HEAT PUMP DEFROST CONTROL

FIELD

The present disclosure relates to water heating systems and, more particularly, to control systems for heat pump water heating systems.

SUMMARY

Heat pump water heating systems operate on the principle of moving heat from an external environment (such as ambient air, the ground, and/or an external water source) to water within the system, rather than generating heat directly (for example, through chemical combustion and/or electrical-resistance heating elements). Because moving heat generally requires less energy than generating heat directly, heat pump water heating systems can be significantly more efficient than other systems that generate heat directly. Heat pump water heating systems may include a heat pump heater unit that extracts heat from an external environment via a heat exchanger (such as an evaporator). For example, heat pump heater units may circulate a cooled working fluid (such as a refrigerant) through the evaporator, and heat from the external environment may transfer into the working fluid via the evaporator. The heat pump heater unit then transfers heat from the heated working fluid into the water in the system via a second heat exchanger.

In certain conditions (for example, during the winter), ambient air of the external environment may be relatively cold (for example, near the freezing point of water) and relatively humid. In order for heat transfer from the external environment to the working fluid to occur, the working fluid (and the surface of the evaporator) must be at a lower temperature than the ambient air. However, in such conditions, when the temperature of the surface of the evaporator drops below the dew point, frost may form on the surface of the evaporator. This frost may form as ice crystals with air trapped between them. Because both frost and air are poor conductors of heat, the frost may function as an insulator between the evaporator and the external environment. In addition, the frost may block the flow of air through the evaporator. Thus, if the frost is not removed, the overall efficiency of the heat pump heater unit may be reduced.

Some techniques for defrosting include positioning electric-resistance heating elements near or adjacent to the evaporator. When electric current passes through these resistive elements, the heating elements generate heat from their resistance to the electrical current. This heat may be transferred to the frost on the evaporator and/or the surface of the evaporator itself, melting the frost. However, because electric-resistance heating elements generate heat directly, they may consume a significant amount of electricity (and are often inefficient).

Other techniques such as hot gas bypass defrosting involve redirecting heated working fluid exiting a compressor to the evaporator so that heat transfers from the heated working fluid to the evaporator, which melts the frost. However, because such techniques change the flow path of the working fluid, they often require adding additional components to the heat pump heater unit, increasing the complexity and cost of the unit.

Additional techniques involve reversing the flow of working fluid in the refrigerant cycle. By reversing the flow of the working fluid, heated working fluid (as opposed to cooled working fluid) is provided to the evaporator (which then effectively functions as a condenser). Heat from the working fluid then transfers to the evaporator, melting the frost. However, reversing the flow of the working fluid may also result in the heat pump heater unit removing heat from the water flowing through the heat pump heater unit and transferring the heat to the external environment, which cools the water to be heated. This cooled water may be returned to the top of a hot water storage tank and drawn for immediate use during a demand draw of hot water.

Systems, apparatuses, methods, and techniques described in the specification provide technical solutions to these and other technical problems by defrosting heat pump heater unit evaporators through efficient heat transfer techniques while minimizing (or eliminating) the cooled water introduced into the hot water storage tank or adding unnecessary complexity to the system.

A water heating system includes a compressor unit fluidly coupled to a first heat exchanger and a controller operatively coupled to the compressor unit. The controller is configured to operate the compressor unit in a heating mode to circulate refrigerant between the compressor unit and the first heat exchanger in a first direction, determine a loss of superheat in the first heat exchanger while the compressor unit is operating in the heating mode, and initiate a defrosting sequence in response to determining the loss of superheat in the first heat exchanger.

In other features, the controller is configured to monitor a temperature of refrigerant exiting the first heat exchanger during the defrosting sequence and initiate a termination sequence in response to the temperature of the refrigerant exiting the first heat exchanger exceeding a threshold. In other features, the controller is configured to operate the compressor unit in a defrosting mode during the defrosting sequence. In other features, the compressor unit is configured to circulate refrigerant between the compressor unit and the first heat exchanger in a second direction in the defrosting mode. The second direction is opposite the first direction.

In other features, the first heat exchanger functions as an evaporator when the compressor unit is operating in the heating mode and the first heat exchanger functions as a condenser when the compressor unit is operating in the defrosting mode. In other features, the controller is configured to determine the loss of superheat in the first heat exchanger based on a comparison of (i) an inlet refrigerant temperature of the first heat exchanger and (ii) an outlet refrigerant temperature of the first heat exchanger. In other features, the controller is configured to determine the loss of superheat in the first heat exchanger in response to the outlet refrigerant temperature of the first heat exchanger not exceeding the inlet temperature of the first heat exchanger over a predetermined time duration.

In other features, the water heating system includes a water pump fluidly coupled to a second heat exchanger and operatively coupled to the controller. The controller is configured to operate the water pump to provide a first water flow rate through the second heat exchanger during the defrosting sequence. The first water flow rate is lower than a second water flow rate associated with normal heating operations. In other features, the compressor unit is fluidly coupled to the second heat exchanger. The controller is configured to initiate a preheating sequence by stopping the water pump to substantially stop water circulation through the second heat exchanger and operating the compressor unit in the heating mode to circulate refrigerant through the first heat exchanger and the second heat exchanger.

In other features, the controller is configured to monitor a characteristic of the refrigerant during the preheating sequence and terminate the preheating sequence in response to the characteristic of the refrigerant exceeding a limit. In other features, the characteristic of the refrigerant is a temperature of refrigerant between the second heat exchanger and the first heat exchanger and the limit is a temperature limit. In other features, the characteristic of the refrigerant is a pressure of refrigerant exiting a compressor of the compressor unit and the pressure is a pressure limit. In other features, the characteristic of the refrigerant is a temperature of refrigerant entering a compressor of the compressor unit and the limit is a temperature limit.

In other features, the controller is configured to initiate the preheating sequence in response to determining the loss of superheat in the first heat exchanger. In other features, the water heating system includes a fan operatively coupled to the controller. The fan is configured to provide airflow to the first heat exchanger. The controller is configured to initiate the termination sequence by operating the fan to provide airflow to the first heat exchanger and configuring the compressor unit to operate in the heating mode. In other features, the controller is further configured to operate the water pump to provide a third water flow rate through the second heat exchanger during the defrosting sequence. The third water flow rate is greater than the first flow rate and lower than the second water flow rate.

In other features, the controller is further configured to progressively increase a water flow rate of the water pump from the first water flow rate to the third water flow rate at a predetermined time rate of change. In other features, the controller is further configured to monitor a characteristic of the refrigerant while progressively increasing the water flow rate of the water pump, compare the characteristic of the refrigerant to a threshold, and terminate the defrosting sequence in response to determining that the characteristic of the refrigerant is below the threshold. In other features, the characteristic of the refrigerant is a temperature of refrigerant between the first heat exchanger and the second heat exchanger and the threshold is a temperature limit.

