Patent Publication Number: US-11639803-B2

Title: System and method for identifying causes of HVAC system faults

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/806,305 filed Mar. 2, 2020, by Amita Brahme et al., and entitled “SYSTEM AND METHOD FOR IDENTIFYING CAUSES OF HVAC SYSTEM FAULTS,” 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 a system and method for identifying causes of HVAC system faults. 
     BACKGROUND 
     Heating, ventilation, and air conditioning (HVAC) systems are used to regulate environmental conditions within an enclosed space. Air is cooled or heated via heat transfer with refrigerant flowing through the system and returned to the enclosed space as conditioned air. 
     SUMMARY OF THE DISCLOSURE 
     In an embodiment, a heating, ventilation and air conditioning (HVAC) system includes a suction-side sensor positioned and configured to measure a suction-side property associated with refrigerant provided to an inlet of a compressor of the system. The system includes a liquid-side sensor positioned and configured to measure a liquid-side property associated with the refrigerant provided from an outlet of the compressor. The system includes a controller communicatively coupled to the suction-side sensor and the liquid-side sensor. The controller monitors the suction-side property and the liquid-side property over a period of time. The controller determines whether the suction-side property has an increasing or decreasing trend over the period of time (e.g., and that the compressor speed and outdoor temperature are not varying over the period of time). The controller determines whether the liquid-side property has an increasing or decreasing trend. In response to determining that both the suction-side property and the liquid-side property have an increasing trend over the period of time, a fan fault is detected. In response to determining that the suction-side property has a decreasing trend and the liquid-side property has an increasing trend over the period of time, a blockage of a refrigerant conduit subsystem is detected. In response to determining that both the suction-side property and the liquid-side property have a decreasing trend over the period of time, a blower fault is detected. 
     In another embodiment, an HVAC system includes a suction-side sensor positioned and configured to measure a suction-side property associated with refrigerant provided to an inlet of a compressor of the system. The system includes a shutoff switch communicatively coupled to the suction-side sensor and configured to be tripped and automatically stop operation of the compressor in response to determining that the suction-side property is less than a predefined minimum value. The system includes a liquid-side sensor positioned and configured to measure a liquid-side property associated with the refrigerant provided from an outlet of the compressor. The system includes a controller communicatively coupled to the shutoff switch and the liquid-side sensor. The controller stores measurements of the liquid-side property over an initial period of time. The controller detects that the shutoff switch is tripped at a first time stamp corresponding to an end of the initial period of time. The controller accesses the measurements of the liquid-side property. The controller determines, based on the measurements of the liquid-side property, whether the liquid-side property has an increasing or a decreasing trend. In response to determining that the liquid-side property has the decreasing trend, a malfunction of a blower of the system is determined to have caused the shutoff switch to trip. In response to determining that the liquid-side property has the increasing trend, a blockage of the refrigerant conduit subsystem is determined to have caused the shutoff switch to trip. 
     In yet another embodiment, an HVAC system includes a liquid-side sensor positioned and configured to measure a liquid-side property associated with the refrigerant provided from an outlet of a compressor of the system. The system includes a shutoff switch communicatively coupled to the liquid-side sensor and configured to be tripped and automatically stop operation of the compressor and fan, in response to determining that the liquid-side property is greater than a predefined maximum value. The system includes a suction-side sensor positioned and configured to measure a suction-side property associated with refrigerant provided to an inlet of the compressor. The system includes a controller communicatively coupled to the shutoff switch and the suction-side sensor. The controller stores measurements of the suction-side property over an initial period of time. The controller detects that the shutoff switch is tripped at a first time stamp corresponding to an end of the initial period of time. The controller accesses the measurements of the suction-side property. The controller determines, based on the measurements of the suction-side property, whether the suction-side property has an increasing or decreasing trend. In response to determining that the suction-side property has the increasing trend, the controller determines that a malfunction of a fan caused the shutoff switch to trip. In response to determining that the suction-side property has the decreasing trend, the controller determines that a blockage of the refrigerant conduit subsystem caused the shutoff switch to trip. 
     HVAC systems include several components which may fail throughout the lifetime of the system, resulting in a system fault. As an example, a system fault may be caused by a loss of refrigerant from the HVAC system, a blockage of the flow of refrigerant through the HVAC system, a malfunction of the fan of an HVAC system, a malfunction of the blower of an HVAC system or the like. Conventional approaches to detecting HVAC system faults generally rely on a user of the system recognizing a loss of system performance (e.g., a user noticing that heating or cooling is no longer being achieved as desired). For example, an occupant of an enclosed space being conditioned by an HVAC system may recognize that the space is not comfortable or is not reaching a desired temperature setpoint. Such approaches result in delayed detection of system faults, such that it may be too late to take effective corrective action once a fault is identified. For instance, by the time a fault is detected using conventional approaches, damage may have occurred to one or more system components, resulting in a need for repairs which may be costly, complex, or even impossible. Moreover, using previous technology, no information is provided with regard to which component of the HVAC system failed or malfunctioned to cause the fault. 
     This disclosure solves problems of previous systems, including those recognized above, by providing systems and methods for detecting a system fault and determining the underlying cause of the detected fault. For example, properties (e.g., or trends in properties) of the refrigerant flowing in different portions of an HVAC system may be used to forecast likely system faults and provide an alert related to the likely fault(s), such that corrective action may be taken before the HVAC system fails or is shut down. In some embodiments, this disclosure provides for determining the underlying causes of system faults (e.g., whether a fault is caused by a blockage of refrigerant flow, a fan malfunction, or a blower malfunction), thereby allowing appropriate corrective actions to be taken more efficiently. As such, the approaches described in this disclosure may incorporated into practical applications to improve the performance of HVAC systems by anticipating malfunctions of components of the system and/or identifying the cause of a failure of the HVAC system. 
     In some cases, an HVAC system may include a high-pressure shutoff switch, which causes the HVAC system to stop operating when a maximum liquid pressure is reached, and/or a low-pressure shutoff switch, which is triggered and causes the HVAC system to stop operating when a minimum suction pressure is reached. There exists an unmet need to (1) identify conditions which would lead to one of these shutoff switches being tripped and (2) identify the underlying components which malfunctioned causing the shutoff switches being tripped. This disclosure encompasses solutions to these unmet needs. For example, some embodiments of this disclosure provide systems, methods and devices for detecting likely system faults and the underlying causes based on trends in monitored system properties (e.g., based on trends in suction and liquid temperature or pressure measurements), as described in greater detail below with respect to  FIGS.  1 - 3   . As another example, this disclosure provides systems, methods and devices for determining the underlying cause of a low-pressure shutoff switch being tripped, as described in greater detail below with respect to  FIGS.  1 ,  2 A -D, and  4 . As yet another example, this disclosure provides systems, methods and devices for determining the underlying cause of a high-pressure shutoff switch being tripped, as described in greater detail below with respect to  FIGS.  1 ,  2 A -D, and  5 . 
