METHOD AND APPARATUS FOR REMOVING ICE FROM OUTDOOR HOUSING FOR AN ENVIRONMENTAL CONDITIONING UNIT

Abstract

A method for removing ice from an outdoor housing for an environmental conditioning unit that includes the housing surrounding a coil and a fan for moving air past the coil includes the steps of: (a) operating the fan at a first speed in a first direction to move air in a first flow past the coil while the conditioning unit is in a heating mode; (b) disengaging the fan when the conditioning unit is in a defrost mode; (c) operating the fan at a second speed in a second direction to move air in a second flow past the coil when an operational parameter attains a first value; and (d) operating the fan in the first direction at the first speed to move air in the first flow when the operational parameter attains a second value.

Description

BACKGROUND OF THE INVENTION

The present invention is directed to environmental conditioning systems, and especially to environmental conditioning systems employing heat pump technology and operated in cold environments tending to ice up outdoor units of the systems.

Environmental conditioning systems, such as heat pump systems, typically include indoor components arranged within a space to be conditioned and outdoor components situated outside the space to be conditioned. Environmental conditioning in the context of this description includes air conditioning of an interior space, such as heating and cooling of the interior space.

In cold climate areas, housing structures, such as louvered housings, surrounding an outside coil of an environmental conditioning system may become iced over. Such icing over reduces air flow available for drawing over the outside coil and reduces efficiency and effectiveness of the environmental conditioning system.

There is a need for a method and apparatus for effecting removal of ice from a housing for an environmental conditioning unit.

In particular, there is a need for a method and apparatus for effecting removal of ice from a housing for an outdoor component for a heat pump system.

SUMMARY OF THE INVENTION

A method for removing ice from an outdoor housing for an environmental conditioning unit that includes the housing surrounding a coil and a fan for moving air past the coil includes the steps of: (a) operating the fan at a first speed in a first direction to move air in a first flow past the coil while the conditioning unit is in a heating mode; (b) disengaging the fan when the conditioning unit is in a defrost mode; (c) operating the fan at a second speed in a second direction to move air in a second flow past the coil when an operational parameter attains a first value; and (d) operating the fan in the first direction at the first speed to move air in the first flow when the operational parameter attains a second value.

An apparatus for effecting ice removal from an outdoor housing for an environmental conditioning unit that includes a coil structure substantially surrounded by the housing, a reversing valve and a fan configured for moving air past the coil structure operates in a heating mode when the reversing valve unit is in a first orientation and operates in a cooling mode when the reversing valve is in a second orientation includes: (a) a control unit coupled with the fan; (b) at least one sensor unit coupled with the control unit and coupled with the coil structure for sensing at least one predetermined operating parameter associated with the fluid. The control unit cooperates with the at least one sensor unit to effect operating the fan at a first speed in a first direction to move air in a first flow past the coil structure while the environmental conditioning unit is in the heating mode. The control unit cooperates with the at least one sensor unit to effect disengaging the fan and orienting the reversing valve in the second orientation when the environmental conditioning unit substantially attains a predetermined first operating condition to place the environmental conditioning unit in a defrost mode. The control unit cooperates with the at least one sensor unit to effect operating the fan at a second speed in a second direction to move air in a second flow past the coil structure when the environmental conditioning unit is in the defrost mode and the environmental conditioning unit substantially attains a predetermined second operating condition. The control unit cooperates with the at least one sensor unit to effect operating the fan in the first direction at the first speed to move air in the first flow when the environmental conditioning unit substantially attains a predetermined third operating condition.

It is, therefore, an object of the present invention to provide a method and apparatus for effecting removal of ice from a housing for an environmental conditioning unit.

It is a further object of the present invention to provide a method and apparatus for effecting removal of ice from a housing for an outdoor component for a heat pump system.

Further objects and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings, in which like elements are labeled using like reference numerals in the various figures, illustrating the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a heat pump environmental conditioning system.

FIG. 2 is a schematic diagram of air flow about an outdoor coil unit for a heat pump environmental conditioning system operating in a heating mode.

FIG. 3 is a schematic diagram of air flow about an outdoor coil unit for a heat pump environmental conditioning system operating in an ice removing mode according to the present invention.