A method for controlling a heat pump water heater includes operating a compressor unit in a heating mode to circulate refrigerant between the compressor unit and a first heat exchanger in a first direction, determining a loss of superheat in the first heat exchanger while the compressor unit is operating in the heating mode, and initiating a defrosting sequence in response to determining the loss of superheat in the first heat exchanger.

In other features, the method includes monitoring a temperature of refrigerant exiting the first heat exchanger during the defrosting sequence and initiating a termination sequence in response to the temperature of the refrigerant exiting the first heat exchanger exceeding a threshold. In other features, the compressor unit operates in a defrosting mode during the defrosting sequence. In other features, the compressor unit is configured to circulate refrigerant between the compressor unit and the first heat exchanger in a second direction in the defrosting mode. The second direction is opposite the first direction. In other features, the first heat exchanger functions as an evaporator when the compressor unit is operating in the heating mode and the first heat exchanger functions as a condenser when the compressor unit is operating in the defrosting mode.

In other features, determining the loss of superheat in the first heat exchanger includes a comparison of (i) an inlet refrigerant temperature of the first heat exchanger and (ii) an outlet refrigerant temperature of the first heat exchanger. In other features, determining the loss of superheat in the first heat exchanger includes a determination that the outlet refrigerant temperature of the first heat exchanger has not exceeded the inlet temperature of the first heat exchanger for a predetermined period of time. In other features, the method includes operating a water pump to provide a first water flow rate through a second heat exchanger during the defrosting sequence. The first water flow rate is lower than a second water flow rate associated with normal heating operations.

In other features, the method includes initiating a preheating sequence by stopping the water pump to substantially stop water circulation through the second heat exchanger and operating the compressor unit in a heating mode to circulate refrigerant through the first heat exchanger and the second heat exchanger. In other features, the method includes monitoring a characteristic of the refrigerant during the preheating sequence and terminating the preheating sequence in response to the characteristic of the refrigerant exceeding a limit. In other features, the characteristic of the refrigerant is a temperature of refrigerant between the second heat exchanger and the first heat exchanger and the limit is a temperature limit.

In other features, the characteristic of the refrigerant is a pressure of refrigerant exiting a compressor of the compressor unit and the pressure is a pressure limit. In other features, the characteristic of the refrigerant is a temperature of refrigerant entering a compressor of the compressor unit and the limit is a temperature limit. In other features, the method includes initiating the preheating sequence in response to the loss of superheat in the first heat exchanger. In other features, the method includes initiating the termination sequence by operating a fan to provide airflow to the first heat exchanger and configuring the compressor unit to operate in the heating mode. In other features, the method includes operating the water pump to provide a third water flow rate through the second heat exchanger during the defrosting sequence. The third water flow rate is greater than the first flow rate and lower than the second water flow rate.

In other features, the method includes progressively increasing a water flow rate of the water pump from the first water flow rate to the third water flow rate at a predetermined time rate of change. In other features, the method includes monitoring a characteristic of the refrigerant while progressively increasing the water flow rate of the water pump, comparing the characteristic of the refrigerant to a threshold, and terminating the defrosting sequence in response to determining that the characteristic of the refrigerant is below the threshold. In other features, the characteristic of the refrigerant is a temperature of refrigerant between the first heat exchanger and the second heat exchanger and the threshold is a temperature limit.

Other examples, embodiments, features, and aspects will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an example heat pump water heating system according to some embodiments.

FIG. 2 shows an exemplary graph illustrating a relationship between enthalpy and pressure of refrigerant as it progresses through a refrigerant circuit of a heat pump water heater unit according to some embodiments.

FIG. 3 is a schematic illustration showing a detail view of the compressor unit of FIG. 1 in a first configuration, according to some embodiments.

FIG. 4 is a schematic illustration showing a detail view of the compressor unit of FIG. 1 in a second configuration, according to some embodiments.

FIG. 5 is a block diagram showing an example interconnected hardware control system of a heater unit, according to some embodiments.

FIG. 6 is a flowchart of an example process for controlling a water heater unit, according to some embodiments.

FIG. 7 is a flowchart of an example process for determining whether an evaporator of a water heater unit has lost superheat, according to some embodiments.

FIG. 8 is a flowchart of a preheating sequence of an example process for defrosting an evaporator of a water heater unit, according to some embodiments.

FIG. 9 is a flowchart of a first defrost stage of an example process for defrosting an evaporator of a water heater unit, according to some embodiments.

FIGS. 10 and 11 are a flowchart of the second defrost stage of an example process for defrosting an evaporator of a water heater unit, according to some embodiments.

FIGS. 12 and 13 are a flowchart of the third defrost stage of an example process for defrosting an evaporator of a water heater unit, according to some embodiments.

In the drawings, reference numbers may be reused to identify similar and/or identical elements.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of an example heat pump water heating system 100. As illustrated in FIG. 1, some examples of the system 100 include one or more hot water storage tanks, such as tank 102, and one or more heat pump heater units, such as heater unit 104. Although a single tank 102 and a single heater unit 104 is shown in FIG. 1, various implementations of the system 100 may include any number of tanks 102 and any number of heater units 104. In some examples, the heater unit 104 may include an air-source heat pump heater unit (for example, as illustrated in FIG. 1), a water-source heat pump heater unit, or a ground-source heat pump heater unit. In various implementations, the tank 102 includes an outer shell, an inner lining, and an insulation layer between the outer shell and the inner lining. The outer shell may include a material such as steel. The inner lining may include a corrosion-resistant material such as glass or a polymer coating and be configured to contain a fluid (such as water). The insulation material may include a material that minimizes and/or reduces heat transfer from the contents of the interior of the tank 102 to an external ambient environment (for example, fiberglass or a polyurethane foam).

The tank 102 may include one or more ports for fluid such as water to enter and/or exit the tank 102. For example, the tank 102 may include a recirculation supply port 106, a return port 108, and a hot water supply port 110. In various implementations, the recirculation supply port 106 is positioned near the bottom of the tank 102 (for example, within the bottom half, third, quarter, fifth, or tenth of the tank 102) so that cooler, less buoyant water within the tank 102 can be supplied to the heater unit 104 via the recirculation supply port 106. In some examples, the return port 108 is located near the top of the tank 102 (for example, within the top half, third, quarter, fifth, or tenth of the tank 102) so that water heated by the heater unit 104 can be returned via the return port 108 near the top of the tank 102. The heated water returned from the heater unit 104 may be near the hot water supply port 110 and supplied via the hot water supply port 110 in response to a hot water demand draw.

The heater unit 104 may include a refrigerant circuit 112 and a water circuit 114. The refrigerant circuit 112 may include a compressor unit 116, a heat exchanger such as a condenser 118, and one or more evaporator sections. In various implementations, each evaporator section may include an expansion device 120 and a heat exchanger such as an evaporator 122. In some examples, the compressor unit 116, condenser 118, and the components of each evaporator section (such as each expansion device 120 and each evaporator 122) may be connected in series so that the output of one component feeds directly into the next component. In various implementations, the refrigerant circuit 112 may include any number of evaporator sections. Each evaporator section may be connected in parallel with each other evaporator section.