     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 system fault prognostic s and/or diagnostics; 
         FIG.  2 A  is a table illustrating trends associated with the prognostics and/or diagnostics of faults of the system of  FIG.  1   ; 
         FIGS.  2 B- 2 D  illustrate examples of approaches to determining the trends shown in the table of  FIG.  2 A ; 
         FIG.  3    is a flowchart illustrating an example method of operating the HVAC system of  FIG.  1    for system fault prognostics and diagnostics; 
         FIG.  4    is a flowchart illustrating an example method of operating the HVAC system of  FIG.  1    for system fault diagnostics after a shutoff switch associated with a low suction property value is tripped; 
         FIG.  5    is a flowchart illustrating an example method of operating the HVAC system of  FIG.  1    for system fault diagnostics following after a shutoff switch associated with a high suction property value is tripped; and 
         FIG.  6    is a diagram of the controller of the example HVAC system of  FIG.  1   . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure and its advantages are best understood by referring to  FIGS.  1  through  6    of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     As described above, prior to the present disclosure, there was a lack of tools for effectively detecting HVAC system faults and for determining the underlying cause of such system faults. The systems and methods described in this disclosure provide solutions to these problems by facilitating prognostics and diagnostics of HVAC system faults. For example, as described with respect to  FIG.  3    below, trends in a suction-side property and a liquid-side property of refrigerant flowing the HVAC system may be monitored to identify upcoming system faults and provide an advanced indication of the suspected underlying cause of the anticipated fault, thereby facilitating preventative maintenance. As described with respect to  FIG.  4    below, if a shutoff switch associated with the suction-side property falling below a minimum value is tripped, trends in the liquid-side property over time may be evaluated to determine the underlying cause of switch&#39;s having been tripped. As described with respect to  FIG.  5    below, if a shutoff switch associated with the liquid-side property increasing above a maximum value is tripped, trends in the suction-side property over time may be evaluated to determine the underlying cause of switch&#39;s having been tripped. 
     As used in this disclosure a “suction-side property” refers to a property (e.g., a temperature or pressure) associated with refrigerant provided to an inlet of the compressor. For example, a suction-side property may be a temperature or pressure of refrigerant provided to a compressor of an HVAC system (e.g., refrigerant flowing into the inlet of the compressor or refrigerant flowing in conduit leading to the inlet of the compressor. As used in this disclosure, a “liquid-side property” refers to a property (e.g., a temperature or pressure) associated with refrigerant provided from an outlet of the compressor. For example, a liquid-side property may be a temperature or pressure of refrigerant provided from a compressor of an HVAC system (e.g., refrigerant flowing out of the outlet of the compressor or refrigerant flowing in conduit leading from the outlet of the compressor. 
     HVAC System 
       FIG.  1    is a diagram of an embodiment of an HVAC system  100  configured for the detection of system faults and the determination of the underlying cause of these faults (e.g., a malfunctioning fan  114 , a malfunctioning blower  132 , or refrigerant flow blockage). The HVAC system  100  conditions air for delivery to a conditioned space. The conditioned 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 the conditioned air is delivered to the interior of the building. In other embodiments, portion(s) of the system 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   . For instance, in some embodiments, the HVAC system  100  may be configured act as a heat pump by reversing flow of the refrigerant through the system. 
     The HVAC system  100  includes a refrigerant conduit subsystem  102 , a condensing unit  104 , an expansion valve  118 , an evaporator  120 , a thermostat  138 , and a controller  144 . The HVAC system  100  is configured to determine anticipated system faults (e.g., anticipated trips of the low-pressure shutoff switch  146  and/or the high-pressure shutoff switch  148 ) by monitoring trends in properties of the HVAC system  100  (e.g., the suction-side property  108   b  and the liquid-side property  110   b ), as described in greater detail below. For instance, trends, over time, of the suction-side property  108   b  and the liquid-side property may be used to diagnose anticipated and already detected faults (see table  200  of  FIG.  2 A  for a summary of trends and/or associated underlying causes of faults). 
     The refrigerant conduit subsystem  102  facilitates the movement of a refrigerant (e.g., a refrigerant) through a cooling cycle such that the refrigerant flows as illustrated by the dashed arrows in  FIG.  1   . 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. 
     The condensing unit  104  includes a compressor  106 , a suction-side sensor  108   a , a liquid-side sensor  110   a,  a condenser  112 , and a fan  114 . In some embodiments, the condensing unit  104  is an outdoor unit while other components of system  100  may be indoors. The compressor  106  is coupled to the refrigerant conduit subsystem  102  and compresses (i.e., increases the pressure of) the refrigerant. The compressor  106  of condensing unit  104  may be a variable speed or multi-stage compressor. 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 refrigerant 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, a multi-stage compressor may include multiple compressors, each configured to operate at a constant speed to increase the pressure of the refrigerant to keep the refrigerant moving along the refrigerant conduit subsystem  102 . 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 a wired or wireless connection. The controller  144  provides commands or signals to control the operation of the compressor  106  and/or receives signals from the compressor  106  corresponding to a status of the compressor  106 . For example, when the compressor  106  is a variable speed compressor, the controller  144  may provide a signal to control the compressor speed. When the compressor  106  operates as a multi-stage compressor, the controller  144  may provide an indication of the number of compressors to turn on and off to adjust the compressor  106  for a given cooling capacity. The controller  144  may operate the compressor  106  in different modes corresponding to load conditions (e.g., the amount of cooling or heating required by the HVAC system  100 ). The controller  144  is described in greater detail below with respect to  FIG.  6   . 
     The suction-side sensor  108   a  is generally positioned and configured to measure a suction-side property  108   b  (e.g., a temperature or pressure) associated with refrigerant provided to an inlet of the compressor  106 . For example, the suction-side sensor  108   a  may be located in, on, or near the inlet of the compressor  106  to measure properties of the refrigerant flowing into the compressor  106 . The suction-side sensor  108   a  is in signal communication with the controller  144  via wired and/or wireless connection and is configured to provide the suction-side property  108   b  to the controller  144 , as illustrated in  FIG.  1   . The suction-side property  108   b  is generally provided as an electronic signal that is interpretable by the controller  144 . In some embodiments, the suction-side property  108   b  is a suction-side pressure (i.e., the pressure of refrigerant flowing into the compressor  106 ). For example, the suction-side sensor  108   a  may provide an indication of the suction-side property  108   b  (e.g., a current or voltage proportional to the measured suction-side property  108   b ) or may provide a signal which may be used by the controller  144  to calculate the suction-side property  108   b.  In some embodiments, the suction-side property  108   b  is a suction-side temperature (i.e., the temperature of refrigerant flowing into the compressor  106 ). The example of  FIG.  1    illustrates the suction-side sensor  108   a  positioned in the refrigerant conduit subsystem  102  proximate to the inlet of the compressor  106 . However, it should be understood that the suction-side sensor  108   a  may be positioned in any other appropriate position (e.g., in the inlet of the compressor  106  or further upstream of the inlet of the compressor  106 ). For instance, in some embodiments, the suction-side sensor  108   a  is located outside of the condensing unit  104  and further upstream (and optionally indoors) in the refrigerant conduit subsystem  102 . 