FIG. 4 is a graphic representation of operation of a heat pump environmental conditioning system employing the present invention for ice removal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a schematic diagram of a heat pump environmental conditioning system. In FIG. 1, a heat pump system 10 is installed for conditioning an interior or inside space 12. A compressor 14 is situated in an exterior or outside space 16 outside a wall 18. An interior or inside coil 20 is situated in interior space 12. Interior coil 20 is in fluid communication with an exterior or outside coil 22 and with compressor 14 in a fluid network involving a reversing valve 24, an expansion valve 21 and an expansion valve 23. Exterior coil 22 and reversing valve 24 are situated in exterior space 16. A blower unit 26 urges air across interior coil 20 in a direction indicated by an arrow 28. A blower unit 30 urges air across exterior coil 22 in a direction indicated by an arrow 32. Exterior coil 22 may be substantially enclosed within a housing 50. Housing 50 may include louvers 52 or other access structures for permitting air flow from outside space 16 about exterior coil 22 in response to blower unit 30.

Heat pump system 10 also includes a thermostat unit 34 and a defrost unit 36 coupled with a control unit 38. Control unit 38 is also coupled with compressor 14 and with reversing valve 24. Some heat pump systems embody defrost unit 36 and control unit 38 in a single circuit board unit.

Reversing valve 24 has a common input port 40, a common output port 42 and bidirectional ports 44, 46. A directing element 48 is situated inside reversing valve 24. Directing element 48 may be situated in a first position spanning common output port 42 and bidirectional port 44 (indicated by a solid line) or in a second position spanning common output port 42 and bidirectional port 46 (indicated by a dotted line). Details regarding how directing element 48 is moved are not illustrated in FIG. 1.

Heat pump system 10 may also include thermostats T₁, T₂. Thermostats T₁, T₂ may be situated in any of several loci within heat pump system 10. Preferably thermostats T₁, T₂ are situated between expansion valve 21 and exterior coil 22. Most preferably thermostats T₁, T₂ are situated between expansion valve 21 and exterior coil 22, and substantially nearer to expansion valve 21 than exterior coil 22. Thermostats T₁, T₂ participate in employing the present invention for controlling a defrost cycle for heat pump system 10, as will be described in greater detail hereinafter in connection with FIG. 4.

During cooling operations when heat pump system 10 operates to cool interior space 12, directing element 48 is in its right-hand (dotted line) position in FIG. 1. In this configuration, refrigerant is exhausted from compressor exhaust 13 and enters reversing valve 24 at common input port 40. Because directing element 48 is in its right-hand position bidirectional port 44 is left unmasked and refrigerant exits reversing valve 24 via port 44. Refrigerant exits via port 44 and enters exterior coil 22 in a compressed high-pressure, high-temperature vapor state. In the cooling operation, exterior coil 22 operates as a condenser and interior coil 20 operates as an evaporator so that as refrigerant passes from compressor exhaust 13 via reversing valve 24 and traverses exterior coil 22 the refrigerant is cooled and condenses to a liquid. Air flowing over exterior coil 22 in response to blower unit 30 removes heat from refrigerant in exterior coil 22. Refrigerant therefore arrives at expansion valve 23 in a high-pressure, high-temperature liquid state. Expansion valve 23 operates to present a low-pressure, low-temperature liquid state refrigerant to interior coil 20. Liquid refrigerant enters interior coil 20 where it is heated by air forced over interior coil 20 by blower unit 26. In this manner refrigerant in interior coil 20 picks up heat from interior space 12, thereby cooling interior space 12. Refrigerant exits interior coil 20 in a vapor state and enters reversing valve 24 via bidirectional port 46. Because directional element 48 is in a position spanning bidirectional port 46 and output port 42, refrigerant entering reversing valve 24 via bidirectional port 46 is directed to exit reversing valve 24 via output port 42. Thereafter refrigerant in its vapor state returns to compressor 14 via an intake port 15.

