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A microprocessor based control system for controlling frost accumulation on the outdoor evaporator coil of a heat pumping system during normal heat mode operations. A demand defrost concept is implemented to increase efficiency and maximize time intervals between defrost cycles by initiating the def

A microprocessor based control system for controlling frost accumulation on the outdoor evaporator coil of a heat pumping system during normal heat mode operations. A demand defrost concept is implemented to increase efficiency and maximize time intervals between defrost cycles by initiating the defrost cycle only when an optimal maximum amount of frost has accumulated on the coil. The control system also facilitates monitoring of the rate of frost accumulation to make the outdoor coil conditions less favorable to frost buildup.

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1. A control system for controlling the accumulation of frost on the outdoor coil of a heat pumping system, comprising: first sensor means thermally decoupled from said heat pumping system for measuring a first parameter indicative of the temperature of the outdoor ambient air; second sensor mea

1. A control system for controlling the accumulation of frost on the outdoor coil of a heat pumping system, comprising: first sensor means thermally decoupled from said heat pumping system for measuring a first parameter indicative of the temperature of the outdoor ambient air; second sensor means thermally coupled to said heat pumping system for measuring a second parameter indicative of the temperature of said outdoor coil; processor means electrically coupled to said first and second sensor means for controlling the initiation of a defrost cycle wherein hot gas is pumped through said outdoor coil to melt said frost when the difference between said first and second parameters exceeds a predefined amount; and wherein said heat pump system includes an electrically controllable expansion valve for metering the flow of refrigerant into said evaporator coil and said control system further comprises means for continually monitoring said difference between said first and second parameters, said processor means being electrically coupled to said monitoring means and said expansion valve for controlling said valve in response to a change in said monitored difference. 2. The control system of claim 1 wherein said predefined amount is based on outdoor air temperature. 3. The control system of claim 1 wherein said processor means controls initiation of said defrost cycle based at least in part upon testing whether said outdoor coil temperature falls below a permitted level. 4. The control system of claim 3 wherein said permitted level is 32° F. 5. The control system of claim 1 wherein said valve is controlled based upon the rate of change in said monitored difference. 6. The control system of claim 1 wherein said expansion valve is controlled to open to an orifice size larger than that used in a normal cooling mode of the heat pump system when the defrost cycle is initiated. 7. The control system of claim 1 wherein said processor means is further electrically coupled to a fan for delivering air across said outdoor coil, said fan being turned off upon the initiation of said defrost cycle and controlled so as to be turned back on prior to terminating said defrost cycle. 8. A method of operating a heat pump system to control frost that accumulates on the outdoor evaporator coil during normal heat mode operations comprising the steps of: determining a maximum allowable amount of frost accumulation; measuring a first parameter indicative of a current amount of frost accumulation; using said first parameter to control the rate of refrigerant flow through the outdoor evaporator coil to raise the coil temperature and decrease frost accumulation; further decreasing frost accumulation by initiating a defrost cycle wherein hot gas is pumped into said evaporator coil to melt said frost when said current amount of frost is substantially equal to said maximum allowable amount; terminating said defrost cycle; and returning said heat pump system to the normal heat mode operations. 9. The method of claim 8 further comprising the steps of sensing the outdoor ambient air temperature and sensing the temperature of the outdoor evaporator coil. 10. The method of claim 9 wherein said maximum allowable amount of frost accumulation is determined as a function of the outdoor ambient air temperature and the outdoor evaporator coil temperature. 11. The method of claim 10 wherein said maximum allowable amount of frost accumulation is determined as a function of the difference between the outdoor ambient air temperature and the temperature of the outdoor coil. 12. The method of claim 9 wherein said current amount of frost accumulation is determined as a function of the outdoor ambient air temperature and the outdoor evaporator coil temperature. 13. The method of claim 9 wherein said current amount of frost accumulation is determined as a function of the difference between the outdoor ambient air temperature and the outdoor evaporator coil temperature. 14. The method of claim 11 wherein the defrost cycle is initiated when said difference between said outdoor ambient air temperature and said outdoor coil temperature exceed a predefined amount and said temperature of said outdoor evaporator coil falls below a predetermined level. 15. The method of claim 14 wherein said predetermined evaporator coil temperature level is about 32° F. 16. The method of claim 14 wherein said predetermined difference between said outdoor ambient air and evaporator coil temperature is based on outdoor air temperature. 17. The method of claim 8 wherein said defrost cycle is initiated only when a predetermined amount of time has elapsed since the previous defrost cycle. 