     The liquid-side sensor  110   a  is generally positioned and configured to measure a liquid-side property  110   b  (e.g., a temperature or pressure) associated with refrigerant provided from an outlet of the compressor  106 . For example, the liquid-side sensor  110   a  may be located in, on, or near the outlet of the compressor  106  to measure properties of the refrigerant flowing out of the compressor  106  (e.g., in a compressed, liquid form). The liquid-side sensor  110   a  is in signal communication with the controller  144  via wired and/or wireless connection and is configured to provide the liquid-side property  110   b  to the controller  144 , as illustrated in  FIG.  1   . Similarly to the suction-side property  108   b,  the liquid-side property  110   b  is generally provided as an electronic signal that is interpretable by the controller  144 . In some embodiments, the liquid-side property  110   b  is a liquid-side pressure (i.e., the pressure of refrigerant flowing into the compressor  106 ). For example, the liquid-side sensor  110   a  may provide an indication of the liquid-side property  110   b  (e.g., a current or voltage proportional to the measured liquid-side property  110   b ) or may provide a signal which may be used by the controller  144  to calculate the liquid-side property  110   b.  In some embodiments, the liquid-side property  110   b  is a liquid-side temperature (i.e., the temperature of refrigerant flowing into the compressor  106 ). The example of  FIG.  1    illustrates the liquid-side sensor  110   a  positioned in the refrigerant conduit subsystem  102  proximate to the outlet of the compressor  106 . However, it should be understood that the liquid-side sensor  110   a  may be positioned in any other appropriate position (e.g., in the outlet of the compressor  106  or further downstream from the outlet of the compressor  106 ). For instance, in some embodiments, the liquid-side sensor  110   a  is located nearer the inlet of the condenser  112 . 
     The condenser  112  is configured to facilitate movement of the refrigerant through the refrigerant conduit subsystem  102 . The condenser  112  is generally located downstream of the compressor  106  and is configured to remove heat from the refrigerant. The fan  114  is configured to move air  116  across the condenser  112 . For example, the fan  114  may be configured to blow outside air through the condenser  112  to assist in cooling the refrigerant flowing therethrough. The fan  114  may in signal communication with the controller  144  via wired and/or wireless communication. For instance, the fan  114  may receive signals from the controller  144  causing the fan to turn on or off based on a cooling need. However, in some embodiments, the fan  114  is not configured to provide any operational information to the controller  144  (i.e., such that the controller  144  is not informed of an operational status or malfunction of the fan  114 ). The compressed, cooled refrigerant flows from the condenser  112  toward an expansion device  118 . 
     The expansion device  118  is coupled to the refrigerant conduit subsystem  102  downstream of the condenser  112  and is configured to remove pressure from the refrigerant. In this way, the refrigerant is delivered to the evaporator  120  and receives heat from airflow  122  to produce a conditioned airflow  124  that is delivered by a duct subsystem  126  to the conditioned space. In general, the expansion device  118  may be a valve such as an expansion valve or a flow control valve (e.g., a thermostatic expansion valve (TXV) valve) or any other suitable valve for removing pressure from the refrigerant while, optionally, providing control of the rate of flow of the refrigerant. The expansion device  118  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 at which refrigerant flows through the refrigerant subsystem  102 . However, in some embodiments, the expansion device  118  is not configured to provide any operational information to the controller  144  (i.e., such that the controller  144  is not informed of an operational status or malfunction of the expansion device  118 ). 
     The evaporator  120  is generally any heat exchanger configured to provide heat transfer between air flowing through the evaporator  120  (i.e., contacting an outer surface of one or more coils of the evaporator  120 ) and refrigerant passing through the interior of the evaporator  120 . The evaporator  120  is fluidically connected to the compressor  106 , such that refrigerant generally flows from the evaporator  120  to the compressor  106 . A portion of the HVAC system  100  is configured to move air  122  across the evaporator  120  and out of the duct sub-system  126  as conditioned air  124 . Return air  128 , which may be air returning from the building, fresh air from outside, or some combination, is pulled into a return duct  130 . 
     The blower  132  pulls the return air  128  and discharges airflow  122  into a duct  134  from where the airflow  122  crosses the evaporator  120  or heating elements (not shown) to produce the conditioned airflow  124 . The blower  132  is any mechanism for providing a flow of air through the HVAC system  100 . For example, the blower  132  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 types of blowers. The blower  132  is in signal communication with the controller  144  using any suitable type of wired or wireless connection. The controller  144  is configured to provide commands or signals to the blower  132  to control its operation. For example, the controller  144  may be configured to signals to the blower  132  to control the speed of the blower  132 . In some embodiments, the controller  144  may be configured to receive operational information from the blower  132  (e.g., associated with a status of the blower  132 ). However, in other embodiments, the blower  132  is not configured to provide operational information to the controller  144  (i.e., such that the controller  144  is not informed of an operational status or a malfunction of the blower  132 ). 
     The HVAC system  100  includes one or more sensors  136   a,b  in signal communication with the controller  144 . The sensors  136   a,b  may include any suitable type of sensor for measuring air temperature and/or other properties of the conditioned space (e.g. a room or building) and/or the surrounding environment (e.g., outdoors). The sensors  136   a,b  may be positioned anywhere within the conditioned space, the HVAC system  100 , and/or the surrounding environment. As an example, the HVAC system  100  may include a sensor  136   a  positioned and configured to measure a return air temperature (e.g., of airflow  128 ) and/or a sensor  136   b  positioned and configured to measure a supply or treated air temperature (e.g., of airflow  124 ). As another example, the HVAC system  100  may include a sensor (not shown for clarity and conciseness) positioned and configured to measure an outdoor air temperature and provide this information to the controller  144 . In other cases, the HVAC system  100  may include sensors positioned and configured to measure any other suitable type of air temperature and/or other property (e.g., the temperature of air at one or more locations within the conditioned space, e.g., an indoor and/or outdoor humidity). 
     The HVAC system  100  includes one or more thermostats  138 , which may be located within the conditioned space (e.g. a room or building). A thermostat  138  is generally in signal communication with the controller  144  using any suitable type of wired or wireless communication. The thermostat  138  may be a single-stage thermostat, a multi-stage thermostat, or any suitable type of thermostat for the HVAC system  100 . The thermostat  138  is configured to allow a user to input a desired temperature or temperature setpoint  140  for a designated space or zone such as a room in the conditioned space. The controller  144  may use information from the thermostat  138  such as the temperature setpoint  140  for controlling the compressor  106 , the fan  114 , the expansion device  118 , and/or the blower  132 . In some embodiments, the thermostat  138  includes a user interface 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 input of the temperature setpoint  140  and display of a fault alert  142  related to any faults anticipated and/or detected by the controller  144  and the determined underlying cause of the fault, as described in greater detail below. 
     As described in greater detail below, the controller  144  is configured to monitor the suction-side property  108   b  and/or the liquid-side property  110   b,  and use this monitored information for system fault prognostics and/or diagnostics.  FIG.  2 A  illustrates the relationship between various trends in properties  108   b,    110   b  and the associated causes of a system fault. For example, determined trends may be used to determine whether a system fault is anticipated and identify an underlying cause of the anticipated fault (e.g., whether the anticipated fault is associated with a malfunction of the fan  114 , a blockage of the refrigerant conduit subsystem  102 , or a malfunction of the blower  132 ), as described in greater detail with respect to  FIG.  3    below. As another example, the controller  144  may be configured to determine that the low-pressure shutoff switch  146  has been tripped (e.g., because the suction-side property  108   b  fell below a minimum value) and determine whether the switch  146  was tripped because of a blockage of the refrigerant conduit subsystem  102  or a malfunction of the blower  132 , as described in greater detail with respect to  FIG.  4    below. As a further example, the controller  144  may be configured to determine that the high-pressure shutoff switch  148  has been tripped (e.g., because the liquid-side property  110   b  exceeded a maximum value) and determine whether the switch  146  was tripped because of a malfunction of the fan  114  or a blockage of the refrigerant conduit subsystem  102 , as described in greater detail with respect to  FIG.  5    below. 
