Abstract:
An HVAC system includes a variable speed electric motor driven fan for circulating air to an enclosed space through an indoor unit for providing heating or cooling effect to air circulated through the indoor unit. A controller with user or system installer actuatable switches includes a programmable microcontroller providing a pulse width modulated (PWM) signal to a motor control unit to select a predetermined air flowrate less than full air flowrate during system startup and shutdown. User input parameters include selected percentages of full air flowrate to satisfy requirements for reduced noise level, lower humidity and for improved sensible heating of air being circulated, the latter being effective particularly for heat pump applications. Selection of pre-run, short run and shutdown cycles may be provided.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     In the art of heating, ventilating and air conditioning (HVAC) equipment there has been a continuing desire and need for a system and method for controlling indoor air flowrate due to varying circumstances and the types of equipment used in HVAC systems. For example, in many residential HVAC systems, the occupants of the house or dwelling being air conditioned may have a desire to vary the air flowrate, hence speed, of the indoor blower or fan which circulates the conditioned air throughout the dwelling. In situations where the humidity being sensed by the dwelling occupants needs to be reduced by varying amounts, such can be accomplished by changing the air flowrate over the air cooling heat exchanger, such as a vapor compression system evaporator coil. In other circumstances, such as for systems which utilize heat pumps for both heating and cooling, the occupants of the dwelling may desire to reduce air flow in the heating mode in order to have a sense of warmer air being circulated during system operation.  
         [0002]     Still further, the occupants of a dwelling or other facility being air conditioned by a forced air HVAC system, may desire to operate the indoor air circulation blower or fan continuously and at selected variable speeds to control air flowrate and noise level, for example. Of course, the occupants of a dwelling or building being air conditioned by a forced air HVAC system may also have the desire to control indoor blower or fan speed and air circulation rate during startup and shutdown phases of operation of the system so as to be able to capture residual heating or cooling effect after system shutoff and to minimize the loss of heated or cooled air due to heat transfer in the system ductwork during periods of system shutoff. Accordingly, it is to provide the capability of changing air flowrates in an HVAC system to satisfy several needs and desires, including those mentioned above, that the present invention has been developed.  
       SUMMARY OF THE INVENTION  
       [0003]     The present invention provides an improved system and method of controlling air flow in a forced air HVAC system and wherein the user of the system may select an indoor air flowrate according to the user&#39;s wishes to modify the sensed humidity and temperature of the circulating air.  
         [0004]     In accordance with one aspect of the present invention, an HVAC system controller is provided which is advantageously embodied in a so-called thermostat device which may be operated by the user or occupant of the dwelling, building or other space being served by the HVAC system, whereby the user may provide temperature setpoints in various modes of operation of the system and may also make selections of indoor air flowrate during selected phases of operation. For example, if humidity control is required, the system user or installer may enable the reduction of indoor air flowrate to improve reduction in relative humidity when the system is operating to cool the indoor air. Still further, the equipment installer for the space being air conditioned may select reduced air flow in certain phases of operation of an HVAC system to provide warmer or cooler air flow sensed by the user.  
         [0005]     The present invention still further provides an improved controller for an HVAC system which may be incorporated in a wall mountable thermostat type control device which may be operated by the user to set temperatures in both heating and cooling mode, to set the mode itself and to control indoor air blower speeds under selected operating conditions.  
         [0006]     In accordance with another aspect of the invention, an HVAC system is provided which includes an indoor unit which is characterized by a motor driven blower or fan and one or more heat exchangers for providing warmed or cooled air or both, wherein the blower drive motor is a variable speed type and the operating speed may be selected by a pulse width modulated (PWM) electrical signal from a thermostat or controller to select the blower speed and provide the desired air flow. The thermostat or controller includes a programmable microcontroller which is operably connected to the indoor unit of the HVAC system, including the blower motor, and to the outdoor unit by way of selected circuit connections.  
         [0007]     Further in accordance with the invention, the programmable microcontroller may be programmed to operate in a default mode if the user has not selected blower air flowrates for startup and/or shutdown of the HVAC system. The programmable microcontroller may also be programmed to provide a pulse width modulated output signal to the blower motor to control air flowrates from a predetermined minimum air flowrate to a maximum air flowrate in different operating modes of the HVAC system. Air flowrate “profiles” may be set, such as a two step, time delay profile at startup of the heat exchangers of the indoor portion of the system and, preferably, a single time delay flowrate profile at turnoff or shutdown of the heat exchangers of the indoor portion of the system. Still further, air flowrate profiles may be selected for applications wherein multiple stages of heating or cooling are provided wherein multiple stage compressors systems are operating at a low or first stage and when operating on a second (or third) high stage when a humidity sensor indicates humidity above a setpoint, for a heating mode of a heat pump system and for cooling efficiency (mild outdoor temperatures), for example. Accordingly, a relatively wide range of blower operating speeds may be selected, depending on the configuration of the HVAC system.  
