HEAT PUMP SYSTEM VARIABLE DEFROST

Description

FIELD OF THE INVENTION

The present subject matter relates generally to heat pump systems, and more particularly to heat pump systems configured for defrosting an outdoor coil thereof and related methods for defrosting the outdoor coil of a heat pump system.

BACKGROUND OF THE INVENTION

Heat pump systems and other air conditioner systems or air conditioning systems are conventionally utilized to adjust the temperature within structures such as dwellings and office buildings. A typical air conditioner or air conditioning system includes an indoor portion comprising one or more indoor units and an outdoor portion comprising at least one outdoor unit. Each of the indoor unit(s) and the outdoor unit(s) generally includes a heat exchanger by which thermal energy is transferred between refrigerant in a sealed system and air. The indoor portion generally communicates (e.g., exchanges air) with the area within a building, and the outdoor portion generally communicates (e.g., exchanges air) with the area outside a building. Generally, the indoor unit may include a fan operable to rotate to motivate air through the indoor unit, or each indoor unit (when the indoor portion includes multiple indoor units) may include such a fan. Another fan (or fans) may be operable to rotate to motivate air through the outdoor portion. The sealed system includes a compressor, and the sealed system is generally coupled to the indoor unit(s) and the outdoor unit(s) to treat (e.g., cool or heat) air as it is circulated through the units. One or more control boards are typically provided to direct the operation of various elements of the particular air conditioner system.

Some air conditioner systems include a reversing valve coupled in line with the sealed system, whereby the reversing valve selectively directs the flow of vapor refrigerant from the compressor to one or the other of the indoor heat exchanger (e.g., for a heating mode or heat pump mode in which the indoor heat exchanger acts as the condenser in the sealed system) or the outdoor heat exchanger (e.g., for a cooling mode in which the outdoor heat exchanger operates as the condenser in the sealed system). Such systems, e.g., which include a reversing valve, may be referred to as heat pump systems or heat pump units.

When operating in the heating mode, e.g., when the outdoor heat exchanger is operating as an evaporator, condensation may accumulate on the outdoor heat exchanger and may freeze thereon. Accordingly, the outdoor heat exchanger may be periodically defrosted, which generates a flow of meltwater that is drained from the system. Under certain outdoor ambient conditions, the meltwater may freeze in the system before draining out. Some heat pump systems thus include an additional heater, such as in a basepan of the system, to maintain the meltwater in a liquid state. Such additional heaters add complexity and increase energy consumption of the heat pump systems.

Accordingly, improved defrost features and operations for heat pump systems that addresses one or more of the challenges noted above are desired in the art, such as defrost features and operations that are responsive to outdoor ambient environmental conditions and/or which avoid or minimize usage of additional heaters.

BRIEF DESCRIPTION OF THE INVENTION

Aspects and advantages of the invention will be set forth in part in the following description, or may be apparent from the description, or may be learned through practice of the invention.

In an example embodiment, a method of operating a heat pump system is provided. The heat pump system includes an outdoor unit, an indoor unit, and a sealed system coupled between the outdoor unit and the indoor unit to circulate refrigerant through an indoor heat exchanger of the indoor unit and an outdoor heat exchanger of the outdoor unit. The sealed system includes a reversing valve to selectively reverse flow direction of the refrigerant. The method includes measuring an ambient condition of an outdoor environment around the outdoor unit and determining a termination criterion for a defrost operation based on the measured ambient condition. The method also includes initiating the defrost operation and exiting the defrost operation when the determined termination criterion is satisfied.

In another example embodiment, a heat pump system is provided. The heat pump system includes an outdoor unit, an indoor unit, and a sealed system coupled between the outdoor unit and the indoor unit to circulate refrigerant through an indoor heat exchanger of the indoor unit and an outdoor heat exchanger of the outdoor unit. The sealed system includes a reversing valve to selectively reverse flow direction of the refrigerant. The heat pump system further includes a controller. The controller is configured for measuring an ambient condition of an outdoor environment around the outdoor unit and determining a termination criterion for a defrost operation based on the measured ambient condition. The controller is also configured for initiating the defrost operation and exiting the defrost operation when the determined termination criterion is satisfied.

These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures.

FIG. 1 is a perspective view of a heat pump system according to an example embodiment of the present subject matter.

FIG. 2 is an interior perspective of the example heat pump system of FIG. 1 installed in a window.