     The low-pressure shutoff switch  146  is generally any appropriate device configured to communicate with the suction-side sensor  108   a  and the controller  144  and stop operation of the HVAC system  100  under certain conditions. The low-pressure shutoff switch  146  is generally configured to receive suction-side property  108   b  from the suction-side sensor  108   a,  determine whether the suction-side property  108   b  is less than a minimum value (e.g., a minimum threshold value of the threshold(s)  612  of  FIG.  6   ), and cause the HVAC system  100  to stop operating if the suction-side property  108   b  is less than the minimum value. In other words, if the suction-side property  108   b  is less than the minimum value, the switch  146  is tripped, causing the HVAC system  100  to stop operation. Stopping operation of the HVAC system  100  may include stopping operation of the compressor  106  (e.g., turning the compressor off or adjusting the speed of the compressor  106  to zero hertz), stopping operation of the fan  114 , and/or stopping operation of the blower  132 . The low-pressure shutoff switch  146  may provide an indication that the switch  146  has been tripped to the controller  144  (e.g., such that the controller  144  may subsequently determine the underlying cause of the trip, as described with respect to  FIG.  4    below). While illustrated as a separate device in the example of  FIG.  1   , functions of the low-pressure shutoff switch  146  may be implemented by the controller  144  (i.e., the controller  144  may include instructions for implementing functions of the low-pressure shutoff switch  146  described above). 
     The high-pressure shutoff switch  148  is generally any appropriate device configured to communicate with the liquid-side sensor  110   a  and the controller  144  and stop operation of the HVAC system  100  under certain conditions. The high-pressure shutoff switch  148  is generally configured to receive liquid-side property  110   b  from the liquid-side sensor  110   a,  determine whether the liquid-side property  110   b  is greater than a maximum value (e.g., a maximum threshold value of the threshold(s)  612  of  FIG.  6   ), and cause the HVAC system  100  to stop operating if the liquid-side property  110   b  is greater than the maximum value. In other words, if the liquid-side property  110   b  is greater than the maximum value, the switch  148  is tripped, causing the HVAC system  100  to stop operation. Stopping operation of the HVAC system  100  may include stopping operation of the compressor  106  (e.g., turning the compressor off or adjusting the speed of the compressor  106  to zero hertz), stopping operation of the fan  114 , and/or stopping operation of the blower  132 . The high-pressure shutoff switch  148  may provide an indication that the switch  146  has been tripped to the controller  144  (e.g., such that the controller  144  may subsequently determine the underlying cause of the trip, as described with respect to  FIG.  5    below). While illustrated as a separate device in the example of  FIG.  1   , the high-pressure shutoff switch  148  may be implemented by the controller  144  (i.e., the controller  144  may include instructions for implementing functions of the high-pressure shutoff switch  148  described above). 
     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 suction-side sensor  108   a,  the liquid-side sensor  110   a,  the expansion device  118 , the blower  132 , sensor(s)  136   a,b , and thermostat(s)  138 . 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  154  to other components of the HVAC system  100 . 
     In an example operation of HVAC system  100 , the system  100  starts up to provide cooling to an enclosed space based on temperature setpoint  140 . For example, in response to the indoor temperature exceeding the temperature setpoint  140 , the controller  144  may cause the compressor  106 , the fan  114 , and the blower  132  to turn on to “startup” the HVAC system  100 . While the HVAC system  100  is cooling the space, the controller  144  may monitor values of the suction-side property  108   b  and the liquid-side property  110   b.  In some embodiments, the controller may wait a predefined delay time (e.g., of about 5 to 15 minutes) before the suction-side property  108   b  and liquid-side property  110   b  are monitored (e.g., to allow the HVAC system to stabilize prior to detecting an anticipated system fault). 
     The monitored suction-side property  108   b  and liquid-side property  110   b  may be used to determine whether an anticipated fault (e.g., a likely future fault) or currently occurring fault is detected and identify the underlying cause of the fault.  FIGS.  2 B- 2 D  illustrate the determination of an anticipated fault related to the various trends identified in table  200  of  FIG.  2 A . For instance, as illustrated in plot  210  of  FIG.  2 B , if both the suction-side property  108   b  and the liquid-side property  110   b  display an increasing trend, the controller  144  may detect an anticipated fan error-induced system fault. For example, the controller  144  may determine that the fan  114  is likely experiencing a malfunction (e.g., such that an expected or desired rate of airflow  116  is not being provided). Trends in the suction-side and liquid-side properties  108   b,    110   b  may be determined, for example, based on a rate of change of the suction-side and liquid-side properties  108   b,    110   b,  an extent to which the suction-side and liquid-side properties  108   b,    110   b  change during a predetermined time interval, and/or whether the suction-side and liquid-side properties  108   b,    110   b  consistently increase or decrease during sub-intervals of a larger time interval, as described in greater detail below with respect to the examples of  FIGS.  2 B- 2 C . 
     Plot  210  of  FIG.  2 B  shows values of the suction-side property  108   b  and the liquid-side property  110   b  over time for the example case of a malfunction of fan  114 . At an initial time (to)  212 , the fan  114  stops functioning (i.e., such that airflow  116  of  FIG.  1    is no longer provided across the condenser  112 ). Following the malfunction of the fan  114  at time  212 , the values of the suction-side property  108   b  and liquid-side property  110   b  increase. 
     In order to determine whether the suction-side property  108   b  and the liquid-side property  110   b  are increasing or decreasing, the controller  144  may evaluate changes in the properties  108   b,    110   b  over a time period  214 . In some embodiments, over the time period  214 , the controller  144  calculates a rate of change  216  (e.g., a time derivative) of the liquid-side property  110   b.  If the rate of change  216  is positive (i.e., greater than zero) and greater than a threshold value  218 , the controller  144  determines that the liquid-side property  110   b  has an increasing trend. In some embodiments, the controller  144  calculates a difference  220  between values of the liquid-side property  110   b  at the end and beginning of the time period  214 . In such embodiments, if the difference  220  is positive (i.e., greater than zero) and greater than a threshold value  222 , the controller  144  determines that the liquid-side property  110   b  has an increasing trend. In some cases, the controller  144  may determine the difference  220  for at least three sequential subintervals of time period  214 , and an increasing trend is only determined if the differences  220  calculated in these sequential subintervals is greater than the threshold value  222 . A similar approach may be used to determine whether the suction-side property  108   b  has an increasing trend. For instance, if a rate of change  224  (e.g., time derivative) of the suction-side property  108   b  is greater than a positive threshold  226 , the controller  144  may determine that the suction-side property  108   b  is increasing. As another example, if a difference  228  between values of the suction-side property  108   b  at the end and beginning of the time period  214  (e.g., or during at least three sequential subintervals of the time period  214 ) is greater than a threshold value  230 , the controller  144  may determine that the suction-side property  108   b  has an increasing trend. 
     Following detection of a fan error-induced fault (e.g., as illustrated in  FIG.  2 B ), the controller  144  may cause a fan fault alert  142  to be displayed on an interface of the thermostat  138 . In some embodiments, the controller  144  may cause the HVAC system  100  to stop operating (e.g., to stop operation of the compressor  106 , fan  114 , and blower  132 ) such that damage to the HVAC system  100  is avoided. In some embodiments, the fan fault alert  142  may be provided to a third-party (e.g., an administrator or maintenance provider of the HVAC system  100 ). This may provide for more rapid correction of the possible malfunction of the fan  114 . In some cases, the advanced detection of an anticipated malfunction may allow appropriate corrective action to be taken (e.g., repair or replacement of the fan  114 ), before a more catastrophic failure of the malfunctioning device or the HVAC system  100  occurs. Thus, the HVAC system  100  may be able to provide continued air conditioning with fewer down times during which air conditioning is not possible. 