18. The method of claim 8 further comprising the step of determining a defrost cycle termination temperature of the outdoor evaporator coil. 19. The method of claim 18 wherein said defrost termination temperature is about 65° F. 20. The method of claim 8 wherein said heat pumping system includes a fan for delivering air across said outdoor coil and said method further comprises the step of turning on the outdoor fan prior to terminating said defrost cycle and returning to the normal heat mode. 21. A method of operating a heat pump system to control frost that accumulates on the outdoor evaporator coil during normal heat mode operations comprising the steps of: detecting the rate of accumulation of frost on said coil; monitoring the rate of increase of said accumulation; causing an increased rate of refrigerant flow through the outdoor coil to raise the coil temperature and decrease frost accumulation. 22. The method of claim 21 wherein said accumulation rate is detected by monitoring the temperature difference between the outdoor air and the outdoor evaporator coil. 23. The method of claim 21 wherein said heat pump is of the type having an expansion valve and said refrigerant flow is increased by modulating said expansion valve. 24. The method of claim 23 wherein said expansion valve is modulated to create a opening larger than that used in the normal cooling mode of the system. 25. An air-conditioning system having improved defrost performance, comprising: a heat pumping fluid circuit having an outdoor heat exchanger, an indoor heat exchanger and a compressor; an electrically adjustable expansion valve disposed in said heat pumping fluid circuit; first sensor means for sensing a value indicative of outdoor air temperature; second sensor means for sensing a value indicative of outdoor heat exchanger temperature; and a processor system coupled to said first and second sensor means and to said expansion valve for adjusting the setting of said expansion valve; said processor system including frost control strategy determining means including means for modulating the setting of said expansion valve based on difference between outdoor air temperature and outdoor heat exchanger temperature. 26. The air-conditioning system of claim 25 wherein said frost control strategy determining means modulates the setting of said expansion valve based on the time rate of change in the difference between outdoor air temperature and outdoor heat exchanger temperature. 27. The air-conditioning system of claim 25 wherein said frost control strategy determining means modulates the setting of said expansion valve to increase flow through said expansion valve in response to accumulation of frost on said outdoor heat exchanger, thereby raising the outdoor heat exchanger temperature making conditions less favorable to frost buildup. 28. An air-conditioning system having improved defrost performance, comprising; a heat pump fluid circuit having an outdoor heat exchanger, an indoor heat exchanger and a compressor; an electrically adjustable expansion valve disposed in said heat pumping fluid circuit; means for sensing the rate of frost buildup on the outdoor heat exchanger; and a processor system coupled to said sensing means and to said expansion valve for adjusting the setting of said expansion valve; said processor system including frost control strategy determining means including means for modulating the setting of said expansion valve to increase flow through said heat pumping fluid circuit in response to the rate of frost buildup. 29. An air-conditioning system having improved defrost performance, comprising: a heat pumping fluid circuit having an outdoor heat exchanger, an indoor heat exchanger and a compressor operable to provide a normal heat exchange cycle and a defrost cycle in which flow through the fluid circuit is reversed to melt accumulated frost; an electrically adjustable expansion valve disposed in said heat pumping fluid circuit; processor system coupled to said expansion valve for adjusting the setting of said expansion valve; said processor system including defrost strategy determining means (a) for causing said expansion valve to substantially fully open during at least a portion of said defrost cycle and (b) for computing a rapid recovery expansion valve setting used at termination of said defrost cycle, said rapid recovery expansion valve setting being computed to cause said heat pumping fluid circuit to reach steady state operation at a rate more rapid than the rate achieved using the valve setting used during the normal heat exchange cycle. 30. A control system for controlling the accumulation of frost on the outdoor coil of a heat pumping system, comprising: first sensor means thermally decoupled from said heat pumping system for measuring a first parameter indicative of the temperature of the outdoor ambient air; second sensor means thermally coupled to said heat pumping system for measuring a second parameter indicative of the temperature of said outdoor coil; processor means electrically coupled to said first and second sensor means for controlling the initiation of a defrost cycle wherein hot gas is pumped through said outdoor coil to melt said frost when the difference between said first and second parameters exceeds a predefined amount; and wherein said heat pump system includes an electrically controllable expansion valve for metering the flow of refrigerant into said evaporator coil and said control system further comprises means for monitoring said difference between said first and second parameters, said processor means being electrically coupled to said monitoring means and said expansion valve for controlling said valve in response to a change in said monitored difference. 31. The control system of claim 30 wherein said predefined amount is based on outdoor air temperature. 32. The control system of claim 30 wherein said processor means controls initiation of said defrost cycle based at least in part upon testing whether said outdoor coil temperature falls below a permitted level. 33. The control system of claim 32 wherein said permitted level is 32° F. 34. The control system of claim 30 wherein said valve is controlled based upon the rate of change in said monitored difference. 35. The control system of claim 30 wherein said expansion valve is controlled to open to an orifice size larger than that used in a normal cooling mode of the heat pump system when the defrost cycle is initiated. 36. The control system of claim 30 wherein said processor means is further electrically coupled to a fan for delivering air across said outdoor coil, said fan being turned off upon the initiation of said defrost cycle and controlled so as to be turned back on prior to terminating said defrost cycle.