         [0008]     Those skilled in the art will further appreciate the above-mentioned advantages and superior features of the invention together with other important aspects thereof upon reading the detailed description which follows in conjunction with the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a general schematic diagram of an HVAC system including a controller for the system in accordance with the present invention;  
         [0010]      FIG. 2  is a front elevation of a thermostat type controller in accordance with the invention and showing a schematic representation of the control signal conductor terminals associated with the thermostat;  
         [0011]      FIG. 3  is a diagram illustrating a typical example of an air flowrate profile selectable by a user of the system shown in  FIGS. 1 and 2 ;  
         [0012]      FIGS. 4 through 8  are diagrams indicating electrical connections made between the thermostat controller, an indoor unit of an HVAC system and an outdoor unit of the HVAC system; and  
         [0013]      FIG. 9  is a table of air flowrates for given electrical control signals to a blower motor controller in accordance with the method and system of the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     In the description which follows like elements are marked throughout the specification and drawing with the same reference numerals, respectively. The drawing figures are not necessarily to scale and certain features may be shown exaggerated in scale or in schematic or generalized form in the interest of clarity and conciseness.  
         [0015]     Referring to  FIG. 1 , the present invention includes a controller for a HVAC system  18  which may be characterized as a thermostat, generally designated by the numeral  20 . The thermostat  20  not only includes certain temperature sensors, such as for sensing the temperature of an enclosed space  22  by way of a sensor  24 , but may also include a humidity sensor  26  for sensing the relative humidity in the enclosed space  22 . The thermostat controller  20  may also include a temperature sensor  28  for sensing the so-called outdoor temperature surrounding the enclosed space  22 . The enclosed space  22  may be a private residential dwelling or one or more rooms or spaces in a commercial building, for example. The temperature and humidity within the enclosed space  22  is controlled by HVAC system  18  comprising an indoor unit  30  which may include a forced airflow fan or blower  32  disposed within a suitable cabinet  34  and operable to be driven by an electric motor  36  for circulating air to and from the enclosed space in a conventional manner. Motor  36  is operably connected to a source of electrical power by way of a motor controller or control unit  37 . The indoor unit  30  of the HVAC system  18  may include one or more heat exchangers  38 ,  39  and  40  arranged in series or parallel with respect to air flow through the cabinet  34  and characterized as electric resistance type grids, or heat exchangers associated with a heat pump, a conventional vapor compression type air conditioning system, other chilled or heated fluids or a combination of such types of heat exchangers. In other words, the heat exchangers  38 ,  39  and/or  40  may provide for heating air being circulated through the cabinet  34 , cooling air being circulated through the cabinet  34  and/or dehumidifying air flowing through cabinet  34 .  
         [0016]     The HVAC system  18 , which the controller or thermostat  20  may be operable to control, also includes a so-called outdoor unit  42  which may include one or more vapor compression compressor units  44  and  46 . The outdoor unit  42  may also include conventional condenser/evaporator type heat exchangers, not shown, operably connected to the heat exchangers  38 ,  39  and/or  40  making up a conventional single or multistage vapor compression air conditioning or heat pump system. The thermostat controller  20 , the indoor unit  30  and the outdoor unit  42  are operably interconnected by way of respective electrical conductor terminal strips  21 ,  31  and  41  whereby control signals may be transmitted to and from the controller  20  for controlling operation of the motor  36  and associated control components normally found in an HVAC system.  
         [0017]     Referring further to  FIG. 1  and also  FIG. 2 , the thermostat or controller  20  includes a suitable housing  23  in which is disposed a microcontroller  48  operably connected to a visual display  50  and to respective user controlled pushbutton type switch actuators  52 ,  54 ,  56 ,  58 ,  60  and  62 . The visual display  50  is operable to display various indicia, such as indicated in  FIG. 2 , including the actual sensed temperature  64 , as well as indicia, not shown, which would appear on the display as a number during installation and setup. Such numbers would correspond to a selected one of various sets of operating profiles, or portions of such profiles, including blower turn-on and turnoff times and a percent of maximum airflow. The pushbutton actuator  52  may be utilized to cause the microcontroller  48  to operate in a default mode or a temperature setback mode for energy savings. As indicated in  FIG. 2 , the pushbutton switch actuator  54  may be used to display the remaining prescribed life of the system air filter, the actuator  56  may be used to control operation of the fan or blower  32  and the actuator  58  may be used to control the mode of operation of the HVAC system  18 , either a heating mode or a cooling mode, for example. The pushbutton switch actuators  60  and  62  may be utilized to decrease or increase, respectively, the numerical value of the parameter being set by the controller  20  and displayed on the display  50 .  