     As another example illustrated in table  200  of  FIG.  2 A , if the suction-side property  108   b  has a decreasing trend and the liquid-side property has an increasing trend, the controller  144  may detect an anticipated fault associated with a blockage of refrigerant flow in the refrigerant conduit subsystem  102 . Such a fault may be associated with a malfunction of the expansion device  118  and/or the accumulation of debris in the conduit subsystem  102 . 
       FIG.  2 C  shows a plot  240  of values of the suction-side property  108   b  and the liquid-side property  110   b  over time for the example case of a blockage of the refrigerant conduit subsystem  102 . At an initial time (t 0 )  242 , the blockage of the conduit subsystem  102  occurs (e.g., debris blocks flow of refrigerant through the conduit subsystem  102 , the expansion device  118  closes or malfunctions, or the like). Following the blockage of the refrigerant conduit subsystem  102  at time  242 , the values of the suction-side property  108   b  decrease and values of the liquid-side property  110   b  increase, as illustrated in plot  240 . 
     Similarly to as described above with respect to  FIG.  2 B , in order to determine whether the suction-side property  108   b  and the liquid-side property  110   b  are increasing or decreasing, the controller  144  may evaluate changes in the properties  108   b,    110   b  over a time period  244 . For instance, if a rate of change  246  (e.g., time derivative) of the liquid-side property  110   b  determined over the time period  244  (e.g., or a portion of the time period  244 ) is greater than a positive threshold  248 , the controller  144  may determine that the liquid-side property  110   b  has an increasing trend. As another example, if a difference  250  between values of the liquid-side property  110   b  at the end and beginning of the time period  244  (e.g., or during at least three sequential subintervals of the time period  244 ) is greater than a threshold value  252 , the controller  144  may determine that the liquid-side property  110   b  has an increasing trend. Likewise, if a rate of change  254  (e.g., time derivative) of the suction-side property  108   b  determined over the time period  244  (e.g., or a portion of the time period  244 ) is less than a negative threshold  256 , the controller  144  may determine that the suction-side property  108   b  has a decreasing trend. As another example, if a difference  258  between values of the suction-side property  108   b  at the end and beginning of the time period  244  (e.g., or during at least three sequential subintervals of the time period  244 ) is less than a negative threshold value  260 , the controller  144  may determine that the suction-side property  108   b  has a decreasing trend. The negative thresholds  256 ,  260  are threshold values (e.g., thresholds  612  of  FIG.  6   ) that are less than zero. 
     In this example case of an anticipated blockage of refrigerant in the conduit subsystem  102 , the controller  144  may cause a refrigerant blockage-related fault alert  142  to be displayed on an interface of the thermostat  138  and/or be provided to a third party for proactive correction. In some embodiments, the controller  144  may attempt to open the expansion device  118  further and determine whether this corrects the fault (i.e., determine whether the trends associated with this fault are no longer observed). If the fault is no longer detected, the alert  142  may be rescinded. However, if the trend remains, the alert  142  may be maintained, and, in some cases, operation of the HVAC system  100  (i.e., of the compressor  106 , the fan  116 , and the blower  132 ) may be stopped to prevent damage to the HVAC system  100 . 
     As another example illustrated in table  200  of  FIG.  2 A , if both the suction-side property  108   b  and the liquid-side property  110   b  have a decreasing trend, the controller  144  may detect an anticipated fault associated with a malfunction of the blower  132 . 
     For instance, the blower  132  may provide a lower than expected airflow  122  across the evaporator  120 . In this example case of an anticipated malfunction of the blower  132 , the controller  144  may cause operation of the HVAC system  100  (i.e., of the compressor  106 , the fan  116 , and the blower  132 ) to be stopped in order to prevent damage to the HVAC system  100 . 
       FIG.  2 D  shows a plot  270  of values of the suction-side property  108   b  and the liquid-side property  110   b  over time for the example case of a malfunction of the blower  132  At an initial time (t 0 )  272 , the malfunction of the blower  132  occurs (e.g., such that airflow  122  is not provided as expected). Following the malfunction of the blower  132  at time  272 , the values of the suction-side property  108   b  and the liquid-side property  110   b  decrease, as illustrated in plot  270 . 
     Similar to as described above with respect to  FIGS.  2 B and  2 C , in order to determine whether the suction-side property  108   b  and the liquid-side property  110   b  are increasing or decreasing, the controller  144  may evaluate changes in the properties  108   b,    110   b  over a time period  274 . For instance, if a rate of change  276  (e.g., time derivative) of the liquid-side property  110   b  determined over the time period  274  (e.g., or a portion of the time period  274 ) is less than a negative threshold  278 , the controller  144  may determine that the liquid-side property  110   b  has a decreasing trend. As another example, if a difference  280  between values of the liquid-side property  110   b  at the end and beginning of the time period  274  (e.g., or during at least three sequential subintervals of the time period  274 ) is less than a negative threshold value  282 , the controller  144  may determine that the liquid-side property  110   b  has a decreasing trend. Likewise, if a rate of change  284  (e.g., time derivative) of the suction-side property  108   b  determined over the time period  274  (e.g., or a portion of the time period  274 ) is less than a negative threshold  286 , the controller  144  may determine that the suction-side property  108   b  has a decreasing trend. As another example, if a difference  288  between values of the suction-side property  108   b  at the end and beginning of the time period  274  (e.g., or during at least three sequential subintervals of the time period  274 ) is less than a negative threshold value  290 , the controller  144  may determine that the suction-side property  108   b  has a decreasing trend. The negative thresholds  278 ,  282 ,  286 ,  290  are threshold values (e.g., thresholds  612  of  FIG.  6   ) that are less than zero. 
     Further details of the determination of an anticipated fault and the identification of an underlying cause of the fault (e.g., whether the anticipated fault is associated with a malfunction of fan  114 , a blockage of the conduit subsystem  102 , or a malfunction of the blower  132 ) are described below with respect to  FIG.  3   . 
     As another example of the operation of the system  100 , the low-pressure shutoff switch  146  may be tripped because the suction-side property  108   b  fell below a minimum value (e.g., a threshold of threshold(s)  612  described in  FIG.  6    below). When the switch  146  is tripped, the HVAC system  100  generally stops operating (e.g., the compressor  106 , fan  114 , and blower  132  shut off). The controller  144  may use previously monitored values of the liquid-side property  110   b  (i.e., values obtained before switch  146  was tripped) to determine whether the fault associated with tripping switch  146  was caused by a blockage of the refrigerant conduit subsystem  102  or a malfunction of the blower  132 . 
     As illustrated in table  200  of  FIG.  2 A , an increasing trend in the liquid-side property  110   b  following a trip of the low-pressure shutoff switch  146 , corresponds to detection of a fault associated with a blockage of conduit subsystem  102 . Meanwhile, a decreasing trend in the liquid-side property  110   b  following a trip of the low-pressure switch  146 , corresponds to detection of a fault associated with a malfunction of the blower  132 . Trends in the property values  108   b,    110   b  may be determined as described above with respect to  FIGS.  2 B- 2 D . The alert  142  presented on an interface of the thermostat  138  for this example case may include an indication that the low-pressure shutoff switch  146  was tripped and an indication of the determined cause of the fault (i.e., whether caused by blockage of conduit subsystem  102  or malfunction of the blower  132 ). Further details of the determination of the cause of system fault following the tripping of low-pressure shutoff switch  146  are described below with respect to  FIG.  4   . 