FIG. 3 is a schematic view of a sealed system of the example heat pump system of FIG. 1.

FIG. 4 is a flow diagram of an exemplary method of operating a heat pump system according to one or more exemplary embodiments of the present disclosure.

FIG. 5 is a flow diagram of another exemplary method of operating a heat pump system according to one or more exemplary embodiments of the present disclosure.

DETAILED DESCRIPTION

Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.

FIG. 1 is a perspective view of a saddle window air conditioner 100 according to an example embodiment of the present subject matter, e.g., saddle window air conditioner 100 is one example embodiment of a heat pump system. FIG. 2 is an interior perspective of saddle window air conditioner 100 installed in a window 10. Saddle window air conditioner 100 is operable to generate chilled and/or heated air in order to regulate the temperature of an associated room or building. As will be understood by those skilled in the art, saddle window air conditioner 100 may be installed within window 10 to cool and/or heat air on an interior side of window 10 to a selected temperature. As discussed in greater detail below, a sealed system 120 (FIG. 3) of saddle window air conditioner 100 is disposed within a casing assembly 110. Thus, saddle window air conditioner 100 may be a self-contained or autonomous system for heating and/or cooling air. Saddle window air conditioner 100 defines a vertical direction V, a lateral direction L and a transverse direction T that are mutually perpendicular and form an orthogonal direction system.

As used herein, the term “saddle window air conditioner” is used broadly. For example, saddle window air conditioner 100 may include a supplementary electric heater (not shown) for assisting with heating air within the associated room or building without operating the sealed system 120. However, as discussed in greater detail below, saddle window air conditioner 100 is also operable in a heat pump heating mode that utilizes sealed system 120 to heat air within the associated room or building.

With reference to FIGS. 1 and 2, casing assembly 110 includes an interior casing 112, an exterior casing 114 and a bridge 130. Interior casing 112 and exterior casing 114 are spaced apart from each other, e.g., along the transverse direction T. Thus, interior casing 112 may be positioned at or contiguous with an interior atmosphere on one side of window 10, and exterior casing 114 may be positioned at or contiguous with an exterior atmosphere on the other side of window 10. Bridge 130 extends between interior casing 112 and exterior casing 114, e.g., through window 10.

Turning to FIG. 3, sealed system 120 is disposed or positioned within casing assembly 110, and sealed system 120 includes components for transferring heat between the exterior atmosphere and the interior atmosphere. In particular, various components of sealed system 120 are positioned within interior casing 112 while other components of sealed system 120 are positioned within exterior casing 114.

Saddle window air conditioner 100 further includes a controller 180 (FIG. 1) with user inputs, such as buttons, switches and/or dials. The controller 180 regulates operation of saddle window air conditioner 100. Thus, the controller 180 is in operative communication with various components of saddle window air conditioner 100, such as components of sealed system 120 and/or a temperature sensor, such as a thermistor or thermocouple, for measuring the temperature of the interior atmosphere. In particular, the controller 180 may selectively activate sealed system 120 in order to chill or heat air within sealed system 120, e.g., in response to temperature measurements from the temperature sensor.

The controller 180 includes memory and one or more processing devices such as microprocessors, CPUs or the like, such as general or special purpose microprocessors operable to execute programming instructions or micro-control code associated with operation of saddle window air conditioner 100. The memory can represent random access memory such as DRAM, or read only memory such as ROM or FLASH. The processor executes programming instructions stored in the memory. The memory can be a separate component from the processor or can be included onboard within the processor. Alternatively, the controller 180 may be constructed without using a microprocessor, e.g., using a combination of discrete analog and/or digital logic circuitry (such as switches, amplifiers, integrators, comparators, flip-flops, AND gates, and the like) to perform control functionality instead of relying upon software.

Sealed system 120 generally operates in a heat pump cycle. Sealed system 120 includes a compressor 122, an interior heat exchanger or coil 124 and an exterior heat exchanger or coil 126. As is generally understood, various conduits may be utilized to flow refrigerant between the various components of sealed system 120. Thus, e.g., interior coil 124 and exterior coil 126 may be between and in fluid communication with each other and compressor 122.