         [0018]      FIG. 2  illustrates the terminal strip  21  somewhat schematically, which strip may be mounted on the backside of the housing  23  of the controller  20  and include conventional wire connection terminals appropriately identified with alphanumeric designations according to HVAC system code standards, such as prescribed by the American National Standards Institute (ANSI). For example, the terminals OT 1  and OT 2  are terminals for the conductors for the temperature sensor  28 . The terminal Y 2  is for controlling a second stage compressor, such as the compressor  46 . The terminal W 3  is for sending a signal to a third stage heat exchanger or heating element, such as one of the heat exchangers  38 ,  39  or  40 . The terminal R is for a connection to a low voltage power supply or transformer, not shown. Terminals W 1  and W 2  are for transmitting signals to the indoor unit  30  for controlling heat exchangers  38  and  39 , respectively. Terminal Y/Y 1  is for transmitting signals to the indoor unit  30  and outdoor unit  42  for controlling a first or only compressor stage, such as the compressor stage  44 . Terminal G is for sending an on-off control signal to control unit  37  for blower motor  36 . Terminal B is for a common conductor for the transformer signal and terminal BK is for transmitting a pulse width modulated (PWM) signal from the microcontroller  48  to control unit  37  for motor  36 , see  FIG. 1 . Terminals H 1  and H 2  are for connecting the humidity sensor  26  to the controller  20  and terminals RS 1  and RS 2  are for connecting indoor temperature sensors, such as the temperature sensor  24  to the microcontroller  48 .  
         [0019]     In accordance with the invention, the user of the HVAC system  18  or, preferably, system installation personnel may control the operation of the motor  36  and fan or blower  32  to vary the air flow through the cabinet  34  in various predetermined sequences and for various periods of time for adjusting the humidity in the enclosed space  22  and for adjusting the sensed temperature of airflow into and through the enclosed space due to certain characteristics of the HVAC system as well as the heating and cooling effect provided by the heat exchangers or heating elements which are in use, including one or more of the heating elements or heat exchangers  38 ,  39  and  40 . The motor  36  may be a brushless, permanent magnet, variable speed motor of a type commercially available, such as a Model ECM 2.3 motor available from the General Electric Company. Motor  36  includes the built-in control unit  37  which is adapted to receive a low voltage PWM signal from the microcontroller  48  for controlling the speed of motor and the fan or blower  32 . However, other motors which will accept a PWM signal or its equivalent may be adapted to be controlled to provide the advantageous features of the present invention.  
         [0020]     As previously discussed, the HVAC system  18  may be set or its operation modified by the controller  20  to provide a PWM signal to the control unit  37  for motor  36  during operation of the system to set a particular “profile” of circulation of indoor air at selected rates. The user may set operation of the blower  32 , in the blower continuous mode, at a predetermined percentage of full air flowrate during periods when there is no call to system  18  for heating or cooling. The continuous air flowrate may be about twenty percent, for example, see  FIG. 3 , and as represented by the line  70 . The diagram of  FIG. 3  shows percent full air flow on the vertical scale and time in minutes, for example, on the horizontal scale. Zero time is that at which a call for heating or cooling is provided at the controller  20  due to correlation between the setpoint temperature for heating or cooling effect input by the user of thermostat controller  20  and the temperature sensed by the sensor  24  and/or sensor  28 .  