For example, as shown in FIG. 1, the refrigerant circuit may include a first evaporator section (having expansion device 120-1 and evaporator 122-1) and a second evaporator section (having expansion device 120-2 and evaporator 122-2), with the components of each evaporator section being connected in parallel with the components of each other evaporator section while the components of each evaporator section are connected in parallel with the compressor unit 116 and the condenser 118. In various implementations, the compressor unit 116, condenser 118, expansion devices 120, and evaporators 122 may be fluidly coupled by one or more refrigerant lines (for example, as illustrated in FIG. 1). The refrigerant may include hydrofluorocarbon (HFC) refrigerants (such as R-134a, R404A, R-407C, R-410A, R-32, etc.), hydrofluoroolefin (HFO) refrigerants (such as R-1234yf, R-1234ze, R-1233zd, etc.), or natural refrigerants (such as carbon dioxide, propane, etc.).

FIG. 2 shows an exemplary graph 200 illustrating a relationship between an enthalpy and a pressure of refrigerant as it progresses through the refrigerant circuit 112. The enthalpy axis 202 represents the enthalpy of the refrigerant at a given point in the cycle while the pressure axis 204 represents the pressure of the refrigerant at that given point. The liquid-vapor dome 206 represents a transition between different phases of the refrigerant. At points to the left of the dome 206, the refrigerant exists in a purely liquid state. At points to the right of the dome 206, the refrigerant exists in a purely vapor state. At points below the dome 206, the refrigerant exists in a mixed two-phase state (the refrigerant exists as a mixture of liquid and vapor states).

Points 208, 210, 212, and 214 represent states of the refrigerant as it progresses through the refrigerant circuit 112 in a water heating operating mode of the system 100. Referring collectively to FIGS. 1 and 2, point 208 represents the refrigerant after it exits the compressor unit 116 and is delivered to the condenser 118. At point 208, the refrigerant is in a vapor state at a first pressure. At the condenser 118, the refrigerant transitions from point 208 to point 210. Within the condenser 118, the refrigerant transfers heat energy to water passing through the condenser 118 (for example, to water in the water circuit 114). As the refrigerant transfers heat energy to water passing through the condenser 118, the enthalpy of the refrigerant decreases and the refrigerant is condensed from a vapor state to a liquid state and exits the condenser 118 in a subcooled liquid state at the first pressure (indicated by point 210). The refrigerant is then expanded at the expansion device(s) 120 of the evaporator section(s) to a second pressure (lower than the first pressure) at point 212. The expansion of the refrigerant at the expansion device(s) 120 may be substantially adiabatic, such that the enthalpy of the refrigerant remains substantially unchanged between points 210 and 212. As a result, the refrigerant at point 212 is within the liquid-vapor dome 206, and therefore exists in a two-phase state at a substantially lower temperature than it had at point 210.

The refrigerant exits the expansion device(s) 120 (at point 212) and is heated to a superheated vapor state in the evaporator(s) 122 of the evaporator section(s). In various implementations, the evaporator(s) 122 are air-to-refrigerant heat exchanger(s), with ambient air directed through each heat exchanger by way of a blower or fan 124 while the refrigerant flows through tubes of the heat exchanger of the evaporator 122. In some examples, the evaporator(s) 122 are liquid-to-refrigerant heat exchanger(s) (similar to condenser 118). Due to the lower pressure of the refrigerant at and between points 212 and 214 (as the refrigerant exits the expansion device 120, flows to the evaporator(s) 122, flows through the heat exchanger of the evaporator(s) 122, and exits the evaporator(s) 122), heat can be transferred into the refrigerant from even a relatively low-temperature heat source (such as the ambient air).

After exiting the evaporator 122 at point 214, the refrigerant (which may be a slightly superheated low-pressure vapor) flows to the compressor unit 116. The compressor unit 116 compresses the refrigerant (which is a vapor) back to the higher first pressure, moving the refrigerant's state from point 214 back to point 208. This compression does not occur adiabatically, resulting in an increase in the enthalpy of the refrigerant between point 214 and point 208. Thus, as shown by the graph 200, the heat energy available to be transferred into the water of the water circuit 114 via the condenser 118 (represented by the enthalpy change from point 208 to point 210) is substantially greater than the work put into the system by the compressor unit 116 (represented by the enthalpy change from point 214 to point 208). Accordingly, the water heating process of the heater unit 104 is very energy efficient.

Returning to FIG. 1, in various implementations, water enters the water circuit 114 via an inlet port 126 and exits the water circuit 114 via an outlet port 128. The inlet port 126 may be fluidly coupled to the recirculation supply port 106 via one or more manifolds, lines, and/or pipes, and the outlet port 128 may be fluidly coupled to the return port 108 via one or more manifolds, lines, and/or pipes. In some examples, the water circuit 114 includes a pump 130, the condenser 118, and a flow control valve 132. The inlet port 126 and the outlet port 128 may be fluidly coupled to the pump 130, condenser 118, and the flow control valve 132 via one or more manifolds, lines, and/or pipes. In various implementations, the pump 130 draws water (such as cold water) in from the tank 102 via the recirculation supply port 106 and the inlet port 126 and pumps the water to the condenser 118. At the condenser 118, the water enters the heat exchanger of the condenser 118 and heat from the refrigerant is transferred to the water (between points 208 and 210 of graph 200), heating the water. The water exits the condenser 118, flows through the flow control valve 132, and exits the heater unit 104 via the outlet port 128.

The heated water may return to the tank 102 via the return port 108. In various implementations, the order of the components in the water circuit 114 may be varied from that illustrated in FIG. 1. For example, the pump 130 and/or the flow control valve 132 may be placed before and/or after the condenser 118. Heated water supplied from the heater unit 104 to the tank 102 may be delivered to the tank via the hot water supply port 110. Cold water may be resupplied to the tank 102 or provided to the inlet port 126 via a cold water port 146. In various implementations, the cold water port 146 may be fluidly coupled to the recirculation supply port 106 and/or the inlet port 126 via one or more manifolds, lines, and/or pipes. In some examples, the cold water port 146 may be directly coupled to the tank.

The heater unit 104 may also include a variety of sensors that measure one or more variables such as temperature, pressure, flow rate, etc. In various implementations, the heater unit 104 includes one or more temperature sensors. For example, the heater unit 104 may include a temperature sensor 134 positioned between the compressor unit 116 and the evaporator section(s) to measure a refrigerant liquid line temperature T_{liq_line}. The refrigerant liquid line temperature T_{liq_line}may be a temperature of compressed liquid refrigerant after it transfers heat from the refrigerant circuit 112 to the water circuit 114 via the condenser 118. Accordingly, in various implementations, the temperature sensor 134 may be positioned between the condenser 118 and the evaporator section(s).

