Abstract:
An improvement in defrosting an air-to-air heat pump system when in the heating mode. A bypass loop transfers refrigerant that is at a higher temperature and pressure than refrigerant normally cycled through the outdoor unit and transfers it to the outdoor coil. This higher temperature refrigerant can then defrost the outdoor coil and any ice that has been formed on the outdoor coil by heating it as the refrigerant passes through it. The bypass loop includes a valve that is capable of being controlled remotely, the valve being movable from a closed position to an open position. A sensor is positioned to monitor a preselected condition indicative of performance of the outdoor unit. The performance of the outdoor unit is an effective way of determining whether icing is inhibiting its operation. A controller is in communication with both the valve and the sensor. Once the controller determines that a preselected set point of a preselected condition indicative of deteriorating performance has been reached and while the compressor is still operating, based on signals received from the sensor, the controller sends a signal to open the valve to allow warm refrigerant to bypass expansion valves and flow directly to the outdoor unit, where it can defrost or assist in defrosting the outdoor unit. Once the controller determines that defrosting has been accomplished, again based on a second predetermined condition having been achieved as determined by the controller, the valve can be moved into a closed position and the normal operation of the air-to-air heat pump unit can be resumed.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is directed to a defrost mechanism for air-to-air heat pump systems operating in the heating mode for defrosting the outdoor coil of the outdoor unit based on predetermined conditions of the outdoor coil, thereby reducing the need for de-icing electric heating elements or decreasing the amount of time required for defrosting the outdoor coil, or both.  
         BACKGROUND OF THE INVENTION  
         [0002]    Air-to-air heat pump systems are heat moving devices used in residential and commercial applications. Heat is absorbed in an evaporator in a first location and released in a condenser in a second location. The systems are designed so that operations can be reversed so that an area can be either cooled or heated. Thus, on reversal of the heat flow direction, the evaporator at the first location becomes a condenser; and the condenser at the second location becomes an evaporator.  
           [0003]    During the heating cycle, the outdoor unit acts as an evaporator and the indoor unit acts as a condenser. Moisture from the outdoor air will condense on the outdoor coil. As the ambient temperature decreases below about 45° F., the outdoor coil temperature will rapidly approach 32° F. or lower, causing the condensed moisture to turn to ice. The ice restricts the airflow across the coil, which in turn affects the ability of the evaporator to efficiently perform its function of absorbing heat from the ambient air as the refrigerant fluid undergoes a phase change when at least a portion of the refrigerant fluid is converted from a liquid state into a gaseous state. The formation of the ice thus reduces the performance or efficiency of the heat pump system. In order to restore performance, the system will enter an evaporator defrosting cycle. The defrosting cycle on some heat pumps begins with a timed period of supplemental electric heat applied to the frosted or iced coil by de-icing electric heating elements. Also in common use today are defrost controls. These are based upon temperature differentials, pressure differentials or a combined time/temperature differential. These units reverse the operation of the heat pump so that the flow of hot refrigerant is reversed, flowing in the opposite direction than required for heating, that is, flowing directly from the compressor to the outdoor unit in order to heat the outdoor unit. There are many variations of how this is accomplished. One such device is described in Trask, U.S. Pat. No. 4,843,838 issued Jul. 4, 1989. However, while the unit is in such a defrost cycle, it is not providing heat as the refrigerant flow is in the direction for cooling. If there is still a heat demand required in the space being heated, the heat demand typically is satisfied with supplemental electric resistance heat, which is expensive in comparison to the cost of running a heat pump.  
           [0004]    Different bypass methods and apparatus for defrosting or de-icing have been taught. McCarty, U.S. Pat. No. 4,158,950 issued Jun. 26, 1979, discloses a bypass arrangement in which defrosting is accomplished by refrigerant after the compressor has stopped operation and any pressure differential within the system is equalized. Thus, operation of the heat pump system cannot be accomplished during the de-ice cycle and auxiliary heat solely must be relied upon to heat any designated areas during the de-icing operation.  
