US 20030116637 A1
A heat pump system is provided having an exterior coil, an exterior fan and fan motor, an interior blower, a reversing valve, and an auxiliary heater element. A method for defrosting the exterior coil includes the steps of initiating a defrost cycle by activating the auxiliary heater element for a period of time, and thereafter deactivating the exterior fan and fan motor and shifting the reversing valve to a cooling mode. After the defrost cycle has ended, the reversing valve is shifted to a heating mode, the exterior fan and fan motor are activated, and the auxiliary heater element is deactivated. The heat pump system results in less “cold blow” during the defrost cycle of a typical heat pump system, by providing additional heat to the interior space and interior duct work before the system is operated in a cooling mode to defrost the exterior coil.
1. For a heat pump system having an exterior coil, an exterior fan and fan motor, an interior blower, a reversing valve, and an auxiliary heater element, a method for defrosting the exterior coil comprising the steps of:
a. first activating the auxiliary heater element for a period of time; and
b. after the period of time has passed, deactivating the exterior fan and fan motor and shifting the reversing valve to a cooling mode.
2. The method of
3. The method of
4. For a heat pump system having an exterior coil, an exterior fan and fan motor, an interior blower, a reversing valve, and an auxiliary heater element, a method for defrosting the exterior coil comprising the steps:
a. initiating a defrost cycle;
b. activating the auxiliary heating element for a period of time during the period of time the auxiliary heater element is activated, operating the interior blow to pass air over the auxiliary heater element;
c. deactivating the exterior fan motor and shifting the reversing valve to end the defrost cycle; and
d. shifting the reversing valve, activating the exterior fan and fan motor, and deactivating the auxiliary heater element:
5. The method of
6. The method of
7. A heat pump system comprising:
at least one exterior coil;
an exterior fan and fan motor;
an interior blower;
a reversing valve;
an auxiliary heater unit; and
a control unit associated with the exterior fan and fan motor, reversing valve, and auxiliary heating unit, the control unit configured to defrost the at least one coil by activating the auxiliary heating unit for a period of time prior to shifting the reversing valve and prior to deactivating the fan and fan motor.
8. The heat pump system of
9. The heat pump system of
 This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/334,731, filed Nov. 15, 2001, and entitled “Heat Pump Defrost Control.”
 1. Field of the Invention
 The present invention relates generally to heating, ventilation, and air conditioning control. More specifically, the present invention relates to the control of the defrost cycle of a heat pump.
 2. Description of the Related Art
 In a heat pump system running in a heating mode, it is common for ice to form on the exterior coil of the system. As the system is operating in a heating mode, the exterior coil can become very cool as it attempts to transfer heat from the exterior ambient air to the refrigerant in the exterior coil. As the coil cools below the dew point of the ambient exterior air, condensation will occur on the coil. If the coil cools below freezing, or if the ambient exterior air is below the freezing point of water, the condensation will form ice on the coil. This is common in most areas where heat pumps are used.
 The formation of ice on the exterior coil reduces the effectiveness of the coil as a heat transfer unit. The exterior coil is designed to transfer heat from the ambient exterior air to the refrigerant inside the coil. To achieve this function an exterior fan draws ambient exterior air across the metallic coil. When ice forms on the coil the fan can no longer draw air across the coil and the heat transfer process is interrupted.
 Therefore, methods have been developed to defrost the exterior coil of common heat pump systems. The primary method is to switch the system into the air conditioning mode so that the heat from the interior may be used to defrost the exterior coil. The system then operates as a typical air conditioner, transferring heat from the interior to the exterior coil via a compressor and expansion valve system. The refrigerant in the exterior coil becomes very warm and removes the ice on the exterior coil, while the refrigerant in the interior coil becomes very cool. Interior air that is then passed over the cool interior coil blows out into the heated space. This is known in the industry as “cold blow.”
 “Cold blow” is typically counteracted by using auxiliary heating elements. When the heat pump system is switched to defrost the exterior coil, three events typically occur simultaneously: the exterior fan is deactivated; the reversing valve shifts from the heat to the cool mode; and the auxiliary heating element or elements are activated. The fan is deactivated to stop the cooling effect on the formed ice and to allow the ice to defrost. The reversing valve is shifted to provide hot refrigerant to the exterior coil to defrost it. The auxiliary heating elements are activated to heat the cool air that is blown over the cool interior coil and into the heated area.
 Various systems have been proposed for the control of the auxiliary heating elements to prevent “cold blow,” while also not overheating the air to create a “hot blow” effect. In one proposed system the auxiliary heat is provided by several discrete elements that are activated as needed to maintain a comfortable temperature, while in another system the discrete elements are activated based on the need in the prior defrost cycle and then adjusted as needed. Such systems require multiple discrete heating elements and a more complex control circuit than standard heat pump systems. Such systems also tend to take a few minutes to heat up causing an initial “cold blow” followed by a warm or comfortable air supply.
 It would be advantageous to prevent “cold blow” using the existing equipment in a typical heat pump system. It would be cost effective to avoid “cold blow” without requiring the use of extra heating elements or more expensive circuitry.
 In accordance with the present invention, a heat pump system is provided having an exterior coil, an exterior fan and fan motor, an interior blower, a reversing valve, and an auxiliary heater element. A method for defrosting the exterior coil may include the steps of: activating the auxiliary heater element for a period of time; and after the period of time has passed, activating the exterior fan and fan motor and shifting the reversing valve to a cooling mode.
 Another feature of the present invention is that the delay between activating the auxiliary heater element and shifting the reversing valve and deactivating the exterior fan and fan motor may either be predetermined or calculated based on variable factors. The timing of this delay and the other steps of this method are determined by a control unit associated with the various parts of the heat pump system.