During heating operations when heat pump system 10 operates to heat interior space 12, directing element 48 is in its left-hand (solid line) position in FIG. 1. In this configuration, refrigerant is exhausted from compressor exhaust 13 and enters reversing valve at common input port 40. Because directing element 48 is in its left-hand position bidirectional port 46 is left unmasked and refrigerant exits reversing valve 24 via port 46. Refrigerant exits reversing valve 24 via port 46 and enters interior coil 20 in a compressed vapor state. In the heating operation, interior coil 20 operates as a condenser and exterior coil 22 operates as an evaporator so that as refrigerant traverses interior coil 20 it is cooled and condenses to a liquid. Air flowing over interior coil 20 because of blower unit 26 picks up heat given up by refrigerant in interior coil 20 as the refrigerant condenses and cools. It is this heat that warms interior space 12. Refrigerant therefore arrives at expansion valve 21 in a high-pressure, high-temperature liquid state. Expansion valve 21 operates to present a low-pressure, low-temperature liquid state refrigerant to exterior coil 22. In exterior coil 22 refrigerant is heated by air forced over exterior coil 22 by blower unit 30. In this manner refrigerant in exterior coil 22 picks up heat from exterior space 16 thereby returning refrigerant in exterior coil 22 to a vapor state. Refrigerant exits exterior coil 22 in a vapor state and enters reversing valve 24 via bidirectional port 44. Because directional element 48 is in a position spanning bidirectional port 44 and output port 42, refrigerant entering reversing valve 24 via bidirectional port 44 is directed to exit reversing valve 24 via output port 42. Thereafter refrigerant in its vapor state returns to compressor 14 via an intake port 15.

One may observe that there are characteristic loci within heat pump system 10 that exhibit temperatures related to the operation being performed by heat pump system 10. That is, when heat pump system 10 is performing a cooling operation, temperatures at certain loci in heat pump system 10 will be relatively cool and other loci in heat pump system 10 will be relatively warm. Providing a temperature sensing device such as, by way of example and not by way of limitation, a thermostat device at one or more such characteristic loci permits one to check whether heat pump system 10 is actually performing a cooling operation or a heating operation. As mentioned earlier herein, thermostats T₁, T₂ may be situated in any of several loci within heat pump system 10. Preferably thermostats T₁, T₂ are situated between expansion valve 21 and exterior coil 22. Most preferably thermostats T₁, T₂ are situated between expansion valve 21 and exterior coil 22, and substantially nearer to expansion valve 21 than exterior coil 22. Thermostats T₁, T₂ are preferably coupled with at least one of control unit 38 and defrost unit 36 to participate in employing the present invention for controlling a defrost cycle for heat pump system 10.

Most heat pump systems have an installed defrost control unit, such as defrost unit 36. Defrost units are typically configured for deciding when to cease compressor operation in response to predetermined system conditions. The system conditions that may occasion a compressor shut down vary from system to system. Defrost units also typically include a defrost timer (not shown in detail in FIG. 1) that times out after a predetermined defrost period to limit defrost operations to a duration no greater than the predetermined defrost period. The present invention enables an alternative employment of a defrost operation to effect ice removal from housing 50 during heating operations by heat pump system 10.

FIG. 2 is a schematic diagram of air flow about an outdoor coil unit for a heat pump environmental conditioning system operating in a heating mode. In FIG. 2, a housing 50 substantially surrounding an exterior coil 22 presents louvers 52 for permitting air flow from outside space 16 about exterior coil 22 in response to a blower unit 30. Blower unit 30 may be substantially contained within housing 50 and may include a blower motor 33 driving a fan blade structure 35. When blower motor 33 drives fan blade structure 35 in a first direction 37, air is moved in a first flow F₁ from outside space 16 through louvers 52 past exterior coil 22 and blower unit 30.

FIG. 3 is a schematic diagram of air flow about an outdoor coil unit for a heat pump environmental conditioning system operating in an ice removing mode according to the present invention. In FIG. 3, housing 50 substantially surrounds exterior coil 22 and presents louvers 52 for permitting air flow from outside space 16 about exterior coil 22 in response to blower unit 30. In the outdoor coil unit illustrated in FIG. 3, blower unit 30 is substantially contained within housing 50 and includes blower motor 33 driving fan blade structure 35. When blower motor 33 drives fan blade structure 35 in a second direction 39, air is moved in a first flow F₂ past blower unit 30 and exterior coil 22 through louvers 52 to outside space 16. Preferably, rotation directions 37, 39 are substantially opposite each other and flows F₁, F₂ are substantially equal and opposite each other.