As may be seen in FIG. 3, sealed system 120 may also include a reversing valve 152. Reversing valve 152 selectively directs compressed refrigerant from compressor 122 to either interior coil 124 or exterior coil 126. For example, in a cooling mode, reversing valve 152 is arranged or configured to direct compressed refrigerant from compressor 122 to exterior coil 126. Conversely, in a heating mode, reversing valve 152 is arranged or configured to direct compressed refrigerant from compressor 122 to interior coil 124. Thus, reversing valve 152 permits sealed system 120 to adjust between the heating mode and the cooling mode, as will be understood by those skilled in the art.

During operation of sealed system 120 in the cooling mode, refrigerant from interior coil 124 flows through compressor 122. For example, refrigerant may exit interior coil 124 as a fluid in the form of a superheated vapor. Upon exiting interior coil 124, the refrigerant may enter compressor 122. Compressor 122 is operable to compress the refrigerant. Accordingly, the pressure and temperature of the refrigerant may be increased in compressor 122 such that the refrigerant becomes a more superheated vapor.

Exterior coil 126 is disposed downstream of compressor 122 in the cooling mode and acts as a condenser. Thus, exterior coil 126 is operable to reject heat into the exterior atmosphere at exterior side portion 114 of casing 110 when sealed system 120 is operating in the cooling mode. For example, the superheated vapor from compressor 122 may enter exterior coil 126 via a first distribution conduit 154 that extends between and fluidly connects reversing valve 152 and exterior coil 126. Within exterior coil 126, the refrigerant from compressor 122 transfers energy to the exterior atmosphere and condenses into a saturated liquid and/or liquid vapor mixture. An exterior air handler or fan 148 is positioned adjacent exterior coil 126 and may facilitate or urge a flow of air from the exterior atmosphere across exterior coil 126 in order to facilitate heat transfer.

Sealed system 120 also includes a capillary tube 128 disposed between interior coil 124 and exterior coil 126, e.g., such that capillary tube 128 extends between and fluidly couples interior coil 124 and exterior coil 126. Refrigerant, which may be in the form of high liquid quality/saturated liquid vapor mixture, may exit exterior coil 126 and travel through capillary tube 128 before flowing through interior coil 124. Capillary tube 128 may generally expand the refrigerant, lowering the pressure and temperature thereof. The refrigerant may then be flowed through interior coil 124.

Interior coil 124 is disposed downstream of capillary tube 128 in the cooling mode and acts as an evaporator. Thus, interior coil 124 is operable to heat refrigerant within interior coil 124 with energy from the interior atmosphere at interior side portion 112 of casing 110 when sealed system 120 is operating in the cooling mode. For example, the liquid or liquid vapor mixture refrigerant from capillary tube 128 may enter interior coil 124. Within interior coil 124, the refrigerant from capillary tube 128 receives energy from the interior atmosphere and vaporizes into superheated vapor and/or high quality vapor mixture. An interior air handler or fan 150 is positioned adjacent interior coil 124 may facilitate or urge a flow of air from the interior atmosphere across interior coil 124 in order to facilitate heat transfer. The vapor refrigerant may then return to the compressor 122 via a second distribution conduit 156 that extends between and fluidly connects interior coil 124 and reversing valve 152.

During operation of sealed system 120 in the heating mode, reversing valve 152 reverses the direction of refrigerant flow through sealed system 120. Thus, in the heating mode, interior coil 124 is disposed downstream of compressor 122 and acts as a condenser, e.g., such that interior coil 124 is operable to reject heat into the interior atmosphere at interior side portion 112 of casing 110. In addition, exterior coil 126 is disposed downstream of capillary tube 128 in the heating mode and acts as an evaporator, e.g., such that exterior coil 126 is operable to heat refrigerant within exterior coil 126 with energy from the exterior atmosphere at exterior side portion 114 of casing 110.

Interior coil 124 and interior fan 150 may be positioned within interior casing 112. Conversely, compressor 122, exterior coil 126, reversing valve 152 and exterior fan 148 may be positioned within exterior casing 114. In such a manner, certain noisy components of sealed system 120 may be spaced from the interior atmosphere, and saddle window air conditioner 100 may operate quietly. Various fluid passages, such as refrigerant conduits, liquid runoff conduits, etc., may extend through bridge 130 to fluidly connect components within interior and exterior casings 112, 114.

It should be understood that sealed system 120 described above is provided by way of example only. In alternative example embodiments, sealed system 120 may include any suitable components for heating and/or cooling air with a refrigerant. Sealed system 120 may also have any suitable arrangement or configuration of components for heating and/or cooling air with a refrigerant in alternative example embodiments.