         [0021]     The HVAC system  18  may, for example, be controlled by the microcontroller  48  to control the motor  36  to provide a predetermined percent of full air flow for a predetermined time period at the time of energization or startup of the system in heating or cooling mode to provide heating or cooling effect at the heat exchangers  38 , and/or  39  and/or  40 . The time periods for reduced airflow may, as previously discussed, be selected by the system installer, or preset by microcontroller  48 . A minimum thirty-five percent air flowrate, for example, could be provided from the call for heating or cooling effect out to time T 1  in  FIG. 3 . This period of time from  0  to T 1  in  FIG. 3 , may be known as the prerun delay period. The system  18  may also, for example, be set at an increased air flowrate known as short run over a time period between time T 1  and T 2  in  FIG. 3  which is a flowrate (fifty percent of maximum is shown) to minimize the sensed temperature of the flowing air due to cooling or heating of the air in the system air flow delivery ducts, or stratification of the air in such ducting, or within the space  22 , all often occurring during periods of system shutdown. Predetermined limits may be imposed on the time T 2  in accordance with the type of heating or cooling equipment being utilized and provided at the heat exchangers  38 ,  39  or  40  to minimize an over temperature condition or to minimize the chance of condensate freezing on the heat exchangers, for example. Accordingly, time T 2  may be a predetermined maximum beyond which the microcontroller  48  will automatically transmit a signal to the motor controller  37  to require substantial or full motor speed and air flow, indicated as one hundred percent, by way of example, in the diagram of  FIG. 3 . Accordingly, the full or one hundred percent air flowrate, shown in  FIG. 3  between times T 2  and T 3 , is also known as the normal air flowrate.  
         [0022]     The system  18  may also be set to a predetermined air flowrate by selecting a reduced speed for the blower  32 , after the call for heating or cooling is satisfied at time T 3 , by providing for a so-called blower run-on or turnoff cycle at a reduced air flowrate of from fifty percent to sixty-five percent of maximum, for example, as shown in  FIG. 3 . Air will be circulated at this percentage of full air flowrate for a period of time between time T 3  and T 4 . At time T 4 , motor  36  is deenergized or may be operated continuously at, for example, twenty percent of full air flowrate, if that mode of operation has been selected by the system user.  
         [0023]     Accordingly, the system  18  may be operated at various blower turn-on and turnoff time delay profiles, in minutes, at a specific percent of air flow as compared with full or one hundred percent air flow. When the controller  20  is sensing temperature from the sensors  24  and  28  and humidity from the sensor  26  and operating according to the setpoints for heating and cooling, certain air flowrates, generally in accordance with the diagram of  FIG. 3 , may be selected within preset conditions or limitations programmed into the microcontroller  48 , for example. When microcontroller  48  determines a call for heating or cooling effect to be provided by the system  18 , the microcontroller  48  will impose a signal at terminals G and BK in accordance with a preprogrammed percent of air flow.  
         [0024]     For example, with a zero percent PWM signal provided at terminal BK, the air flowrate may be preset at thirty-three percent and when a one hundred percent PWM signal is imposed by the microcontroller  48  on controller terminal BK, the motor  36  will be commanded by control unit  37  to provide one hundred percent air flow. Zero percent PWM signal means no current flowing at terminal BK, fifty percent PWM signal means current flowing for half of a designated time period (i.e., a square wave signal) and one hundred percent PWM signal is current flowing constantly.  
         [0025]     Still further, air flowrate profiles may be selected for predetermined periods of time in minutes, for example, at a particular percent of full or maximum air flow. A factory default setting for the pre-run delay period and short run delay period of zero minutes at zero percent air flowrate is typically selected. In other words, in the factory default setting, when a call for heating or cooling is received at thermostat  20 , motor  36  is commanded to immediately accelerate to full or one hundred percent air flow. Other preprogrammed flowrates in the pre-run delay and short run delay may be, for example, one minute at fifty percent air flowrate and seven and a half minutes at eighty percent air flowrate. Other selected pre-run and short run delay periods may be, for example, one minute at fifty percent air flowrate and four minutes at eighty percent air flowrate. Further pre-run and short run flowrates may be selected as zero minutes at zero percent air flowrate and seven and a half minutes at eighty percent flowrate, zero minutes at zero percent flowrate and four minutes at eighty percent flowrate, one minute at fifty percent flowrate and zero minutes at zero percent air flowrate.  
         [0026]     Blower run on or so-called turnoff delay profiles may be selected (these are times T 3  to T 4  in  FIG. 3 ) of one and one half minutes at one hundred percent air flowrate as a factory default setting or zero minutes at zero percent flowrate. Still further, preset rates of one-half minute at fifty percent air flowrate, one and one half minutes at fifty percent air flowrate, three minutes at fifty percent air flowrate, or one half minute at thirty-five percent air flowrate. These settings may all be preprogrammed into the microcontroller  48  and given a numerical code for use in setting the operating cycle or profile.  