           [0005]    In Chrostowski et al., U.S. Pat. No. 4,389,851 issued Jun. 28, 1983, a combination of reverse and nonreverse defrost is utilized to de-ice the heat exchanger. During de-icing, a three way valve directs gas from the compressor to an outdoor coil. The only heat exchange path during the defrost mode is from the compressor to the outdoor unit. A valve closes to prevent the flow of refrigerant between the indoor unit and the outdoor unit. This valve and a reversing valve isolate the indoor unit from the outdoor unit as refrigerant from the compressor defrosts the outdoor coil.  
           [0006]    Bonne, U.S. Pat. No. 4,441,335, issued Apr. 10, 1984, is similar to Chrostowski et al. in that the bypass arrangement moves discharge refrigerant from the compressor directly to the outdoor coil. In addition to utilizing a plurality of three way valves to direct the flow of the refrigerant, Bonne provides no circuit between the indoor unit and the outdoor unit in which the refrigerant is not first required to pass through an expansion valve, thereby lowering its pressure.  
           [0007]    Sato et al., U.S. Pat. No. 4,519,214 issued May 28, 1985, utilizes a branch circuit for the defrost cycle that passes hot compressor refrigerant through the outdoor unit to de-ice the outdoor coil. However, to accomplish this task, the cycle is first reversed, thereby causing the air-to air heat pump to be placed into the cooling mode and converting the outdoor unit into a condenser. The refrigerant fluid passes through the outdoor coil/condenser and back to the compressor until defrost is accomplished.  
           [0008]    Aoki et al,, U.S. Pat. No. 4,760,709 issued Aug. 2, 1988, utilizes a five-way valve to direct a portion of hot refrigerant gas from the compressor to the outdoor unit to accomplish defrost of the outdoor unit, while continuing a flow of the remaining refrigerant from the compressor to the indoor unit so that the heat pump can continue to provide heat during the defrost cycle. After the refrigerant leaves the indoor unit, it passes to the outdoor unit/evaporator through an expansion valve in the usual manner. There is no other connection or branch between the indoor and outdoor unit.  
           [0009]    An arrangement of utilizing refrigerant leaving the indoor unit and indoor coil for a defrost/de-ice cycle would be effective in making use of relatively high pressure refrigerant having a temperature significantly higher than that of the outdoor ambient temperature or the outdoor coil. Such an arrangement would not seriously impact the heating functions of the air-to-air heat pump and would eliminate the need to reverse the operation of the heat pump. It would also eliminate or reduce the need to rely on supplemental auxiliary heat during the defrost cycle. A simple arrangement that utilizes minimal and readily available equipment is desirable to keep manufacturing costs low. Furthermore, a unit having predetermined set points that can be changed simply by a user is also desirable to increase the flexibility of the system as a result of the environment in which it is installed.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention is directed to an improvement in defrosting an air-to-air heat pump system when in the heating mode. The present invention utilizes a bypass loop that takes refrigerant that is at a higher temperature and pressure than refrigerant normally cycled through the outdoor unit and transfers the refrigerant to the outdoor coil. This higher temperature refrigerant can then defrost any ice that has been formed on the outdoor coil by heating the outdoor coil. The bypass loop includes a valve that is capable of being controlled remotely, the valve being movable from a closed position to an open position. A sensor is positioned to monitor a preselected condition indicative of performance of the outdoor unit. The performance of the outdoor unit is an effective way of determining whether icing or frosting is inhibiting its operation. A controller is in communication with both the valve and the sensor. Once the controller determines that a preselected set point of a first preselected condition has been reached and while the compressor is still operating, based on signals received from the sensor, the controller sends a signal to open the valve to allow warm refrigerant to bypass expansion valves and flow directly to the outdoor unit, where it can defrost or assist in defrosting the outdoor unit. Once the controller determines that defrosting has been accomplished, again based on a second predetermined condition having been achieved as determined by the controller, the valve can be moved into a closed position and the normal operation of the air-to-air heat pump unit can be resumed.  
           [0011]    An advantage of the present invention is that the de-icing electric heating elements and the cost associated with its operation may be eliminated.  
           [0012]    A further advantage of the present invention is that the heat pump system can remain in the heating mode during the defrost/de-ice operation, so that the indoor unit continues to operate as a condenser and the outdoor unit continues to operate as an evaporator. It is not necessary to reverse the cycle of the heat pump to place it into the cooling mode to accomplish defrost/de-ice.  