     As yet another example of the operation of the HVAC system  100 , the high-pressure shutoff switch  148  may be tripped because the liquid-side property  110   b  increases above a maximum value (e.g., a threshold of threshold(s)  612  described in  FIG.  6    below). When the switch  148  is tripped, the HVAC system  100  generally stops operating (e.g., the compressor  106 , fan  114 , and blower  132  shut off). The controller  144  may use previously monitored values of the suction-side property  108   b  (i.e., values obtained before switch  148  was tripped) to determine whether the fault associated with the tripping of switch  148  was caused by a malfunction of the fan  114  or a blockage of the refrigerant conduit subsystem  102 . 
     As illustrated in table  200  of  FIG.  2 A , an increasing trend in the suction-side property  108   b  following a trip of the high-pressure switch  148 , corresponds to detection of a fault associated with a malfunction of the fan  114 . Meanwhile, a decreasing trend in the suction-side property  108   b  following a trip of the high-pressure switch  148 , corresponds to detection of a fault associated with a blockage of conduit subsystem  102 . Trends in the property values  108   b,    110   b  may be determined as described above with respect to  FIGS.  2 B- 2 D . The alert  142  presented on an interface of the thermostat  138  for this example case may include an indication that the high-pressure shutoff switch  148  was tripped and an indication of the determined cause of the fault (i.e., whether caused by malfunction of fan  114  or blockage of conduit subsystem  102 ). Further details of the determination of the cause of system fault following the tripping of high-pressure shutoff switch  148  are described below with respect to  FIG.  5   . 
     Trend-Based Prognostics and Diagnostics 
       FIG.  3    is a flowchart of an example method  300  of operating the HVAC system  100  of  FIG.  1    for system prognostics and diagnostics. The method  300  generally facilitates the determination of an anticipated system fault and the identification of the underlying cause of the fault, based on trends in the suction-side property  108   b  and liquid-side property  110   b  over time. At step  302 , the suction-side property  108   a  is monitored by the controller  144  over time. For example, the controller  144  may receive the suction-side property  108   b  from the suction-side sensor  108   a  intermittently (e.g., several times per second, each second, or the like) and store the suction-side property  108   b  measurements (e.g., as measurements  608  of  FIG.  6   , described below). At step  304 , the liquid-side property  110   a  is monitored by the controller  144  over time. For example, the controller  144  may receive the liquid-side property  110   b  from the liquid-side sensor  110   a  intermittently and store the liquid-side property  110   b  measurements (e.g., as measurements  610  of  FIG.  6   , described below). 
     At step  306 , the controller  144  determines whether the suction-side property  108   b  has an increasing trend. The controller  144  determines whether the suction-side property  108   b  generally increases or decreases in value over a period of time, as illustrated in the examples of  FIGS.  2 A- 2 D  described above. In some embodiments, a trend in the suction-side property  108   b  is determined based on a rate of change of the suction-side property  108   b  (e.g., a time derivative of stored values and/or instantaneous values of the suction-side property  108   b ). For example, the controller  144  may determine a rate of change of the suction-side property  108   b  over a period of time. For example, several values of the rate of change may be determined over time. The controller  144  may determine if the rate of change is positive (i.e., greater than zero) for a predefined period of time (e.g., for 30 seconds or more). In some embodiments, if the rate of change has been positive for the period of time, the controller  144  may determine that the suction-side property  108   b  has an increasing trend at step  306 . In some embodiments, in order to determine that the suction-side property  108   b  has an increasing trend, the controller  144  may determine that the rate of change of the suction-side property  108   b  is both positive and greater than a threshold value for a minimum period of time. In some embodiments, in order for a trend to be established (e.g., based on a rate of change or a difference, as described above), the trend must be consistent over a minimum number of sequential time subintervals as described, for example, with respect to  FIG.  2 B  above. In some embodiments, the controller  144  may also determine that the compressor speed and outdoor temperature are not varying (e.g., not changing by more than a corresponding threshold amount), before determining a trend in the suction-side property  108   b.  For example, if one or both of the compressor speed and the outdoor temperature vary by more than a corresponding threshold amount, the controller  144  may end method  300 . 
     If, at step  306 , the controller  144  determines that the suction-side property has an increasing trend, the controller  144  proceeds to step  308  to determine whether the liquid-side property  110   b  has an increasing trend. Whether the liquid-side property  110   b  has an increasing trend may be determined as described above with respect to  FIGS.  2 B . If the liquid-side property  110   b  is not determined to have an increasing trend, the controller  144  may return to monitoring the suction-side property  108   b  and liquid-side property  110   b  at steps  302  and  304 . 
     Otherwise, if the suction-side property  108   b  is determined to have an increasing trend at step  306  and the liquid-side property  110   b  is determined to have an increasing trend at step  308 , the controller  144  determines that a fault is anticipated related to a malfunction of the fan  114  (see also the second row of table  200  of  FIG.  2 A ). This disclosure encompasses the recognition that conditions resulting to an increasing trend in the suction-side property  108   b  and the liquid-side property  110   b  may be associated with a malfunction of the fan  114  (e.g., and an inadequate supply of airflow  116  across the condenser  112 ). At step  312 , an alert  142  may be provided indicating the anticipated malfunction of the fan  114 . This alert  142  may be provided for display on an interface of the thermostat  138  and/or to a third party (e.g., a maintenance provider or administrator of the HVAC system  100 ), as described above with respect to  FIG.  1   . 
     At step  314 , the controller  144  may stop operation of the HVAC system  100  (e.g., stop operation of the compressor  106 , the fan  114 , and the blower  132 ). Stopping operation of the HVAC system  100  may prevent damage to the HVAC system  100  caused by a malfunction of the fan  114 . In some embodiments, the HVAC system  100  may be allowed to operate briefly after a fan malfunction is determined at step  310  (e.g., to ascertain whether the trends determined at steps  306  and  308  are maintained). However, in other embodiments, the HVAC system may be shut down at step  314  without delay following determination of a fan fault at step  310 . This disclosure encompasses the recognition that a malfunction of fan  114  may lead to a relatively rapid decrease in system performance, such that operation of the HVAC system  100  should be stopped rapidly after determination of the fan-related fault at step  310  to prevent damage to the HVAC system  100 . 
     If, at step  306 , the suction-side property  108   b  is not determined to have an increasing trend, the controller  144  determines whether the suction-side property  108   b  has a decreasing trend at step  316 . Whether the suction-side property  108   b  has an increasing trend may be determined, for example, as described above with respect to  FIGS.  2 B  (e.g., based on a rate of change of the suction-side property  108   b  or a difference of values of the suction-side property  108   b  between the end and start of a predefined period of time). 
     If the suction-side property  108   b  does not have a decreasing trend at step  316 , the controller  144  may return to monitoring the suction-side property  108   b  and liquid-side property  110   b  at steps  302  and  304 . Otherwise, if the controller  144  determines that the suction-side property has a decreasing trend at step  316 , the controller  144  proceeds to determine whether the liquid-side property  110   b  has an increasing trend at step  318 . The determination at step  318  may be performed as explained above with respect to step  308 . 