As shown in FIG. 3, saddle window air conditioner 100 also includes a basepan, e.g., drain pan or bottom tray 138. Components of sealed system 120 within interior casing 112 are positioned on bottom tray 138. Thus, liquid runoff from components of sealed system 120 within interior casing 112 may flow into and collect within bottom tray 138. In particular, interior coil 124 may be positioned over bottom tray 138 along the vertical direction, and liquid runoff from interior coil 124, e.g., generated during a defrost of interior coil 124, may flow downwardly from interior coil 124 into bottom tray 138. Thus, bottom tray 138 may collect defrost melt water from interior coil 124 within interior casing 112. As discussed in greater detail below, saddle window air conditioner 100 also includes features for flowing the liquid runoff in bottom tray 138 out of interior casing 112, e.g., and to exterior casing 114.

As shown in FIG. 3, saddle window air conditioner 100 may also include a pump 142 and a float switch 144. Pump 142 is coupled to condensate tube 140 and is operable to flow the liquid runoff from interior coil 124 within bottom tray 138 to exterior casing 114 through condensate tube 140. Float switch 144 is coupled to pump 142 and is operable to activate/deactivate pump 142 in response to a fill level of liquid runoff from interior coil 124. For example, float switch 144 may be positioned within bottom tray 138, and liquid runoff from interior coil 124 may flow into bottom tray 138 with float switch 144. As bottom tray 138 fills with liquid runoff from interior coil 124, float switch 144 trips and activates pump 142 when bottom tray 138 is filled with a predetermined fill level of liquid runoff. In such a manner, liquid runoff from interior coil 124 may be evacuated from bottom tray 138 by pump 142 when triggered by float switch 144.

Also as may be seen in FIG. 3, the saddle window air conditioner 100 may optionally include a defrost heater 160 coupled to and in conductive thermal communication with the exterior coil 126. When in the heating mode, the exterior coil 126 acts as an evaporator, e.g., liquid phase refrigerant absorbs heat from the outside air and vaporizes at the exterior coil 126 in the heating mode. Thus, the exterior coil 126 may be colder than ambient outdoor temperature such that frost may accumulate on the exterior coil 126 during heating mode operation. Accordingly, the heat pump system, e.g., saddle window air conditioner 100, may be configured for performing a defrost operation to remove frost from the outdoor heat exchanger. In various embodiments, the defrost operation may include running the sealed system in a defrost mode (as will be described further below) and/or activating the defrost heater 160. For example, in some embodiments, the defrost heater 160 may be activated to melt the frost from the exterior coil 126. Additionally, as will be described further below, the defrost operation may also be used to melt or prevent freezing of condensate and/or other liquid water from the interior casing 112, e.g., that is flowed to the exterior casing 114 by pump 142.

As mentioned, the defrost operation may include performing a defrost cycle and/or running the heat pump system in a defrost mode. In at least some embodiments, the defrost cycle or defrost mode may be performed without activating a defrost heater in the outdoor portion of the heat pump system, e.g., defrost heater 160. Further, such embodiments may include heat pump systems in which an outdoor defrost heater is omitted entirely. During the defrost mode, the refrigerant is flowed through the sealed system as in cooling mode, e.g., whereby the outdoor heat exchanger acts as a condenser and the indoor heat exchanger acts as an evaporator, however, unlike cooling mode, the indoor fan is disabled and is not activated during the defrost mode. Accordingly, where the defrost mode may be initiated when the heat pump system is otherwise in heating mode and/or there is a call for heating, chilled air from the evaporator (indoor coil during defrost mode) is not flowed into the indoor space, and/or such air flow is limited, by disabling the indoor fan. The defrost mode thus promotes melting of frost from the outdoor heat exchanger via heat provided to the outdoor heat exchanger from refrigerant therein, as the refrigerant releases heat and changes phase from vapor to liquid in the outdoor heat exchanger. Further, the defrost mode may be exited by returning to heating mode, which may include moving the reversing valve to heating mode position (whereby vapor refrigerant is directed from the compressor to indoor heat exchanger) and re-enabling both fans, e.g., permitting the indoor fan to be activated as may be called for during the heating mode operation.