           [0013]    Another advantage of the present invention is that, when used in conjunction with conventional defrosting methods, the defrost cycle can be significantly shortened, thereby reducing the cost of operation of the defrost cycle. An associated advantage is that heat pump heating operations will be restored more rapidly, thereby reducing the amount of time that the heat pump system must utilize supplemental electric heat, further reducing costs and increasing the Heating Seasonal Performance Factor (HSPF) of the heat pump system.  
           [0014]    Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a schematic of a prior art air-to-air heat pump system.  
         [0016]    [0016]FIG. 2 is a schematic of a general embodiment of the air-to-air heat pump system of the present invention having a defrost bypass loop with a sensor associated with the outdoor unit.  
         [0017]    [0017]FIG. 3 is a schematic of a first embodiment of the heat pump system of the present invention with a temperature sensor attached to the outdoor coil.  
         [0018]    [0018]FIG. 4 is a schematic of a second embodiment of the heat pump system of the present invention with a temperature sensor monitoring ambient temperature within the outdoor coil.  
         [0019]    [0019]FIG. 5 is a schematic of a third embodiment of the heat pump system of the present invention with a sensor monitoring a preselected condition of the refrigerant fluid after the fluid has entered the evaporator unit.  
         [0020]    [0020]FIG. 6 is a schematic of a fourth embodiment of the heat pump system of the present invention with a defrost bypass loop that receives its refrigerant fluid from the compressor prior to the refrigerant passing into the condenser.  
         [0021]    [0021]FIG. 7 is a schematic of a fifth embodiment of the heat pump system of the present invention having two defrost bypass loops, each loop receiving refrigerant fluid at different temperatures to increase the defrost capability of the system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    A typical prior art air-to air heat pump system  102  is shown in FIG. 1. A compressor  104  compresses refrigerant fluid and transmits the refrigerant as high pressure vapor via line  120  to a reversing valve  118 . The reversing valve  118  allows the heat pump system  102  to switch between heating and cooling mode by reversing the flow of the refrigerant through the system. For the purposes of this invention, heat pump system  102  is in the heating mode. However, the defrost scheme of the present invention is effective whether the heat pump system is in a heating or cooling mode. When in the heating mode, refrigerant flows along line  122  through indoor coil  112  of indoor unit or condenser  110  where it loses heat as it changes phase to high pressure liquid. The heat is distributed through the area to be heated by an air distribution system. The high pressure liquid flows from the condenser  110  through line  130  and into at least one expansion means  126  where it undergoes a pressure loss. FIG. 1 shows a second expansion means  126  which is utilized by the system  102  during the cooling cycle. For simplicity, these are shown in the same line, but they may be in separate lines. Alternatively, one of the expansion means  126  may be disconnected from the loop during the cycle that it is not required. The expansion means  126  is typically a device such as a valve that is located between indoor unit  110  and the outdoor unit  114 . For heating cycles, high pressure liquid refrigerant leaving condenser  110  passes through the expansion device where it is converted into a low pressure liquid at a lower temperature. The low pressure liquid is transported along line  132  to manifold  134  the evaporator  114 , passing through the outdoor coil  116  (which may be a plurality of finned tubes as is known in the industry) where at least a portion of the low pressure liquid undergoes a phase change from the low pressure liquid state to a gaseous state. The low pressure gas is transported along line  124  through expansion valve  118  to accumulator  106  where liquid refrigerant accumulates while gaseous refrigerant passes along line  108  to the compressor. The normal heating function of the heat pump typically cease during the de-icing cycle, and auxiliary heat is provided to the areas requiring heat while the de-icing is completed.  