In various implementations, the heater unit 104 may include one or more temperature sensors that measure refrigerant temperatures before and/or after the refrigerant enters each evaporator 122 of each evaporator section. For example, the heater unit 104 may include temperature sensor(s) 136 that measure the evaporator section refrigerant temperature(s) T_{ev_A}between the expansion device(s) 120 and the evaporator(s) 122. In the example of FIG. 1 where there are two evaporator sections, the heater unit 104 may include a temperature sensor 136-1 that measures the first evaporator section refrigerant temperature T_{ev1_A}between the expansion device 120-1 and the evaporator 122-1 of the first evaporator section and a temperature sensor 136-2 that measures the second evaporator section refrigerant temperature T_{ev2_A}between the expansion device 120-2 and the evaporator 122-2 of the second evaporator section.

In some examples, the heater unit 104 may include temperature sensor(s) 138 that measure the evaporator section refrigerant temperature(s) T_{ev_B}between the evaporator(s) 122 and the compressor unit 116. In the example of FIG. 1 where there are two evaporator sections, the heater unit 104 may include a temperature sensor 138-1 that measure the first evaporator section refrigerant temperature T_{ev1_B}between the evaporator 122-1 of the first evaporator section and the compressor unit 116 and a temperature sensor 138-2 that measure the second evaporator section refrigerant temperature T_{ev2_B}between the evaporator 122-2 of the second evaporator section and the compressor unit 116. In various implementations, the heater unit 104 includes a flow meter 140 to measure a water flow rate V_wwithin the water circuit 114. The flow meter 140 may be positioned at any point in the water circuit 114, such as between the inlet port 126 and the pump 130, between the pump 130 and the condenser 118, between the condenser 118 and the flow control valve 132, or between the flow control valve 132 and the outlet port 128.

FIG. 3 is a schematic illustration showing a detail view of the compressor unit 116 of FIG. 1 in a first configuration, in accordance with some embodiments. FIG. 4 is a schematic illustration showing a detail view of the compressor unit 116 of FIG. 1 in a second configuration, in accordance with some embodiments. In various implementations, the compressor unit 116 includes a compressor 302 and a four-way reversing valve. The four-way reversing valve may include a valve body 304 having four refrigerant ports. For example, the valve body 304 may include a refrigerant port 306 fluidly coupled to the compressor 302 for receiving refrigerant from the compressor 302 and a refrigerant port 308 fluidly coupled to the compressor 302 for providing refrigerant to the compressor 302. The valve body 304 may include a refrigerant port 310 fluidly coupling the compressor unit 116 to the evaporator(s) 122 and a refrigerant port 312 fluidly coupling the compressor unit 116 to the condenser 118. A slide 314 may be positioned within the valve body 304 and movable between a first position (shown in the first configuration of FIG. 3) and a second position (shown in the second configuration of FIG. 4). For example, a solenoid coil may be positioned within the valve body 304 to move the slide 314 between the first position and the second position.

In various implementations, the compressor unit 116 operates in a heating mode when configured in the first configuration and operates in a defrosting mode when configured in the second configuration. For example, in the first configuration (e.g., FIG. 3), the slide 314 fluidly couples the refrigerant port 308 and the refrigerant port 310. Thus, in the first configuration, refrigerant flows from the compressor 302 and enters the valve body 304 via the refrigerant port 306. The refrigerant flows through the valve body 304 and exits the valve body 304 through the refrigerant port 312. The refrigerant flows from the refrigerant port to the condenser 118, from the condenser 118 through the expansion device(s) 120, from the expansion device(s) 120 through the evaporator(s) 122, and from the evaporator(s) 122 back to the refrigerant port 310. The refrigerant flows from the refrigerant port 310 to the refrigerant port 308 (through the slide 314) and returns back to the compressor 302. Thus, in the first configuration, the refrigerant flows through the refrigerant circuit 112 in a manner that transfer heat from the evaporator(s) 122 to the condenser 118.

In the second configuration, the slide 314 fluidly couples the refrigerant port 308 and the refrigerant port 312. Thus, in the second configuration (e.g., FIG. 4), refrigerant flows from the compressor 302 and enters the valve body 304 via the refrigerant port 306. The refrigerant flows through the valve body 304 and exits the valve body 304 via the refrigerant port 310. The refrigerant flows from the refrigerant port 310 to the evaporator(s) 122, through the evaporator(s) 122 to the expansion device(s) 120, through the expansion device(s) 120 to the condenser 118, and through the condenser 118 to the refrigerant port 312. The refrigerant flows from the refrigerant port 312 to the refrigerant port 308 (through the slide 314) and returns back to the compressor 302. Thus, in the second configuration, the evaporator 122 effectively functions as a condenser and the condenser 118 effectively functions as an evaporator, and the refrigerant flows through the refrigerant circuit 112 in a manner that transfers heat from the condenser 118 to the evaporator(s) 122.

The compressor unit 116 may also include a variety of sensors that measure one or more variables such as temperature, pressure, flow rate, etc. In various implementations, the compressor unit 116 includes a pressure sensor 316 positioned between the compressor 302 and the refrigerant port 306, a temperature sensor 318 positioned between the compressor 302 and the refrigerant port 306, and/or a pressure sensor 320 positioned between the refrigerant port 308 and the compressor 302. The pressure sensor 316 may measure a discharge refrigerant pressure P_dis(e.g., a “high side” refrigerant pressure) of the refrigerant output from the compressor 302. The temperature sensor 318 may measure a discharge refrigerant temperature T_dis(e.g., a “high side” refrigerant temperature) of the refrigerant output from the compressor 302. The pressure sensor 320 may measure a suction refrigerant pressure P_suc(e.g., a “low side” refrigerant pressure) of the refrigerant returning to the compressor 302.

FIG. 5 is a block diagram showing an example interconnected hardware control system of the heater unit 104. As shown in FIG. 5, the heater unit 104 may include a heater unit controller 502. In various implementations, the controller 502 includes one or more electronic processors and non-transitory computer-readable storage media containing instructions executable by the one or more electronic processors to perform the various processes, methods, and techniques described in this specification (for example, the processes of FIGS. 6-10). The controller 502 may be operatively coupled to and communicate with the compressor unit 116, the fan 124, the pump 130, the flow control valve 132, the temperature sensor 134, the temperature sensor(s) 136 (such as temperature sensor 136-1 and/or temperature sensor 136-2), the temperature sensor(s) 138 (such as temperature sensors 138-1 and/or temperature sensor 138-2), flow meter 144, pressure sensor 316, temperature sensor 318, and/or pressure sensor 320.

In various implementations, the controller 502 controls the operation of the compressor unit 116 (such as operation of the compressor 302 and/or the position of the slide 314), the fan 124, the pump 130, and/or the flow control valve 132. In some examples, the controller 502 receives signals indicative of the refrigerant liquid line temperature T_{liq_line}from the temperature sensor 134. In various implementations, the controller 502 receives signals indicative of the evaporator section refrigerant temperature(s) T_{ev_A}from the temperature sensor(s) 136. For example, in implementations where the heater unit 104 includes multiple evaporator sections, the controller 502 may generate a single evaporator section refrigerant temperature T_{ev_A}from the multiple temperature sensors 136-1 and/or 136-2 and/or the multiple evaporator section refrigerant temperatures T_{ev1_A}and/or T_{ev2_A}generated from the multiple sensor signals. In some examples, the evaporator section refrigerant temperatures T_{ev_A}may include each of the multiple evaporator section refrigerant temperature T_{ev1_A}and/or T_{ev2_A}.