Embodiments of the present disclosure may also include methods of operating a heat pump system, such as the example methods 400 and 500 illustrated in FIGS. 4 and 5. Such methods may be used with any suitable heat pump system, such as heat pump system 100 as described above, or any other heat pump system, e.g., any air conditioning unit or system, Heating Ventilation and Air Conditioning (HVAC) system, etc., which includes indoor and outdoor portions and which is operable in a heating mode where an outdoor heat exchanger operates as an evaporator, such as a Packaged Terminal Air Conditioner (PTAC), e.g., a vertical PTAC, among numerous other possible systems.

Referring generally to FIGS. 4 and 5, the methods 400 and 500 may be interrelated and/or may have one or more steps from one of the methods 400 and 500 combined with the other method 400 or 500. Thus, those of ordinary skill in the art will recognize that the various steps of the exemplary methods described herein may be combined in various ways to arrive at additional embodiments within the scope of the present disclosure.

FIGS. 4 and 5 depict steps in a particular order for purpose of illustration and discussion. Those of ordinary skill in the art, using the disclosures provided herein, will understand that (except as otherwise indicated) methods 400 and 500 are not mutually exclusive. Moreover, the steps of the methods 400 and 500 can be modified, adapted, rearranged, omitted, interchanged, or expanded in various ways without deviating from the scope of the present disclosure.

Furthermore, the skilled artisan will recognize the interchangeability of various features from different embodiments. Similarly, the various method steps and features described, as well as other known equivalents for each such methods and feature, can be mixed and matched by one of ordinary skill in this art to construct additional systems and techniques in accordance with principles of this disclosure. Of course, it is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.

Still referring to FIGS. 4 and 5 collectively, some exemplary methods according to the present disclosure may include measuring an ambient condition of an outdoor environment around the outdoor unit, e.g., as indicated at (410) in FIG. 4, and determining a termination criterion for a defrost operation based on the measured ambient condition, e.g., as indicated at (420) in FIG. 4. For example, as illustrated in FIG. 5, the termination criterion may be determined based on the outdoor ambient condition by comparing the outdoor ambient condition to a predetermined threshold or limit, e.g., determining whether the outdoor ambient condition is greater than the limit, as indicated at (510) in FIG. 5, and setting a termination criterion based on whether or not the outdoor ambient condition is greater than the limit. For example, the termination criterion may be set to a first value when the outdoor ambient condition is greater than the limit (520) or may be set to a second value when the outdoor ambient condition is not greater than (i.e., is less than or equal to) the limit (522). The defrost operation may then be initiated (as indicated, for example, at 430 and 530), e.g., methods according to the present disclosure may include initiating a defrost cycle or defrost mode of the sealed system and/or activating the defrost heater in the outdoor unit of the heat pump system (such as at the outdoor coil) until the termination criterion is satisfied. For example, such methods may include exiting the defrost operation (e.g., exiting the defrost mode and/or deactivating the defrost heater), when the determined termination criterion is satisfied as indicated at (440) in FIG. 4 and at (550) in FIG. 5.

Referring briefly to FIG. 5 in particular, in some embodiments method 500 may begin at start (502), and may then proceed to the defrost operation (530) as described above, e.g., via (510) and (520) or (522). As illustrated in FIG. 5, exemplary methods may include a decision operation, e.g., checking whether the determined termination criterion has been satisfied, as indicated at (540) in FIG. 5, after initiating the defrost operation at (530). When the outcome of check (540) is negative, e.g., the determined termination criterion has not been satisfied, method 500 may return to (530) and continue the defrost operation (e.g., defrost mode and/or keep defrost heater activated). When the outcome of check (540) is positive, e.g., the determined termination criterion has been satisfied, method 500 may proceed to (550), whereupon the defrost operation is exited and the method 500 ends.

In some embodiments, the ambient condition, e.g., which is measured and compared to the predetermined limit and on the basis of which the termination criteria is determined, may include temperature and/or humidity. For example, in some embodiments, the ambient condition may be an outdoor temperature. In such embodiments, exemplary methods may further include measuring a humidity of the outdoor environment around the outdoor unit, and the termination criterion may also be determined based on the measured humidity, such as both the temperature and the humidity may be compared to predetermined limits to determine which termination criterion to set.