         [0023]    When ice forms on the coils of the outdoor unit as humidity condenses on them at low temperatures, typically below about 45° F., the ability of the outdoor unit to properly operate by allowing evaporation of the low pressure liquid is inhibited. The present invention is an alternative method for defrosting the evaporator. The present invention defrosts the coils on the outdoor unit either without using conventional defrost methods thereby reducing the cost associated with such methods, or by working in conjunction with such elements thereby reducing the time and the expense associated with defrosting. Instead, the present invention utilizes a bypass defrost loop  240  as shown in FIG. 2. This loop  240  is connected to the refrigerant line, shown in FIG. 2 at  230 , to draw high pressure refrigerant to the outdoor coil before it reaches expansion device  226 . The loop  240  is controlled by a valve  250  which in turn is connected to a controller  270  that controls the operation of valve  250 . Controller  270  in turn is connected to a sensor  260  that is located to sense a preselected condition of outdoor coil  216  or refrigerant in outdoor coil  216  or as it leaves outdoor coil  216  in supply line  224 .  
         [0024]    Sensor  260  can be located in a variety of positions to sense any one of several conditions in outdoor unit  214  that are associated with its performance. Sensor  260  can be, for example, a temperature sensor or a pressure sensor. If it is a temperature sensor, it can readily be located on outdoor coil  216  to determine for example, when a temperature of about 32° F. is reached. If this temperature is reached, it is indicative of the formation of ice on outdoor coil  216 . The temperature sensor can also be located within the outdoor unit  214 , but not specifically on the coil, to sense, for example, the ambient temperature within the environment of outdoor unit  214 . The temperature sensor can also be located outside outdoor unit  214  to measure the ambient atmospheric air temperature. The sensor can also be located in return line  224  between outdoor coil  216  and compressor  204  or associated accumulator  206  to monitor a preselected condition of the refrigerant fluid indicative of performance leaving outdoor unit  214 . If sensor  260  is a pressure sensor, it can be located in return line  224  between outdoor coil  216  and compressor  204  or associated accumulator  206  to monitor the gas pressure of the refrigerant leaving outdoor coil  216 .  
         [0025]    The controller  270  controls the operation of bypass defrost loop  240  by controlling operation of valve  250  in the bypass defrost loop  240 . When the heat pump system  202  is operating normally, supplying heat to the areas to be heated, valve  250  is in the closed position, causing refrigerant to flow through the expansion device  226  to be converted from a high pressure liquid to a low pressure liquid, and then be moved to outdoor unit  214  which is acting as an evaporator. However, controller  270 , which is receiving and monitoring signals from the sensor  260  indicative of a condition that is associated with the performance of outdoor coil  214 , will open valve  250  once a signal from sensor  260  indicates that a first predetermined set point has been reached. This set point can be preprogrammed into controller  270 , but may be changed by a user if desired. There are several different ways that controller  270  can operate to defrost outdoor coil  216 . If desired, all of these modes can be preprogrammed into controller  270  and can be selected by the user, as will be discussed. The controller  270 , however, must be capable of performing at least one of these modes.  
         [0026]    Regardless of which mode is chosen, the basic operation of the loop is the same. Once valve  250  is opened, a portion of high temperature, high pressure liquid refrigerant flows through defrost bypass loop  240 , bypassing the expansion device  226 , and then through the coils  216  of outdoor unit  214 . The liquid refrigerant passing through the defrost bypass loop  240 , being of higher temperature, depending upon the configuration, from 70° F. to as high as 185° F., but typically about 70° to about 90° F., than the temperature of the liquid refrigerant passing through the expansion device, typically from about 48°-56° F. transfers its heat to coil  216  causing defrosting and melting of any ice formed on the coil  216 . The cooled refrigerant fluid is then returned to the accumulator  206  or the compressor  204 . Valve  250  can remain open unit a second predetermined condition is obtained. For example, this predetermined condition can be a preselected passage of time. Alternatively, it can be a signal from the sensor to the controller indicating that a second predetermined set point has been reached.  
       EXAMPLE 1  
       [0027]    An air-to-air heat pump system  302 , shown in the heating mode, includes a defrost bypass loop  340  as depicted in FIG. 3. A defrost bypass loop  340  connects discharge line  330  from the indoor coil unit  310  to the inlet line  332  of the outdoor unit. A bypass line having a first end  352  and a second end  354  connects to discharge line  330  at its first end  352  between indoor coil unit  310  and expansion device  326 . A valve  350  is located in bypass line. Bypass line  352 ,  354  connects to inlet line  332  at its second end  354 .  