Reversing direction of air flow during a defrost operation for a heat pump is disclosed in U.S. Pat. No. 5,095,711 to Marris et al. for “Method and Apparatus for Enhancement of Heat Pump Defrost”, issued Mar. 17, 1992 (hereinafter referred to as “Marris”). Marris describes a heat pump with a reversible fan motor in its outdoor coil and, after a delay period following initiation of the defrost cycle, the fan is caused to operate in a reverse direction to thereby cause the surrounding air to flow through the outdoor coil in a direction opposite to that in which it flows during the heating mode operation. During this time, according to Marris, the fan is operated at a relatively slow speed to thereby prevent the convective flow of heat upwardly, while at the same time causing little, if any, flow of ambient air downwardly into the coil.

Marris pointedly limits the flow of air in the reverse direction. As a result, no clearing of an air-flow-accommodating structure such as louvers is effected using Marris' method. The problem of icing over of air-flow-accommodating structure and consequent restriction of air flow past the outdoor coil remains a problem when using Marris' method.

FIG. 4 is a graphic representation of operation of a heat pump environmental conditioning system employing the present invention for ice removal. In FIG. 4, a graphic plot 70 illustrates temperature of refrigerant displayed as a function of time. As mentioned earlier herein, heat pump system 10 (FIG. 1) may include thermostats T₁, T₂. Thermostats T₁, T₂ may be situated in any of several loci within heat pump system 10. Preferably thermostats T₁, T₂ are situated between expansion valve 21 and exterior coil 22. Most preferably thermostats T₁, T₂ are situated between expansion valve 21 and exterior coil 22, and substantially nearer to expansion valve 21 than exterior coil 22. Thermostats T₁, T₂ participate in employing the present invention for controlling a defrost cycle for heat pump system 10.

Refrigerant temperature is illustrated in FIG. 4 as declining during a time interval t₀-t₁. At time t₁, refrigerant temperature, as measured by one of thermostats T₁, T₂ (FIG. 1) is at a level TEMP₁. Temperature TEMP₁ may be a set point for a defrost operation for heat pump system 10 so that at time t₁ defrost unit 36 may operate as a defrost control unit to start a timer for a defrost delay. By way of example and not by way of limitation, thermostat T₁ may be configured to electrically close at set point temperature TEMP₁ to effect detection of refrigerant attaining temperature TEMP₁. By way of example and not by way of limitation, TEMP₁ may be established substantially at 42° F. Defrost delays are sometimes settable by a field selectable pin on a defrost board or similar unit and may, by way of example and not by way of limitation, be preset for setting at 30, 60 or 90 minutes.

Once the selected defrost delay elapses, defrost control unit 36 may initiate a defrost mode of operation by orienting reversing valve 24 for cooling operation and disengaging outdoor fan motor 33 (FIGS. 2 and 3). This operational mode change is indicated in FIG. 4 as being effected at a time t₂. Refrigerant in heat pump system 10 warms during defrost mode operation and may equal set point temperature TEMP₁ at a time t₃. While heat pump system 10 is in the defrost mode exterior coil 22 warms up, ice that has accumulated on exterior coil 22 may melt and refrigerant temperature may increase. In a normal prior art defrost operation, thermostat T₁ may be set to electrically open at a second set point temperature TEMP₃ to terminate defrost operation by re-engaging fan motor 33 and configuring reversing valve 24 for heating mode operations by heat pump system 10. Second set point temperature TEMP₃ is attained at a time t₅ in a normal prior art defrost operation, as indicated in FIG. 4.