For example, in some embodiments, determining the termination criterion (420) may include comparing the measured ambient condition to a threshold (510). Such embodiments may further include selecting a first termination criterion (520) when the measured ambient condition is greater than the threshold and selecting a second termination criterion (522) when the measured ambient condition is less than or equal to the threshold. For example, the second termination criterion may be greater than the first termination criterion, such as where the ambient condition is temperature (or one of the measured ambient conditions is temperature), the second termination criterion may be larger when the outdoor ambient temperature is lower (colder). Thus, the outdoor coil may be heated to a higher temperature during defrost operation, heated for a longer time during defrost operation, and/or a greater amount of thermal energy may be applied to the outdoor coil during the defrost operation when the outdoor ambient temperature is lower (colder) by activating the defrost heater until the greater second termination criterion is satisfied.

For example, when the outdoor temperature is relatively high (such as at least about 40° F), the condensate and/or meltwater, e.g., from defrosting the outdoor coil, may readily drain from the outdoor unit, whereas lower outdoor temperatures (such as approaching and/or below the freezing point of water) may present an increased risk of the condensate and/or meltwater freezing in the outdoor unit, such as in the basepan thereof and/or in a drain line extending from the outdoor unit. Accordingly, applying higher limits to the defrost operation, e.g., higher termination criteria, when the temperature is lower may reduce or prevent such freezing without an additional heater in the basepan of the outdoor unit and, in some embodiments, without a defrost heater in the outdoor unit at all.

In some embodiments, more than one predetermined limit or threshold may be applied. Thus, in exemplary embodiments where determining the termination criterion (420) includes comparing the measured ambient condition to a threshold (510), the threshold may be a first threshold and the method may further include comparing the measured ambient condition to a second threshold as well as the first threshold. For example, the first threshold may be greater than the second threshold. Such embodiments may also include selecting a first termination criterion when the measured ambient condition is greater than the first threshold, selecting a second termination criterion when the measured ambient condition is less than or equal to the first threshold and greater than the second threshold, and selecting a third termination criterion when the measured ambient condition is less than or equal to the second threshold. In such embodiments, the third termination criterion may be greater than the second termination criterion and the second termination criterion may be greater than the first termination criterion. Thus, for example, as the outdoor temperature gets colder, progressively higher exit conditions (termination criteria) may be applied to the defrost operation of the outdoor coil. Further, additional thresholds beyond three with a corresponding number of additional termination criteria may be applied as well in various embodiments.

In additional embodiments, determining the termination criterion for a defrost operation based on the measured ambient condition, e.g., as indicated at (420) in FIG. 4, may include calculating the termination criterion based on the measured ambient condition (or conditions, such as both ambient temperature and humidity). Thus, the termination criterion may be a function of the outdoor ambient condition such that the termination criterion varies with the ambient condition more continuously and the defrost operation is thereby more responsive to changes in the ambient condition(s). For example, determining the termination criterion based on the measured ambient condition may include calculating the termination criterion by inputting the measured ambient condition into a linear function. As another example, determining the termination criterion based on the measured ambient condition may include calculating the termination criterion by inputting the measured ambient condition into a non-linear function.

As mentioned above, the termination criterion for the defrost operation may relate to the temperature to which the outdoor coil is heated during the defrost operation, how much time the outdoor coil is heated during the defrost operation, and/or what amount of thermal energy is applied to the outdoor coil during the defrost operation. Thus, in some exemplary embodiments, the termination criterion may be or may include a temperature of the outdoor heat exchanger. In additional embodiments, the termination criterion may also or instead include a thermal accumulation limit for the defrost operation. The thermal accumulation term may generally correspond to the outdoor coil being heated to at least a certain level for at least a minimum time during the defrost operation, such as above 32° F for a certain amount of time and/or by a certain amount. Thus, the thermal accumulation limit may reduce or prevent terminating the defrost operation early if the temperature of the outdoor coil briefly spikes. For example, the thermal accumulation limit may be an integral term, such as may represent an area under the curve of a time-temperature plot for the outdoor coil during the defrost operation (or at least a portion thereof, such as the portion that is above the freezing point of water). The thermal accumulation limit generally represents accumulated thermal energy supplied to the outdoor coil by the defrost heater during the defrost operation, such as may be quantified in terms of time and temperature, such as in degree-minutes. In various embodiments, the defrost operation may be terminated when one or both of the temperature limit and the thermal accumulation limit are satisfied, such as exiting the defrost operation at (440) and/or (550) may include activating the defrost heater until the determined termination temperature and the thermal accumulation limit are both satisfied.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.