         [0028]    A temperature sensing device  360  is placed in contact with outdoor coil  316  to periodically or continuously monitor the actual temperature of outdoor coil  316 . Temperature sensing device  360  can be any well known temperature monitoring device such as a thermocouple, thermistor and the like. Temperature sensing device  360  is in communication with controller  370  along path  380 . Communications path  380  may be any convenient method of transferring a signal from temperature sensing device  360  to controller  370 . Thus, temperature sensing device  360  may be hard-wired to controller  370 , so that path  380  is the hard wiring that permits the signal from device  360  indicative of the temperature of outdoor coil  316  to be sent to controller  370 . Alternatively, temperature sensing device  360  may include circuitry that permits a signal indicative of temperature of the outdoor coil  316  to be transferred via RF waves, infrared waves or other suitable electromagnetic transmission to controller  370 , which controller includes means to receive such electromagnetic transmission.  
         [0029]    Controller  370  is in communication with valve  350  along a communication path  382 . As discussed above for the communication path between the temperature sensing device  360  and controller  370 , the communications path  382  between controller  370  and valve  350  may be via hard wiring or electromagnetic wave, it being understood that when communications path  382  is electromagnetic wave communications, controller  370  includes the means to transmit an electromagnetic signal and valve  350  includes the means to receive the electromagnetic signal.  
         [0030]    In operation, valve  350  is normally in the closed position when the heat pump system is running in the normal mode of heating an area. In this mode, all of the liquid refrigerant leaving indoor coil unit  310  passes through refrigerant line  330  into expansion device  326  and then into outdoor unit  314  through manifold  334 . Temperature measuring device  360  attached to outdoor coil transmits a signal indicative of the temperature to controller  370  along path  380 . The controller  370  is programmed for a first predetermined temperature set point indicating that the temperature of the outdoor coil is sufficiently low that a defrosting cycle must be performed. When temperature measuring device  360 , transmits a signal to controller  370  indicating the that temperature of the outdoor unit corresponds to a first predetermined set point, controller  370  causes heat pump unit  302  to reduce or shut off its heating functions and transmits a signal along path  382  activating valve  350  to an open configuration. This permits a portion of the refrigerant at elevated temperatures in line  330  to be diverted through valve  350  into the second end  354  of the line between the indoor coil unit  310  and outdoor unit  314 . This refrigerant then can flow into outdoor coil  316  through manifold  334 . This warm refrigerant will heat outdoor coil  316  causing it to defrost. The defrosting process will continue until controller  370  receives a signal from temperature sensing device  360  that a second predetermined temperature set point higher than the first predetermined temperature set point has been reached. The controller then transmits a signal to valve  350  causing valve  350  to close. Controller  370  simultaneously signals heat pump system  302  to resume normal heating operations, shutting down any auxiliary heat that may have been activated. It should be noted that although this embodiment shows the defrost bypass loop as the only means of defrosting the outdoor coil, it will be understood by those skilled in the art that this defrost loop can be combined with conventional defrosting elements, such as for example electric heating elements, to accomplish a more rapid defrost cycle, if desired.  