However, when employing the present invention a second thermostat T₂ is preferably configured to sense attainment of an interim set point temperature TEMP₂. Interim set point temperature TEMP₂ is attained at a time t₄ in FIG. 4. When interim set point temperature TEMP₂ is detected as behaving been attained, thermostat T₂ electrically closes and an enhanced defrost routine is initiated by which fan motor 33 is re-energized and run in a reverse rotational direction (e.g., direction 39; FIG. 3) at substantially the same speed at which fan motor 33 is operated during a heating mode of operation by heat pump system 10. The enhanced defrost routine may be initiated at any time after first temperature set point TEMP₁ is attained. However, delaying commencement of the enhanced defrost routine may be advantageous; after having no fan rotation during a defrost interval such as substantially from time t₂ to substantially time t₄, external coil 22 may build up heat so that relatively warm air is extant about external coil 22. The warm air accumulated or built up about external coil 22 may therefore be available for movement toward housing 50 in an enhanced defrost operation during time interval t₄-t₅ or longer. When fan rotation is reversed, refrigerant temperature may tend to warm at a slower rate, as indicated by a line segment 80 during times following time t₄ in FIG. 4. In such conditions when fan rotation is reversed, refrigerant temperature may not reach second set point temperature TEMP₃ to occasion termination of an extant defrost operation until time t₇. Defrost timer limits may terminate defrost operation at a predetermined defrost period ending at a time t₆ earlier than time t₇. Experiments by the inventors have demonstrated that warm air and increased air velocity impinging a louvered housing structure during an enhanced defrost operation as described herein significantly reduces ice accumulation on the louvered housing structure, even when defrost operations are terminated according to a defrost timer predetermined defrost period.

It is to be understood that, while the detailed drawings and specific examples given describe preferred embodiments of the invention, they are for the purpose of illustration only, that the apparatus and method of the invention are not limited to the precise details and conditions disclosed and that various changes may be made therein without departing from the spirit of the invention which is defined by the following claims:

Claims

1. A method for effecting ice removal from an outdoor housing for an environmental conditioning unit; said environmental conditioning unit including a coil structure and a fan configured for moving air past said coil structure; said housing substantially surrounding said coil; the method comprising the steps of:

(a) operating said fan at a first speed in a first direction to move air in a first flow past said coil structure while said environmental conditioning unit is in a heating operational mode;

(b) disengaging said fan for at least a portion of a time when said environmental conditioning unit is in a defrost operation mode;

(c) operating said fan at a second speed in a second direction to move air in a second flow past said coil structure when at least one selected operational parameter substantially attains a selected first predetermined value; and

(d) operating said fan in said first direction at said first speed to move air in said first flow when said at least one selected operational parameter substantially attains a selected second predetermined value.

2. A method for effecting ice removal from an outdoor housing for an environmental conditioning unit as recited in claim 1 wherein said coil flowingly contains a fluid and wherein said at least one operational parameter is temperature of said fluid as said fluid exits said coil.

3. A method for effecting ice removal from an outdoor housing for an environmental conditioning unit as recited in claim 1 wherein said first speed and said second speed are substantially equal.

4. A method for effecting ice removal from an outdoor housing for an environmental conditioning unit as recited in claim 1 wherein said at least one first predetermined value said at least one second predetermined value are selected appropriately to assure that said environmental conditioning unit is in said defrost operation mode when said at least one first predetermined value said at least one second predetermined value are attained.

5. A method for effecting ice removal from an outdoor housing for an environmental conditioning unit as recited in claim 2 wherein said first speed and said second speed are substantially equal.

6. A method for effecting ice removal from an outdoor housing for an environmental conditioning unit as recited in claim 2 wherein said at least one first predetermined value said at least one second predetermined value are selected appropriately to assure that said environmental conditioning unit is in said defrost operation mode when said at least one first predetermined value said at least one second predetermined value are attained.

7. A method for effecting ice removal from an outdoor housing for an environmental conditioning unit as recited in claim 3 wherein said at least one first predetermined value said at least one second predetermined value are selected appropriately to assure that said environmental conditioning unit is in said defrost operation mode when said at least one first predetermined value said at least one second predetermined value are attained.

8. A method for effecting ice removal from an outdoor housing for an environmental conditioning unit as recited in claim 5 wherein said at least one first predetermined value said at least one second predetermined value are selected appropriately to assure that said environmental conditioning unit is in said defrost operation mode when said at least one first predetermined value said at least one second predetermined value are attained.

9. A method for effecting ice removal from an outdoor housing for a heat pump environmental conditioning unit; said heat pump unit including a coil structure, a reversing valve and a fan configured for moving air past said coil structure; said housing substantially surrounding said coil; said heat pump unit operating in a heating mode when said reversing valve unit is in a first orientation; said heat pump unit operating in a cooling mode when said reversing valve is in a second orientation; the method comprising the steps of:

(a) operating said fan at a first speed in a first direction to move air in a first flow about said coil structure while said heat pump unit is in said heating mode;

(b) disengaging said fan and orienting said reversing valve in said second orientation when said heat pump unit substantially attains a predetermined first operating condition to place said heat pump unit in a defrost mode;

(c) operating said fan at a second speed in a second direction to move air in a second flow past said coil structure when said heat pump unit is in said defrost mode and said heat pump unit substantially attains a predetermined second operating condition; and

(d) engaging said fan and orienting said reversing valve in said first orientation when said heat pump unit substantially attains a predetermined third operating condition to place said heat pump unit in said heating mode.