In various implementations, the controller 502 receives signals indicative of the evaporator section refrigerant temperature(s) T_{ev_B}from the temperature sensor(s) 138. For example, in implementations where the heater unit 104 includes multiple evaporator sections, the controller 502 may generate a single evaporator section refrigerant temperature T_{ev_B}from the multiple temperature sensors 138-1 and/or 138-2 and/or the multiple evaporator section refrigerant temperatures T_{ev1_B}and/or T_{ev2_B}generated from the multiple sensor signals. In some examples, the evaporator section refrigerant temperatures T_{ev_B}may include each of the multiple evaporator section refrigerant temperature T_{ev1_B}and/or T_{ev2_B}. In various implementations, the controller 502 generates a single temperature signal and/or measurement based on multiple signals and/or measurements according to averaging (such as simple averaging or weighted averaging) techniques, median filtering techniques, majority voting techniques, advanced statistical methods that predict actual temperatures (such as Kalman filters and predictive filters), machine learning techniques, consensus techniques, etc. Thus, as used herein, the evaporator section temperature T_{ev_A}may refer to any combination (or aggregation) of evaporator section temperatures from the various evaporator sections (such as T_{ev1_A}and/or T_{ev2_A}). Similarly, the evaporator section temperature T_{ev_B}may refer to any combination (or aggregation) of evaporator section temperatures from the various evaporator sections (such as T_{ev1_B}and/or T_{ev2_B}).

FIG. 6 is a flowchart of an example process 600 for controlling the water heater unit 104. In the process 600, the controller 502 determines whether to initiate a defrost process based on sensor signals and selectively initiates the defrost process. In the process 600, the controller 502 receives sensor signals from the temperature sensor(s) 136 and determines a refrigerant section temperature T_{ev_A}based on the sensor signals (at block 602). The refrigerant section temperature T_{ev_A}may indicate the temperature of the refrigerant entering the evaporator(s) 122. In various implementations, the controller 502 may monitor each temperature sensor (such as temperature sensors 136-1 and 136-2) individually and generate an individual refrigerant section temperature (such as T_{ev1_A}and T_{ev2_A}) for each evaporator (such as evaporator 122-1 and 122-2) or aggregate the sensor signals to generate a single refrigerant section temperature T_{ev_A}(for example, according to previously described techniques). Thus, the refrigerant section temperature T_{ev_A}may refer to a single temperature (for example, where the heater unit 104 has a single evaporator section), collectively to multiple temperatures (for example, where the heater unit 104 has multiple evaporator sections), or a single temperature generated from multiple sensor signals.

In the process 600, the controller 502 determines whether the refrigerant section temperature T_{ev_A}is below a threshold (at decision block 604). Since the refrigerant section temperature T_{ev A}indicates the temperature of refrigerant entering the evaporator(s) 122, the refrigerant section temperature T_{ev_A}may be used to determine whether the evaporator(s) 122 are likely at a temperature where they are at risk of forming frost. In scenarios where the refrigerant section temperature T_{ev_A}is below the threshold, the evaporator(s) 122 may be at risk of forming frost. In scenarios where the refrigerant section temperature T_{ev_A}is not below the threshold, the evaporator(s) 122 may not be at risk of forming frost. In various implementations, the threshold may be set to a value close to the freezing temperature of water. For example, the threshold may be set to about 30° F., 30.5° F., 31° F., 31.5° F., 32° F., 32.5° F., 33° F., 33.5° F., 34° F., 34.5° F., 35° F., or 35.5° F.

In response to the controller 502 determining that the refrigerant section temperature T_{ev_A}is not below the threshold (“N” at decision block 604), the process 600 ends. In response to determining that the refrigerant section temperature T_{ev_A}is below the threshold (“Y” at decision block 604), the controller 502 evaluates sensor signals (such as signals from the temperature sensor(s) 136 and/or the temperature sensors(s) 138) to determine whether at least one of the evaporator(s) 122 has lost superheat (at block 606). Superheat may refer to the temperature rise of the refrigerant above its boiling point (saturation temperature). During normal operation (e.g., without frost formation on the evaporator(s) 122), refrigerant enters the evaporator(s) 122 as low-temperature, low-pressure mixture of liquid and vapor. As the refrigerant passes through the evaporator(s) 122, it absorbs heat from the external environment. The absorbed heat causes the liquid part of the refrigerant to vaporize. After the refrigerant has reached complete vaporization (e.g., it exists in a substantially vapor state), it may continue to absorb heat from the external environment. This continued absorption of heat in the substantially vapor state may be referred to as superheat. In normal operation, the refrigerant exits the evaporator(s) as a superheated vapor (e.g., shown as point 214 in the graph 200) The loss of superheat in at least one of the evaporator(s) 122 may indicate that the evaporator(s) are not efficiently absorbing heat from the external environment (for example, as the result of frost formation on the evaporator(s) 122). Additional details of the controller 502 determining whether at least one of the evaporator(s) 122 has lost superheat are described with reference to FIG. 7.

In response to the controller 502 determining that loss of superheat in at least one of the evaporator(s) 122 has not occurred (“N” at decision block 608), the process 600 ends. In response to the controller 502 determining that loss of heat in at least one of the evaporator(s) 122 has occurred (“Y” at decision block 608), the controller 502 initiates a defrost process (at block 610). Additional details associated with the controller 502 initiating the defrost process are described with reference to FIGS. 8-10.

FIG. 7 is a flowchart of an example process 700 for determining whether an evaporator 122 has lost superheat. In the example process 700, the controller 502 initializes a timer and sets the timer to zero (at block 702). In the example process 700, the controller 502 monitors the evaporator section temperature T_{ev_A}(at block 704). In the example process 700, the controller 502 monitors the evaporator section temperature T_{ev_B}(at block 706). When the compressor unit 116 is operating in the first configuration (e.g., the heating mode), the evaporator section temperature T_{ev_A}computed from sensor signal(s) of the temperature sensor(s) 136 indicate the temperature of refrigerant entering the evaporator(s) 122, and the evaporator section temperature T_{ev_B}computed from sensor signal(s) of the temperature sensor(s) 138 indicate the temperature of refrigerant exiting the evaporator(s) 122. In the example process 700, the controller 502 compares the evaporator section temperatures T_{ev_A}and T_{ev_B}and determines whether the evaporator section temperature T_{ev_B}is greater than the evaporator section temperature T_{ev_A}(at decision block 708). When the refrigerant exits the evaporator(s) 122 at approximately the same temperature as it enters the evaporator(s) 122 (e.g., when T_{ev_B}is not greater than T_{ev_A}), the refrigerant may be exiting the evaporator(s) 122 in an unsaturated state (e.g., in a mixed two-phase state), which may indicate that superheating is not occurring in the evaporator(s) 122.