     If the suction-side property  108   b  is determined to have a decreasing trend at step  316  and the liquid-side property  110   b  is determined to have an increasing trend at step  318 , the controller determines, at step  320 , that a fault related to blockage of the conduit subsystem  102  is anticipated (see also the third row of table  200  of  FIG.  2 A ). At step  322 , the controller  144  may provide an alert  142  indicating the anticipated blockage of the conduit subsystem  102  determined at step  320 . This alert  142  may be provided for display on an interface of the thermostat  138  and/or to a third party (e.g., a maintenance provider or administrator of the HVAC system  100 ), as described above with respect to  FIG.  1   . 
     At step  324 , the controller  144  may, optionally, test operation of the expansion device  118  to ascertain whether the blockage of the conduit subsystem  102  can be compensated for and/or corrected. For example, the controller  144  may send a signal instructing the expansion device  118  to open further and determine whether, following sending this signal, the trends determined at steps  316  and  318  are maintained. If the trends remain, the controller  144  may stop operation of the HVAC system  100  (e.g., stop operation of the compressor  106 , the fan  114 , and the blower  132 ). Stopping operation of the HVAC system  100  may prevent damage to the HVAC system  100  caused by a blockage of refrigerant flow in the conduit subsystem  102 . If the test at step  324  indicates that conduit subsystem  102  blockage was corrected (e.g., if trends at steps  316  and  318  are no longer determined), the controller  144  may allow the HVAC system  100  to continue operating (e.g., providing heating or cooling) for at least a brief period of time. This may allow continued comfort for individuals during a time before maintenance to the conduit subsystem  102  is performed. 
     If at step  318 , the controller  144  does not determine that the liquid-side property  110   b  has an increasing trend, the controller may proceed to step  326  to determine whether the liquid-side property has a decreasing trend. For example, the controller  144  may determine whether the suction-side property  110   b  has a decreasing trend based on a rate of change of the liquid-side property  110   b  or a difference of values of the liquid-side property  110   b  between the end and start of a predefined period of time. Whether the liquid-side property  110   b  has a decreasing trend may be determined as described above with respect to  FIG.  2 D . 
     If the controller  144  determines, at step  326 , that the liquid-side property  110   b  does not have a decreasing trend, the controller  144  may return to monitoring the suction-side property  108   b  and liquid-side property  110   b  at steps  302  and  304 . 
     Otherwise, if the controller  144  determines that the suction-side property  108   b  and the liquid-side property  110   b  have a decreasing trend, the controller  144  may determine that a fault associated with a malfunction of the blower  132  is anticipated (see the fourth row of table  200  of  FIG.  2 A ). At step  330 , the controller  144  may provide an alert  142  indicating the anticipated blower fault determined at step  328 . This alert  142  may be provided for display on an interface of the thermostat  138  and/or to a third party (e.g., a maintenance provider or administrator of the HVAC system  100 ), as described above with respect to  FIG.  1   . At step  314 , the controller  144  may stop operation of the HVAC system  100  (e.g., stop operation of the compressor  106 , the fan  114 , and the blower  132 ). Stopping operation of the HVAC system  100  may prevent damage to the HVAC system  100  caused by malfunction of the blower  132 . 
     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 steps, any suitable HVAC system or components of the HVAC system  100  may perform one or more steps of the method  300 . 
     Diagnostics Following a Low-pressure Switch Trip 
       FIG.  4    is a flowchart of an example method  400  of operating the HVAC system  100  of  FIG.  1    for automatically diagnosing the cause of a trip of the low-pressure shutoff switch  146 . The method  400  generally facilitates the determination (e.g., the automatic determination) of the underlying cause of the low-pressure shutoff switch  146  being tripped. At step  402 , the low-pressure shutoff switch  146  is tripped. The low-pressure shutoff switch  146  may be tripped if the suction-side property  108   b  is less than a minimum value, as described above with respect to  FIG.  1   . Tripping of the low-pressure shutoff switch  146  generally causes the HVAC system to stop operating (e.g., for the compressor  106 , fan  114 , and blower  132  to shut off). At step  404 , the controller  144  accesses previously measured values of the liquid-side property  110   a  (e.g., measurements  610  of  FIG.  6   , described below). 
     At step  406 , the controller  144  determines whether the liquid-side property  110   b  had an increasing trend prior to when the switch  146  was tripped. The controller  144  determines whether the liquid-side property  110   b  generally increases in value over a period of time, as illustrated in the example of  FIG.  2 B  described above. In some embodiments, a trend in the suction-side property  108   b  is determined based on a rate of change of the liquid-side property  110   b  (e.g., a time derivative of stored values of the liquid-side property  110   b ). For example, the controller  144  may determine a rate of change of the liquid-side property  110   b  over a period of time. For example, several values of the rate of change may be determined over time. The controller  144  may determine if the values of the rate of change are positive (i.e., greater than zero) for a predefined period of time (e.g., for 30 seconds or more). In some embodiments, if the rate of change has been positive for the period of time, the controller  144  may determine that the liquid-side property  110   b  has an increasing trend at step  406 . In some embodiments, in order to determine that the liquid-side property  110   b  has an increasing trend, the controller  144  may determine that the rate of change of the liquid-side property  110   b  is both positive and greater than a threshold value for a minimum period of time. In some embodiments, in order for a trend to be established (e.g., based on a rate of change or a difference, as described above), the trend must be consistent over a minimum number of sequential time subintervals as described with respect to  FIG.  2 B  above. 
     If the liquid-side property  110   b  had an increasing trend, the controller  144  determines, at step  408 , that the system fault (e.g., leading to tripping of the switch  146 ) was caused by a blockage of the refrigerant conduit subsystem  102 . At step  410 , the controller  144  may provide an alert  142  indicating that the switch  146  was likely tripped because of a blockage of the refrigerant conduit subsystem  102 . This alert  142  may be provided for display on an interface of the thermostat  138  and/or to a third party (e.g., a maintenance provider or administrator of the HVAC system  100 ), as described above with respect to  FIG.  1   . 
     If the liquid-side property  110   b  had an increasing trend, the controller  144  determines, at step  412 , whether the liquid-side property  110   b  had a decreasing trend prior to when the switch  146  was tripped. The controller  144  determines whether the liquid-side property  110   b  generally decreases in value over a period of time, as illustrated in the example of  FIGS.  2 D  described above. In some embodiments, a trend in the suction-side property  108   b  is determined based on a rate of change of the liquid-side property  110   b  (e.g., a time derivative of stored values of the liquid-side property  110   b ). For example, the controller  144  may determine a rate of change of the liquid-side property  110   b  over a period of time. For example, several values of the rate of change may be determined over time. The controller  144  may determine if the values of the rate of change are negative (i.e., less than zero) for a predefined period of time (e.g., for 30 seconds or more). In some embodiments, if the rate of change has been negative for the period of time, the controller  144  may determine that the liquid-side property  110   b  has a decreasing trend at step  412 . In some embodiments, in order to determine that the liquid-side property  110   b  has a decreasing trend, the controller  144  may determine that the rate of change of the liquid-side property  110   b  is both negative and less than a threshold value for a minimum period of time. In some embodiments, in order for a trend to be established (e.g., based on a rate of change or a difference, as described above), the trend must be consistent over a minimum number of sequential time subintervals as described with respect to  FIG.  2 B  above. 
     If the liquid-side property  110   b  had a decreasing trend, the controller  144  determines, at step  414 , that the system fault (e.g., leading to tripping of the switch  146 ) was caused by a malfunction of the blower  132 . At step  416 , the controller  144  provides an alert  142  indicating the tripping of the switch  146  is likely related to a malfunction of the blower  132 . This alert  142  may be provided for display on an interface of the thermostat  138  and/or to a third party (e.g., a maintenance provider or administrator of the HVAC system  100 ), as described above with respect to  FIG.  1   . 