10. A method for effecting ice removal from an outdoor housing for a heat pump environmental conditioning unit as recited in claim 9 wherein said coil flowingly contains a fluid between a first coil end and a second coil end, and wherein at least one of said first operating condition, said second operating condition and said third operating condition is related to temperature of said fluid substantially adjacent one of said first coil end and said second coil end.

11. A method for effecting ice removal from an outdoor housing for a heat pump environmental conditioning unit as recited in claim 9 wherein said first speed and said second speed are substantially equal.

12. A method for effecting ice removal from an outdoor housing for a heat pump environmental conditioning unit as recited in claim 1 wherein said first operating condition and said second operating condition are selected appropriately to assure that said heat pump unit is in said defrost mode when said second operating condition is attained.

13. A method for effecting ice removal from an outdoor housing for a heat pump environmental conditioning unit as recited in claim 10 wherein said first speed and said second speed are substantially equal.

14. A method for effecting ice removal from an outdoor housing for a heat pump environmental conditioning unit as recited in claim 10 wherein said first operating condition and said second operating condition are selected appropriately to assure that said heat pump unit is in said defrost mode when said second operating condition is attained.

15. A method for effecting ice removal from an outdoor housing for an environmental conditioning unit as recited in claim 11 wherein said first operating condition and said second operating condition are selected appropriately to assure that said heat pump unit is in said defrost mode when said second operating condition is attained.

16. A method for effecting ice removal from an outdoor housing for an environmental conditioning unit as recited in claim 13 wherein said first operating condition and said second operating condition are selected appropriately to assure that said heat pump unit is in said defrost mode when said second operating condition is attained.

17. An apparatus for effecting ice removal from an outdoor housing for an environmental conditioning unit; said environmental conditioning unit including a coil structure, a reversing valve and a fan configured for moving air past said coil structure; said environmental conditioning unit operating in a heating mode when said reversing valve unit is in a first orientation; said environmental conditioning unit operating in a cooling mode when said reversing valve is in a second orientation; said housing substantially surrounding said coil; the apparatus comprising:

(a) a control unit coupled with said fan;

(b) at least one sensor unit coupled with said control unit and coupled with said coil structure for sensing at least one predetermined operating parameter associated with said fluid;

(c) said control unit cooperating with said at least one sensor unit to effect operating said fan at a first speed in a first direction to move air in a first flow past said coil structure while said environmental conditioning unit is in said heating mode;

(e) said control unit cooperating with said at least one sensor unit to effect disengaging said fan and orienting said reversing valve in said second orientation when said environmental conditioning unit substantially attains a predetermined first operating condition to place said environmental conditioning unit in a defrost mode;

(f) said control unit cooperating with said at least one sensor unit to effect operating said fan at a second speed in a second direction to move air in a second flow past said coil structure when said environmental conditioning unit is in said defrost mode and said environmental conditioning unit substantially attains a predetermined second operating condition; and

(g) said control unit cooperating with said at least one sensor unit to effect operating said fan in said first direction at said first speed to move air in said first flow when said environmental conditioning unit substantially attains a predetermined third operating condition.

18. An apparatus for effecting ice removal from an outdoor housing for an environmental conditioning unit as recited in claim 17 wherein said coil flowingly contains a fluid between a first coil end and a second coil end, and wherein at least one of said first operating condition, said second operating condition and said third operating condition is related to temperature of said fluid substantially adjacent one of said first coil end and said second coil end.

19. An apparatus for effecting ice removal from an outdoor housing for an environmental conditioning unit as recited in claim 17 wherein said first speed and said second speed are substantially equal.

20. An apparatus for effecting ice removal from an outdoor housing for an environmental conditioning unit as recited in claim 19 wherein said first operating condition and said second operating condition are selected appropriately to assure that said heat pump unit is in said defrost mode when said second operating condition is attained.