In response to the controller 502 determining that the evaporator section temperature T_{ev_B}is not greater than the evaporator section temperature T_{ev_A}(“N” at decision block 708), the controller 502 runs the timer (at block 710). In the process 700, the controller 502 determines whether the timer has expired (at decision block 712). In various implementations, the timer expiration may be set to a value in a range of between about five minutes and about 15 minutes. For example, the timer may be set to expire at about eight minutes. In response to the controller 502 determining that the timer has not yet expired (“N” at decision block 712), the controller 502 continues monitoring the evaporator section temperatures T_{ev_A}and T_{ev_B}(at blocks 704 and 706) and determining whether the evaporator section temperature T_{ev_B}is greater than the evaporator section temperature T_{ev_A}(at decision block 708). By continuing to monitor the evaporator section temperatures T_{ev_A}and T_{ev_B}for the duration of the timer, the controller 502 ensures that the loss of superheat determination is not made (and the defrost process is not initiated) in response to spurious sensor readings. Accordingly, in some examples, the controller 502 determines the loss of superheat condition only in response to determining that the evaporator section temperature T_{ev_B}is not greater than the evaporator section temperature T_{ev_A}for a period of time (e.g., the duration of the timer). In response to the controller 502 determining that the timer has expired (“Y” at decision block 712), the controller 502 determines that evaporator(s) 122 have lost superheat (at block 714) and the process 700 ends.

In response to the controller 502 determining that the evaporator section temperature T_{ev_B}is greater than the evaporator section temperature T_{ev_A}(“Y” at decision block 708), the controller 502 determines whether the difference between the evaporator section temperature T_{ev_B}and the evaporator section temperature T_{ev_A}(e.g., T_{ev_B}−T_{ev_A}) is greater than a threshold (at decision block 716). In various implementations, the threshold may be in range of between about 1° F. and about 15° F. For example, the threshold may be about 3° F. The controller 502 determining that the evaporator section temperature T_{ev_B}is greater than the evaporator section temperature T_{ev_A}(“Y” at decision block 708) but the difference between T_{ev_B}and T_{ev_A}is not greater than the threshold (“N” at decision block 716) may indicate that a small amount of superheat is occurring. Thus, in response to the controller 502 determining that the difference (T_{ev_B}−T_{ev_A}) is not greater than the threshold (“N” at decision block 716), the controller 502 pauses the timer (at block 718). The controller 502 continues monitoring the evaporator section temperatures T_{ev_A}and T_{ev_B}(at blocks 704 and 706) and determining whether the evaporator section temperature T_{ev_B}is greater than the evaporator section temperature T_{ev_A}(at decision block 708). In response to the controller 502 determining that the difference (T_{ev_B}−T_{ev_A}) is greater than the threshold (“Y” at decision block 716), the controller 502 determines that evaporator(s) 122 have not lost superheat (at block 720) and the process 700 ends.

FIGS. 8-10 are a flowchart of an example defrost process 800. FIG. 8 is a flowchart of a preheating sequence of the example process 800. The preheating sequence may minimize the amount of cold water directed to a hot water circuit or to the tank 102 as a result of the defrost process 800. In the example process 800, the controller 502 initializes and starts a preheat timer (at block 802). In the example process 800, the controller 502 shuts off water flow in the water circuit 114 (at block 804) while continuing to run the compressor unit 116 in the heating mode, continuing to transfer heat from the refrigerant circuit 112 to stagnant water in the condenser 118. For example, the controller 502 stops the pump 130. In the example process 800, the controller 502 measures one or more characteristics of the refrigerant (at block 806). In various implementations, the characteristics include one or more of the refrigerant liquid line temperature T_{liq_line}(e.g., computed based on sensor signals from the temperature sensor 134), the discharge refrigerant pressure P_dis(e.g., computed based on sensor signals from the pressure sensor 316), and the discharge refrigerant temperature T_dis(e.g., computed based on sensor signals from the temperature sensor 318).

In the example process 800, the controller 502 determines whether any of the one or more characteristics of the refrigerant exceeds a limit (at decision block 808). In various implementations, the limit for the refrigerant liquid line temperature T_{liq_line}may be about 150° F. In some examples, the limit for the discharge refrigerant pressure P_dismay be about 310 psi. In various implementations, the limit for the discharge refrigerant temperature T_dismay be about 220° F. Checking whether any of the characteristics of the refrigerant exceeds the relevant limit may ensure that the operational limitations of the refrigerant circuit 112 are not exceeded by the transfer of heat to the stagnant body of water from the water circuit 114 present in the condenser 118. In response to the controller 502 determining that none of the characteristics of the refrigerant exceeds its limit (“N” at decision block 808), the controller 502 determines whether the preheat timer expired (at decision block 810). In various implementations, the preheat timer may expire at a run time in a range of between about five minutes and 15 minutes. For example, the preheat timer may expire at a run time of about eight minutes. In response to determining that the preheat timer has not expired (“N” at decision block 810), the controller 502 increments the preheat timer (at block 812) and continues measuring the one or more refrigerant characteristics (at block 806).

FIG. 9 is a flowchart of a first defrost stage of the example process 800. In the example process 800, the controller 502 shuts off the fan 124 (at block 814). Shutting off the fan 124 stops airflow from the fan 124 flowing over the evaporator(s) 122. In the example process 800, the controller 502 stops the compressor unit 116 from pumping refrigerant (at block 816). For example, the controller 502 stops the compressor 302. In the example process 800, the controller 502 switches the compressor unit 116 from the heating mode to the defrosting mode (at block 818). For example, the controller 502 moves the slide 314 from the first position (e.g., corresponding to the heating mode) to the second position (e.g., corresponding to the defrosting mode). In the example process 800, the controller 502 may initiate a time delay (at block 820). Adding a time delay may allow refrigerant to migrate throughout the refrigerant circuit 112, equalizing the pressure of refrigerant throughout the circuit. Equalizing the refrigerant pressure before reversing the direction of refrigerant flow in the refrigerant circuit 112 may reduce the stress experienced by various components of the circuit (since portions of the circuit previously filled with high-pressure refrigerant will be filled with low-pressure refrigerant and portions of the circuit previously filled with low-pressure refrigerant will be filled with high-pressure refrigerant when the direction of flow is reversed). In various implementations, the time delay may be in a range of between about one minutes and about 15 minutes. In some examples, the time delay may be omitted.