 The heat pump system of the present invention is believed to result in less “cold blow” during the defrost cycle of a typical heat pump system, by providing additional heat into the air in the space to be heated and in the duct work and sufficient warm-up time for the auxiliary heater element, without significantly adding to the equipment or operational costs of the system.
FIG. 1 is a partial, perspective view of heat pump system incorporating the present invention.
FIG. 2 is partial, perspective view of an electric auxiliary heating element incorporating the present invention.
FIG. 3 is flow diagram of an embodiment of the present invention.
 While the invention will be described in connection with the preferred embodiment, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.
FIG. 1 illustrates a typical heat pump system 10 incorporating the present invention. A heat pump system 10 is typically comprised of: an interior coil 12; exterior coil 14, located outside the space to be heated or cooled; compressor 16; reversing valve 18; and expansion valves 20 and 22, all connected by suitable piping 24 as shown. Refrigerant 26 travels through the piping 24 between the coils 12 and 14, valves 18, 20 and 22, and compressor 16. Interior coil 12 and exterior coil 14 each act as either a condensing coil or evaporator coil depending upon the mode of operation. The mode of operation, either a cooling or heating mode, is switched by shifting reversing valve 18. Each expansion valve 20 or 22 operates in only one mode, such that while in the heating, mode only expansion valve 20 is operative and while in the cooling, or air conditioning, mode only expansion valve 22 is operative.
 Also illustrated in FIG. 1 are: an exterior fan 28, also typically located outside the space to be heated or cooled; exterior fan motor 30; interior blower 32; and an interior housing 46, in which interior coil 12, blower 32, and a portion of piping 24 are disposed. Exterior fan 28 draws ambient exterior air over exterior coil 14 to transfer heat efficiently between the exterior air at an ambient outdoor temperature, and the refrigerant 26 in exterior coil 14. Exterior fan motor 30 drives exterior fan 28. As is known in the art coil 14, fan 28 and motor 30 are disposed in a suitable, conventional housing (not shown) disposed outside the interior space to be heated or cooled. Interior blower 32 draws air over interior coil 12 in interior housing 46 to efficiently transfer heat between the return air 48 and the refrigerant 26 in interior coil 12. Blower 32 draws return air 48 into interior housing 46 from a return air plenum 40. After the return air 48 passes through return air plenum 40 into interior housing 46, it passes over interior coil 12 and through blower 32. The return air 48 then exits blower 32, passes through auxiliary heater housing 34, and exits outwardly as supply air 52 through a supply air plenum 54 to a climate controlled interior space (not shown).
 A detailed view of the auxiliary heater housing 34, with its side walls formed by wall portions of housing 46 not shown for drawing clarity, is shown in FIG. 2, including auxiliary heater element 36, control unit 38 and thermistor 42. Auxiliary heater element 36 is activated by control unit 38 dependent upon how control unit 38 is programmed and configured. Thermistor 42 supplies temperature information to control unit 38 via thermistor leads 40 that connect thermistor 42 to control unit 38. Power is supplied to control unit 38 through conventional power leads 44.
FIG. 3 is a flow diagram of the preferred embodiment of the invention. Once the defrost cycle 56 is initiated, as desired or necessary, to remove, or defrost, ice disposed upon exterior coil 14, the auxiliary heat cycle 58 begins, wherein auxiliary heater element 36 is activated. In the preferred embodiment described above, system control unit 38 would activate auxiliary heater element 36. A delay cycle 60 then begins, which is a period of time sufficient to allow the auxiliary heater element to warm up and supply additional heated supply air 52 into the interior space and conventional duct work, by air being blown by blower 32 through heater housing 34 and over heater element 36. Cooling cycle 62 then begins, wherein reversing valve 18 is shifted to cooling, or cooling mode, and exterior fan motor 30 and fan 28 are deactivated. The duration of delay cycle 60 may be a preset amount of time, a calculated amount of time based upon exterior temperature, or it may be calculated based upon other pertinent conditions. After running in this defrost mode, or cooling cycle 62, for a sufficient amount of time to heat up and remove the frozen condensate or ice from the exterior coil 14, the control unit 38 then ends the cooling cycle 60, thus ending the defrost cycle 64. The time required to complete the defrost cycle 64 is determined by the programming of the control unit 38. The duration of the defrost cycle 64 may be a preset amount of time, a calculated amount of time based upon exterior temperature, or it may be calculated based upon other pertinent conditions. Once the defrost cycle 64 is complete, the heating cycle 66 begins, wherein exterior fan motor 30 and fan 28 are activated, auxiliary heater element 36 is deactivated, and reversing valve 18 is shifted for the heating mode of operation. Accordingly, auxiliary heater element 36 has sufficient time to provide additional heated air to the interior space and interior duct work, prior to shifting the reversing valve 18 and deactivating the exterior fan 28 and fan motor 30 to begin a cooling cycle to defrost the exterior coil 14. This prevents “cold blow” by providing additional heated air before the defrosting of the exterior coil, so that a higher air temperature, or warmer interior space is maintained for a period of time before the cooling mode begins to defrost the exterior coil.
 It shall be noted that any type of heater element 36 could be utilized, including using a plurality of heater elements 36. Control Unit 38 could be any suitable type of control device including, among others, a small computer, circuit board, or solid state electronic controls circuitry, or any other device to provide the requisite control signals.
 It is to be understood that the invention is not limited to the exact details of the construction, operation, exact materials or embodiment shown and described, as obvious modifications and equivalents will be apparent to one skilled in the art. Accordingly, the invention is therefore to be limited only by the scope of the appended claims.