       EXAMPLE 2  
       [0031]    Referring now to FIG. 4, a slight variation to the previously described defrost bypass loop  340  is set forth. This variation results in a different operation of the defrost bypass loop  440 . Air-to-air heat pump system  402  is similar to heat pump system shown in FIG. 2. However, in this configuration, temperature sensor  460  is located within outdoor unit  414  to monitor the ambient temperature within outdoor unit  414 , but not attached to outdoor coil  416 . Alternatively, temperature sensor  460  may be located external to outdoor unit  414  to monitor the ambient temperature. When temperature measuring device  460 , transmits a signal to controller  470  indicating the that temperature within outdoor unit  414 , or alternatively the outdoor ambient temperature, corresponds to a predetermined set point, controller  470  activates a timed sequence operation, which may be preprogrammed into a programmable controller, causing heat pump unit  402  to reduce or shut off its heating functions and transmitting a signal along path  480  activating valve  450 , such as a solenoid valve to an open configuration for a preselected time period. Refrigerant at elevated temperatures in line  430  is diverted through valve  450  into the second end  454  of the line between the indoor coil unit  410  and outdoor unit  414 . This refrigerant flows into outdoor coil  416  through manifold  444  for a preselected time. This warm refrigerant will heat outdoor coil  416  causing it to defrost. After the preselected time has expired, the valve  450  closes and the heat pump resumes normal operation. The preselected time can be a fixed time or could vary depending upon the temperature sensed by sensing device  460 , with longer defrosting times required for lower sensed temperatures. Normal operation resumes, but this defrosting process will cycle or repeat periodically at second preselected time intervals until controller  470  receives a signal from temperature sensing device  460  that a second predetermined temperature set point higher than the first predetermined temperature set point has been reached. The second preselected time interval may also be a fixed time interval or may vary depending upon the temperature sensed by the sensing device  460 , with shorter intervals required for lower temperatures (i.e. the defrost cycles occur more often at lower temperatures) or as noted above, the defrost time interval can be longer at lower temperatures. Once the second predetermined temperature set point is reached, controller terminates the timed sequence operation by transmitting a signal to valve  450 , causing valve  450  to close, if it is not already closed, which returns the heat pump to normal operation and resumes normal heating operations while, shutting down any auxiliary heat that may have been activated during the defrost cycle. It should be noted that although this embodiment shows the defrost bypass loop as the only means of defrosting the outdoor coil, it will be understood by those skilled in the art that this defrost loop can be combined with conventional defrosting heat elements to accomplish a more rapid defrost cycle, if desired.  
       EXAMPLE 3  
       [0032]    Referring now to FIG. 5, a different embodiment of the present invention. a slight variation to the previously described defrost bypass loops X 40 , where X 40  represents any of the previously discussed loops, is set forth. This variation results in a different operation of the defrost bypass loop  540 . Air-to-air heat pump system  502  is similar to heat pump system shown in FIG. 2. However, in this configuration, a sensor  560  is located either within outdoor coil  516  or within line  524  leaving the evaporator  514 , as shown in FIG. 5, or within outdoor coil itself. Sensor  560  monitors a condition of the refrigerant. It can be set to monitor, for example the temperature of the refrigerant or the pressure of the refrigerant gas. For a refrigerant, the temperature at which a phase change from liquid to gas is known. If the temperature of the refrigerant is too low, insufficient refrigerant is undergoing a phase transformation from liquid to gas and the refrigerant gas pressure is also low. These conditions will occur when the proper functioning of the evaporator is hindered by icing conditions. Sensing device  560  senses a condition of the refrigerant, either pressure or temperature, and transmits a signal to controller  570  indicating the temperature of the refrigerant or the pressure of refrigerant gas either within return line  524 , as shown, or within outdoor coil  516 . If the sensed condition corresponds to a predetermined set point, controller  570  causes heat pump unit  502  to reduce or shut off its heating functions and transmits a signal along path  582  activating valve  550  to an open configuration. This permits a portion of the refrigerant at temperatures elevated temperatures in line  530  to be diverted through valve  550  into the second end  554  of the line between the indoor coil unit  510  and outdoor unit  514 . This refrigerant then can flow into outdoor coil  516  through manifold  534 . This warm refrigerant will heat outdoor coil  516  causing it to defrost. The defrosting process will continue until controller  570  receives a signal from sensing device  560  that a second predetermined condition set point higher than the first predetermined condition set point has been reached. The controller then transmits a signal to valve  550  causing the valve to close. Controller  570  simultaneously signals heat pump system  502  to resume normal heating operations, shutting down any auxiliary heat source that may have been activated. Alternatively, controller  570  can enter into a timed sequence operation, sending a signal to valve  550  after a first predetermined time to close it. In this configuration, the defrost cycle is a timed defrost cycle. It should be noted that although this embodiment shows the defrost bypass loop as the only means of defrosting the outdoor coil, it will be understood by those skilled in the art that this defrost loop can be combined with conventional defrosting elements such as electric elements, to accomplish a more rapid defrost cycle, if desired.  