     Modifications, additions, or omissions may be made to method  400  depicted in  FIG.  4   . Method  400  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 steps, any suitable HVAC system or components of the HVAC system  100  may perform one or more steps of the method  400 . 
     Diagnostics Following a High-pressure Switch Trip 
       FIG.  5    is a flowchart of an example method  500  of operating the HVAC system  100  of  FIG.  1    for automatically diagnosing the cause of a trip of the high-pressure shutoff switch  148 . The method  500  generally facilitates the determination (e.g., the automatic determination) of the underlying cause of the high-pressure shutoff switch  148  being tripped. At step  502 , the high-pressure shutoff switch  148  is tripped. The high-pressure shutoff switch  148  may be tripped if the liquid-side property  110   b  is greater than a maximum value, as described above with respect to  FIG.  1   . Tripping of the high-pressure shutoff switch  148  generally causes the HVAC system  100  to stop operating (e.g., for the compressor  106 , fan  114 , and blower  132  to shut off). At step  504 , the controller  144  accesses previously measured values of the suction-side property  108   b  (e.g., measurements  608  of  FIG.  6   , described below). 
     At step  506 , the controller  144  determines whether the suction-side property  108   b  had a decreasing trend prior to when the switch  148  was tripped. The controller  144  determines whether the suction-side property  108   b  generally decreases in value over a period of time, as illustrated in the example of  FIGS.  2 D  described above. In some embodiments, a trend in the suction-side property  108   b  is determined based on a rate of change of the suction-side property  108   b  (e.g., a time derivative of stored values of the suction-side property  108   b ). For example, the controller  144  may determine a rate of change of the suction-side property  108   b  over a period of time. For example, several values of the rate of change may be determined over time. The controller  144  may determine if the values of the rate of change are negative (i.e., less than zero) for a predefined period of time (e.g., for 30 seconds or more). In some embodiments, if the rate of change has been negative for the period of time, the controller  144  may determine that the suction-side property  108   b  has a decreasing trend at step  506 . In some embodiments, in order to determine that the suction-side property  108   b  has a decreasing trend, the controller  144  may determine that the rate of change of the suction-side property  108   b  is both negative and less than a threshold value for a minimum period of time. In some embodiments, in order for a trend to be established (e.g., based on a rate of change or a difference, as described above), the trend must be consistent over a minimum number of sequential time subintervals as described with respect to  FIG.  2 B  above. 
     If the suction-side property  108   b  had a decreasing trend at step  506 , the controller  144  determines, at step  508 , that the system fault (e.g., leading to tripping of the switch  148 ) was caused by a blockage of the refrigerant conduit subsystem  102 . At step  510 , the controller  144  may provide an alert  142  indicating that the switch  148  was likely tripped because of a blockage of the refrigerant conduit subsystem  102 . This alert  142  may be provided for display on an interface of the thermostat  138  and/or to a third party (e.g., a maintenance provider or administrator of the HVAC system  100 ), as described above with respect to  FIG.  1   . 
     If the suction-side property  108   b  did not have a decreasing trend at step  506 , the controller  144  determines, at step  512 , whether the suction-side property  108   b  had an increasing trend prior to when the switch  148  was tripped. The controller  144  determines whether the suction-side property  108   b  generally increases in value over a period of time, as illustrated in the example of  FIG.  2 B  described above. In some embodiments, a trend in the suction-side property  108   b  is determined based on a rate of change of the suction-side property  108   b  (e.g., a time derivative of stored values of the suction-side property  108   b ). For example, the controller  144  may determine a rate of change of the suction-side property  108   b  over a period of time. For example, several values of the rate of change may be determined over time. The controller  144  may determine if the values of the rate of change are positive (i.e., greater than zero) for a predefined period of time (e.g., for 30 seconds or more). In some embodiments, if the rate of change has been positive for the period of time, the controller  144  may determine that the suction-side property  108   b  has an increasing trend at step  512 . In some embodiments, in order to determine that the suction-side property  108   b  has an increasing trend, the controller  144  may determine that the rate of change of the suction-side property  108   b  is both positive and greater than a threshold value for a minimum period of time. In some embodiments, in order for a trend to be established (e.g., based on a rate of change or a difference, as described above), the trend must be consistent over a minimum number of sequential time subintervals as described with respect to  FIG.  2 B  above. 
     If the suction-side property  108   b  had an increasing trend at step  512 , the controller  144  determines, at step  514 , that the system fault (e.g., leading to tripping of the switch  148 ) was caused by a malfunction of the fan  114 . At step  516 , the controller  144  provides an alert  142  indicating the tripping of the switch  148  is likely related to a malfunction of the blower  132 . This alert  142  may be provided for display on an interface of the thermostat  138  and/or to a third party (e.g., a maintenance provider or administrator of the HVAC system  100 ), as described above with respect to  FIG.  1   . 
     Modifications, additions, or omissions may be made to method  500  depicted in  FIG.  5   . Method  500  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 steps, any suitable HVAC system or components of the HVAC system  100  may perform one or more steps of the method  500 . 
     Example Controller 
       FIG.  6    is a schematic diagram of an embodiment of the controller  144 . The controller  144  includes a processor  602 , a memory  604 , and an input/output (I/O) interface  606 . 
     The processor  602  includes one or more processors operably coupled to the memory  604 . The processor  602  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  604  and controls the operation of HVAC system  100 . The processor  602  may be a programmable logic device, a microcontroller, a microprocessor, or any suitable combination of the preceding. The processor  602  is communicatively coupled to and in signal communication with the memory  604 . The one or more processors are configured to process data and may be implemented in hardware or software. For example, the processor  602  may be 8-bit, 16-bit, 32-bit, 64-bit or of any other suitable architecture. The processor  602  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  604  and executes them by directing the coordinated operations of the ALU, registers, and other components. The processor 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  FIG.  3   ). The processor  602  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  604  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  604  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  604  is operable to store one or more suction-side property measurements  608 , liquid-side property measurements  610 , and thresholds  612 . The suction-side property measurements  608  generally include values of the suction-side property  108   b  measured by the suction-side sensor  108   a  of  FIG.  1   . For example, the suction-side property measurements  608  may include a record of previous values of the suction-side property  108   b  measured for the HVAC system  100 . The liquid-side property measurements  610  generally include values of the liquid-side property  110   b  measured by the liquid-side sensor  110   a  of  FIG.  1   . For example, the liquid-side property measurements  610  may include a record of previous values of the liquid-side property  110   b  measured for the HVAC system  100 . The threshold values  612  include any of the thresholds used to implement the functions described herein. For instance, the thresholds  612  may include the thresholds  218 ,  222 ,  226 ,  230 ,  248 ,  252 ,  256 ,  260 ,  278 ,  282 ,  286 ,  290  described with respect to  FIGS.  2 B- 2 D . 
     The I/O interface  606  is configured to communicate data and signals with other devices. For example, the I/O interface  606  may be configured to communicate electrical signals with components of the HVAC system  100  including the compressor  106 , the suction-side sensor  108   a,  the liquid-side sensor  110   a,  the expansion device  118 , the blower  132 , sensors  136   a,b , thermostat  138 , and switches  146 ,  148 . The I/O interface may receive, for example, signals associated with the suction-side property  108   b,  signals associated with the liquid-side property  110   b  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  606  may include ports or terminals for establishing signal communications between the controller  144  and other devices. The I/O interface  606  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.