In the example process 800, the controller 502 starts the pump 130 (at block 822). For example, the controller 502 may control the pump 130 to deliver a low water flow rate V_w(e.g., as measured by the flow meter 140) in the water circuit 114. In various implementations, the controller 502 may set the water flow rate V_wto a value in a range of between about 0.5 gallons per minute (GPM) and about 1 GPM. In the example process 800, the controller 502 starts the compressor unit 116 (at block 824). For example, the controller 502 commands the compressor 302 to run, and the compressor unit 116 begins delivering compressed refrigerant to the refrigerant circuit 112 in the defrosting mode. In the example process 800, the controller 502 monitors sensor signals from the temperature sensor 134 (at block 826). For example, the controller 502 may compute the refrigerant liquid line temperature T_{liq_line}based on the sensor signals. Since the compressor unit 116 is operating in the defrosting mode (e.g., refrigerant is flowing in the reverse direction compared to refrigerant flow during the heating mode), the refrigerant line between the evaporator section(s) and the condenser 118 (which is effectively functioning as an evaporator) will start to fill with low-pressure two-phase (e.g., mixed liquid and vapor) refrigerant corresponding to point 212 in graph 200. Thus, the refrigerant liquid line temperature T_{liq_line}may begin to drop.

In the example process 800, the controller 502 determines whether the refrigerant liquid line temperature T_{liq_line}falls below a first threshold (at decision block 828). In various implementations, the first threshold may be set to a temperature near (but above) the freezing point of water. For example, the first threshold may be set in a range of between about 33° F. and about 50° F. In various implementations, the first threshold may be set to about 45° F. The first threshold may be set to prevent ice formation in the water circuit 114 side of the condenser 118. For example, when the liquid line temperature T_{tiq_line}approaches freezing or falls below freezing, the refrigerant may remove enough heat from the condenser 118 such that the temperature of the condenser 118 approaches or falls below freezing, which may cause ice crystals to form in the water circuit 114. In response to the controller 502 determining that the liquid line temperature T_{liq_line}is not below the first threshold (“N” at decision block 828), the controller 502 continues monitoring sensor signals from the temperature sensor 134 (at block 824). In response to the controller 502 determining that the liquid line temperature T_{liq_line}is below the first threshold (“Y” at decision block 828), the controller initiates a second defrost stage (at block 830 of FIG. 10).

FIGS. 10 and 11 are a flowchart of the second defrost stage of the example process 800. In the example process 800, the controller 502 initializes and starts a first timer (at block 830). In the example process 800, the controller 502 increases the speed of the pump 130 to increase the water flow rate Vw in the water circuit 114 by an increment ΔV (at block 832). For example, the controller 502 monitors sensor signals from the flow meter 140 and increases the speed of the pump 130 to achieve incremental increases in the water flow rate ΔV over the old water flow rate V_oldto achieve a new water flow rate V_new. In the example process 800, the controller 502 monitors the liquid line temperature T_{liq_line}(at block 834). In various implementations, the controller 502 computes the liquid line temperature T_{liq_line}based on sensor signals from the temperature sensor 134. In the example process 800, the controller 502 determines whether the liquid line temperature T_{liq_line}falls below a second threshold (at decision block 836). The liquid line temperature T_{liq_line}falling below the second threshold may indicate a fault condition (for example, an abnormally low refrigerant line temperature). In various implementations, the second threshold may be set to about 2° F.

In response to the controller 502 determining that the liquid line temperature T_{liq_line}falls below the second threshold (“Y” at decision block 836), controller 502 may determine a fault condition and the process 800 ends. In response to the controller 502 determining that the liquid line temperature T_{lig_line}does not fall below the second threshold (“N” at decision block 836), the controller 502 determines whether the first timer has expired (at decision block 838). In response to the controller 502 determining that the first timer has not expired (“N” at decision block 838), the controller 502 increments the first timer (at block 840) and continues monitoring the liquid line temperature T_{liq_line}(at block 834). In response to the controller 502 determining that the first timer has expired (“Y” at decision block 838), the controller 502 determines whether the new water flow rate V_newis above a maximum water flow rate V_max(at decision block 842).

In various implementations, maximum water flow rate V_maxmay be set to a value that allows sufficient heat transfer between water entering the water circuit 114 via the inlet port 126 and the condenser 118 to prevent ice buildup in the condenser 118 while minimizing the amount of cooled water provided to the tank 102 via the outlet port 128. In some examples, the maximum water flow rate V_maxis below the water flow rate V_wduring normal heating operations. In various implementations, the maximum water flow rate V_maxmay be in a range of between about 1 GPM and about 5 GPM. For example, the maximum water flow rate V_maxmay be about 2.5 GPM. In response to the controller 502 determining that the new water flow rate V_newis less than the maximum water flow rate V_max(“Y” at decision block 842), the controller reinitializes and restarts the first timer (at block 830) and increases the speed of the pump 130 to achieve another incremental increase in the water flow rate ΔV (at block 832). In response to the controller 502 determining that the new water flow rate V_newis not less than the maximum water flow rate V_max(e.g., the pump 130 is operating at a speed that achieves the maximum water flow rate V_max—“N” at decision block 842), the process 800 continues operating the compressor unit 116 in the defrost mode and the water pump 130 to achieve the maximum water flow rate V_maxuntil a predetermined time period expires or a terminating condition is met (e.g., as shown in FIG. 11, beginning at block 844).

In the example process 800, the controller 502 initializes and starts a second timer (at block 844). In the example process 800, the controller 502 monitors the evaporator section refrigerant temperature T_{ev_A}(at block 846). For example, the controller 502 determines evaporator section refrigerant temperature T_{ev_A}based on sensor signals from the temperature sensor(s) 136. In the example process 800, the controller 502 determines whether the evaporator section refrigerant temperature T_{ev_A}is greater than a limit (at decision block 848). The limit may be set to a temperature substantially above the freezing point of water. For example, the limit may be set to a temperature in a range of between about 43° F. and about 59° F. In various implementations, the terminating condition may be met when the evaporator section refrigerant temperature T_{ev_A}remains above the limit for a preset period of time. The evaporator section refrigerant temperature T_{ev_A}may indicate the temperature of refrigerant exiting from the evaporator(s) 122 (which are functioning as condensers). Thus, the evaporator section refrigerant temperature T_{ev_A}remaining above the limit for a predetermined period of time may indicate that any frost formed on the evaporator(s) 122 has melted.

In response to the controller 502 determining that the evaporator section refrigerant temperature T_{ev_A}is not above the limit (“N” at decision block 848), the controller 502 determines whether the second stage timer has expired (at decision block 850). In response to the controller 502 determining that the second stage timer has not expired (“N” at decision block 850), the controller 502 increments the second stage timer (at block 852) and continues monitoring the evaporator section refrigerant temperature T_{ev_A}(at block 846). In response to the controller 502 determining that the evaporator section refrigerant temperature T_{ev_A}is above the limit (“Y” at decision block 848), the controller 502 waits for a preset delay (at block 854). In various implementations, the preset delay may be about 15 seconds. In the example process 800, after waiting for the preset delay, the controller 502 again determines whether the evaporator section refrigerant temperature T_{ev_A}is above the limit (at decision block 856). After determining that the evaporator section refrigerant temperature T_{ev_A}does not remain above the limit after waiting the preset delay (“N” at decision block 856), the controller 502 determines whether the second stage timer has expired (at decision block 850).