       EXAMPLE 4  
       [0033]    Referring now to FIG. 6, a different embodiment of the present invention is set forth. In this embodiment, air-to-air heat pump system  602  is similar to heat pump systems shown in FIG. 2 or in any of FIGS. 3, 4 or  5 . However, in this configuration, a defrost bypass loop  640  is connected at its first end  652  to line  622  between the compressor  604  and condenser  610 . While the defrost bypass loop can operate by any of the modes set forth in the previous examples, once valve  650  is opened by controller  670 , a portion of refrigerant fluid discharged from the compressor  604 , rather than from the condenser  610 , flows through the bypass loop  640  where it moves through the second end  654  of the discharge line into line  632  and into manifold  634 . Because this refrigerant fluid is significantly higher in temperature than refrigerant from condenser  610 , the temperature ranging from about 160° F.-185° F. on discharge from compressor  604 , the defrost cycle can be accomplished much more quickly. Since the flow of refrigerant to the condenser is reduced once valve  650  is open, it should be readily apparent to those familiar with the operation of such units that the ability of system  602  to provide heat will be reduced during the defrost cycle. It should be noted that although this embodiment shows the defrost bypass loop  640  as the only means of defrosting outdoor coil  616 , it will be understood by those skilled in the art that this defrost loop can be combined with conventional defrosting heat elements to accomplish a more rapid defrost cycle, if desired.  
       EXAMPLE 5  
       [0034]    Referring now to FIG. 7, a more complex arrangement is set forth. This arrangement provides additional defrost capacity by combining the defrost bypass loops of FIG. 2 and FIG. 6. Sensors X 60  where X 60  represents any previously described sensor, may be placed in any of the positions previously discussed to sense preselected conditions. A first defrost bypass loop  740  with valve  750  is shown connected to line  730  from condenser  710 . This defrost bypass loop operates in the same manner as the defrost bypass loops shown in FIGS. 3, 4 and  5  and discussed in greater detail above. Also included is a second defrost bypass loop  741 . Loop  741  includes a second valve operable from a first position to a second position in response to a signal, such as previously discussed valves, a line having a first input end  792  connected to a line  722  from compressor  704 , and a second discharge end  794  connected to a line  732 , which is an inlet line to evaporator  714 . Valve  790  is in communication with controller along path  796 , in a manner similar to path  782 , X 82  where X 82  represents previously described path as previously discussed. In operation, second valve  790  remains in a first closed position during normal operation of the heat pump. A signal from controller  770  is sent to second valve  790  to a second open position when controller determines that a third predetermined set point has been reached. This predetermined set point may be the same set point that opened valve  750 . Alternatively, the controller may include an algorithm that includes a timing function. If, after a predetermined time, valve  750  is still open, controller may send a signal to valve  790  to open it, thereby adding additional defrost capacity to the system. Alternatively, controller  770  may be in communication with a second sensor (not shown) monitoring a second condition of the refrigerant or outdoor unit  714 . If the second sensor provides a signal to controller  770  that a third predetermined set point is reached, or that the third predetermined set point is not reached within a second preselected time period, controller  770  sends a signal to open valve  790  to provide additional defrost capability to the system through second defrost bypass loop  741 . Valve  790  may be closed either in response to a fourth predetermined set point being reached, as signaled by sensor  760 , or after a preselected period of time. After defrost has been accomplishes as determined by controller  770 , a signal can be transmitted to heat pump unit  702  to resume normal operation and to shut off auxiliary heat that may have been activated as a result of the defrost cycle. It should be noted that although this embodiment shows a pair of defrost bypass loops as the means of defrosting the outdoor coil, it will be understood by those skilled in the art that these defrost loops can be combined with conventional defrost elements, such as electric heating elements, to accomplish a more rapid defrost cycle, if needed.  
         [0035]    The present invention sets forth a heat pump system that includes a defrost bypass loop that uses heat within the heat pump system to accomplish a defrost cycle. When used alone, it can eliminate the use of defrost elements, such as electric heating elements. When used in conjunction with conventional defrosting elements, it can reduce the amount of time that the defrosting elements are in use and can shorten the time required for a defrost cycle. The temperature range over which the heat pump system can operate efficiently may also be extended.  
         [0036]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.