rate of airflow  120  is greater than a threshold value  154 ), an indication of possible under-charging of the HVAC system  100  with working fluid (e.g., a loss of working fluid from one or more components of the HVAC system  100 ), and/or an indication that the expansion device  114  may be stuck in an open position. If, at step  316 , the count is not greater than the threshold, the controller  144  may return to the start of method  300 . By requiring the count to be greater than a threshold value, false positive fault detections may be eliminated or reduced in some cases. 
     At step  320  (i.e., for cases where the temperature drop value  152  is not less than the predefined minimum threshold value  154  at step  312 ), the controller  144  determines whether the discharge air temperature  146  is less than a predefined threshold value  154 . The predefined threshold value  154  may be about 40° F. If the discharge air temperature  146  is less than the predefined threshold value  154 , then the controller  144  proceeds to step  322 . Otherwise, if the discharge air temperature  146  is not less than the predefined threshold value  154 , then the controller  144  may return to the start of method  300  to continue checking for possible system faults. 
     At step  322 , the controller  144  may increase a count of detected low discharge air temperature  146 . At step  324 , the controller  144  determines whether this count of detected low discharge air temperature  146  is greater than a threshold (e.g., greater than three). If the count is greater than the threshold, the controller  144  proceeds to step  326  and provides an alert  142  of the detected low discharge air temperature  146 . The alert  142  of the low discharge air temperature  146  may include an indication of one or more faults associated with the low discharge air temperature  146 . For example, the alert  142  may include an indication of a possible low rate of the airflow  120  across the evaporator  116  (e.g., an indication that the rate of airflow  120  is lower than a threshold value  154 ) and/or an indication that the expansion device  114  is possibly stuck in a closed position. If, at step  324 , the count is not greater than the threshold, the controller  144  may return to the start of method  300 . As described above with respect to step  316 , requiring, in some embodiments, the count to be greater than a threshold value may reduce or eliminate false positive fault detections. 
     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  144 , 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 . 
     Example Controller 
       FIG.  4    is a schematic diagram of an embodiment of the controller  144 . The controller  144  includes a processor  402 , a memory  404 , and an input/output (I/O) interface  406 . 
     The processor  402  includes one or more processors operably coupled to the memory  404 . The processor  402  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  404  and controls the operation of HVAC system  100 . The processor  402  may be a programmable logic device, a microcontroller, a microprocessor, or any suitable combination of the preceding. The processor  402  is communicatively coupled to and in signal communication with the memory  404 . The one or more processors are configured to process data and may be implemented in hardware or software. For example, the processor  402  may be 8-bit, 16-bit, 32-bit, 64-bit or of any other suitable architecture. The processor  402  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  404  and executes them by directing the coordinated operations of the ALU, registers, and other components. The processor  402  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  402  is not limited to a single processing device and may encompass multiple processing devices. Similarly, the controller  144  is not limited to a single controller but may encompass multiple controllers. 
     The memory  404  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  404  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  404  is operable (e.g., or configured) to store monitored temperatures  140 ,  146 ,  148 , temperature rise values  150 , temperature drop values  152 , and thresholds  154  (i.e., including any of the threshold values, predefined ranges of values, predefined time periods, and the like described above with respect to  FIGS.  1 - 3   ), and/or any other logic and/or instructions for performing the function described in this disclosure. 
     The I/O interface  406  is configured to communicate data and signals with other devices. For example, the I/O interface  406  may be configured to communicate electrical signals with components of the HVAC system  100  including the compressor  106 , fan  110 , expansion device  114 , heating element  118 , sensors  134   a,b , blower  130 , and thermostat(s)  136 . The I/O interface 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  406  may include ports or terminals for establishing signal communications between the controller  144  and other devices. The I/O interface  406  may be configured to enable wired and/or wireless communications. 
     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.