In the example process 800, the controller 502 begins a third defrost stage (at block 858 of FIG. 12) in response to the controller 502 determining that the evaporator section refrigerant temperature T_{ev_A}remains above the limit after waiting the preset delay (“Y” at decision block 856) or determining that the second timer has expired (“Y” at decision block 856). In various implementations, the controller 502 configures the heater unit 104 for the compressor unit 116 to resume operations in the heating mode during the third defrost stage. FIGS. 12 and 13 are a flowchart of the third defrost stage of the example process 800. In the example process 800, the controller 502 starts the fan 124 (at block 858). For example, the controller 502 runs the fan for a period of time so that airflow from the fan 124 removes any water from the melted frost remaining on the evaporator(s) 122. In the example process 800, the controller 502 shuts off the pump 130 (at block 860). In the example process 800, the controller 502 shuts off the compressor 302 (at block 862). In the example process 800, the controller 502 switches the compressor unit 116 from the defrosting mode to the heating mode (at block 864). For example, the controller 502 moves the slide 314 from the second position (e.g., corresponding to the defrosting mode) to the first position (e.g., corresponding to the heating mode).

In the example process 800, the controller 502 monitors the suction refrigerant pressure P_suc(at block 866). For example, the controller 502 determines the suction refrigerant pressure P_sucbased on sensor signals from the pressure sensor 320. In the example process 800, the controller 502 initializes and starts a third stage timer (at block 868). In the example process 800, the controller 502 determines whether the time t of the running third stage timer is greater than a minimum time t_min(at decision block 870). In various implementations, the minimum time t_minmay be set to specify a minimum time for the fan 124 to run. In some examples, the minimum time t_minmay be set to about 90 seconds. In response to the controller 502 determining that the time t of the running third stage timer is not greater than the minimum time t_min(“N” at decision block 870), the controller 502 increments the third stage timer (at block 872) and continues determining whether the time t of the running third stage timer is greater than the minimum time t_min(at decision block 870).

In response to the controller 502 determining that the time t of the running third stage timer is greater than the minimum time t_min(“Y” at decision block 870), the controller 502 determines whether the suction refrigerant pressure P_sucis within a range (at decision block 874). In various implementations, the range may be a range of acceptable suction refrigerant pressures P_sucfor the compressor unit 116 during normal heating operations. In response to the controller 502 determining that the suction refrigerant pressure P_sucis not within the range (“N” at decision block 874), the controller 502 determines whether the time t of the running third stage timer is less than a maximum time t_max(at decision block 876). In various implementations, the maximum time t_maxmay be set to specify a maximum time for the fan 124 to run. In some examples, the maximum time t_maxmay be set to about 300 seconds. In response to the controller 502 determining that the time t of the running third stage timer is less than the maximum time t_max(“Y” at decision block 876), the controller 502 increments the third stage timer (at block 878) and continues determining whether the suction refrigerant pressure P_sucis within the range (at decision block 874).

In response to the controller 502 determining that the suction refrigerant pressure P_sucis within the range (“Y” at decision block 874) or the controller 502 determining that the time t of the running third stage timer is not less than the maximum time t_max(“N” at decision block 876), the controller 502 initiates the preheat process (at block 880) and controls the heater unit 104 to resume normal heating operations. In various implementations, the preheat process may be the preheating sequence described with reference to FIG. 8. In some examples, block 880 may be omitted from the process 800 and the controller 502 proceeds directly to resuming normal heating operations.

The foregoing description is merely illustrative in nature and does not limit the scope of the disclosure or its applications. The broad teachings of the disclosure may be implemented in many different ways. While the disclosure includes some particular examples, other modifications will become apparent upon a study of the drawings, the text of this specification, and the following claims. In the written description and the claims, one or more processes within any given method may be executed in a different order—or processes may be executed concurrently or in combination with each other—without altering the principles of this disclosure. Similarly, instructions stored in a non-transitory computer-readable medium may be executed in a different order—or concurrently—without altering the principles of this disclosure. Unless otherwise indicated, the numbering or other labeling of instructions or method steps is done for convenient reference and does not necessarily indicate a fixed sequencing or ordering.

Unless the context of their usage unambiguously indicates otherwise, the articles “a,” “an,” and “the” should not be interpreted to mean “only one.” Rather, these articles should be interpreted to mean “at least one” or “one or more.” Likewise, when the terms “the” or “said” are used to refer to a noun previously introduced by the indefinite article “a” or “an,” the terms “the” or “said” should similarly be interpreted to mean “at least one” or “one or more” unless the context of their usage unambiguously indicates otherwise.

Spatial and functional relationships between elements—such as modules—are described using terms such as (but not limited to) “connected,” “engaged,” “interfaced,” and/or “coupled.” Unless explicitly described as being “direct,” relationships between elements may be direct or include intervening elements. The phrase “at least one of A, B, and C” should be construed to indicate a logical relationship (A OR B OR C), where OR is a non-exclusive logical OR, and should not be construed to mean “at least one of A, at least one of B, and at least one of C.” The term “set” does not necessarily exclude the empty set. For example, the term “set” may have zero elements. The term “subset” does not necessarily require a proper subset. For example, a “subset” of set A may be coextensive with set A, or include elements of set A. Furthermore, the term “subset” does not necessarily exclude the empty set.

In the figures, the directions of arrows generally demonstrate the flow of information—such as data or instructions. The direction of an arrow does not imply that information is not being transmitted in the reverse direction. For example, when information is sent from a first element to a second element, the arrow may point from the first element to the second element. However, the second element may send requests for data to the first element, and/or acknowledgements of receipt of information to the first element. Furthermore, while the figures illustrate a number of components and/or steps, any one or more of the components and/or steps may be omitted or duplicated, as suitable for the application and setting.

The term computer-readable medium does not encompass transitory electrical or electromagnetic signals or electromagnetic signals propagating through a medium—such as on an electromagnetic carrier wave. The term “computer-readable medium” is considered tangible and non-transitory. The functional blocks, flowchart elements, and message sequence charts described above serve as software specifications that can be translated into computer programs by the routine work of a skilled technician or programmer.

It should also be understood that although certain drawings illustrate hardware and software as being located within particular devices, these depictions are for illustrative purposes only. In some embodiments, the illustrated components may be combined or divided into separate software, firmware, and/or hardware. For example, instead of being located within and performed by a single electronic processor, logic and processing may be distributed among multiple electronic processors. Regardless of how they are combined or divided, hardware and software components may be located on the same computing device, or they may be distributed among different computing devices—such as computing devices interconnected by one or more networks or other communications systems.

In the claims, if an apparatus or system is claimed as including an electronic processor or other element configured in a certain manner, the claim or claimed element should be interpreted as meaning one or more electronic processors (or other element as appropriate). If the electronic processor (or other element) is described as being configured to make one or more determinations or one or execute one or more steps, the claim should be interpreted to mean that any combination of the one or more electronic processors (or any combination of the one or more other elements) may be configured to execute any combination of the one or more determinations (or one or more steps).

HEAT PUMP DEFROST CONTROL

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