         [0027]     Alternatively, the microcontroller  48  may be programmed to allow the system installer (or the user) to set the percent air flowrate and the time period for each percent air flowrate as long as certain minimums and/or maximums are not exceeded. For example, the system installer or user may be allowed to adjust the air flowrate in five percent increments. However, the percent air flowrate for preset turn on (pre-run and/or short run) and turnoff profiles is typically for a single stage system or for the second stage operation of a two stage system. These percentages of air flow may be required to be adjusted for first stage operation in a two stage system and any user selected percent air flow would be that percent air flow multiplied by an installer selected first stage percent air flowrate.  
         [0028]     If a continuous fan mode has been selected, that is the fan runs constantly at a selected percent of full air flow when there is no call for heating or cooling, then typically, if the system cycles off the signal imposed at terminals BK go immediately to the continuous fan air flow setting. However, during the “turn-on” profiles while the system is operated, a continuous fan air flow mode signal imposed on terminals BK should be one corresponding to the profile selected but at a motor speed not less than the continuous fan mode speed.  
         [0029]     Another preferred mode of operation is, if continuous fan mode is not selected and a blower or fan turnoff delay profile is selected and the system compressor shuts off during a pre-run delay time, the system should go to the blower turnoff delay flowrate which is a certain percent of the turn-on or pre-run air flow percent, for example, fifty percent of fifty percent air flow or twenty-five percent air flow. Under the same conditions, if the compressor cycles off during the short run delay profile the microcontroller  48  should impose a signal on terminals BK to cause the blower or fan  32  to operate at the blower turnoff air flowrate times the short run air flowrate, for example, fifty percent of eighty percent air flow or forty percent air flow. However, if the air flow percentage calculated drops to a minimum, say thirty-five percent, the controller would cause the blower to operate at the minimum continuous air flow percentage.  
         [0030]      FIGS. 4 through 8  show typical wiring connections between the respective terminal strips  21  and  31  of the controller  20  and the indoor unit  30  and between terminal strip  31  and a terminal strip  41  of an outdoor unit  42  for various systems which may be controlled in accordance with the invention and which may utilize the method of the invention. In the diagrams of  FIGS. 4 through 8 , only the connections being used may be shown on terminal strips  31 ,  41  and  41 a.  FIG. 4  illustrates connections between the terminal strips of the controller  20 , the indoor unit  30  and the outdoor unit  42  for three stages of heating and two stages of compressor operation.  FIG. 5  illustrates the connections between the respective terminal strips for the controller  20 , the indoor unit  30  and the outdoor unit  42  for three stages of indoor heaters and two stages of compressor heat pump operation. Typically, this system would provide for two stages of heat pump operation and a third stage of electric heating, for example.  
         [0031]      FIG. 6  illustrates the connections between the terminal strips for the respective components  20 ,  30  and  42  for a single stage compressor in a heat pump system with a dual fuel furnace.  
         [0032]      FIG. 7  illustrates the connections between the respective terminal strips for the controller  20 , the indoor unit  30  and the outdoor unit  42  for a two stage heat pump system with a dual fuel two stage furnace.  
         [0033]      FIG. 8  illustrates the connections between the terminal strips  21 ,  31  and  41 a for the components  20 ,  30  and  42  for one, two or three stages of heat, which can be electric heat, and with two separate single stage air conditioning compressor units operating as the outdoor unit. In  FIG. 8 , separate terminal strips  41 a are shown for respective compressors of the outdoor unit  42 .  
         [0034]     Additional control scenarios are operable to be programmed into the microcontroller  48  depending on the particular system being controlled. For systems utilizing two or three stages of compression, the air flowrate for low stage compressor operation in a two stage system, for example, could be installer selectable from thirty-five percent to seventy-five percent of air flow in increments of five percent with a so-called factory default setting of fifty percent. Moreover, a percentage of total air flowrate for the turn on or startup as well as the turnoff or shutdown profiles may be selected for first stage operation wherein the percent air flow actually programmed and carried out is equal to a user selected profile percent air flow multiplied by the installer selected low stage percent air flow. Reduced air flowrates during the high or highest compressor stage operation would not normally be permitted, except when a humidity sensor would sense relative humidity greater than the setpoint, for heat pump heating modes and for cooling “efficiency” operation during periods of mild outdoor temperatures.  
         [0035]     Still further, for example, when continuous operation of blower or fan  32  is selected and the system has been operating a compressor and/or electric heat and is cycled off, i.e., setpoint has been achieved, the PWM signal from terminal BK output to the control unit  37  may go to a continuous fan air flowrate setting. However, if the continuous air flowrate is less than the regular system full running air flowrate, the air flowrate is reduced to the running air flowrate at the start of the profile and delay times are monitored since the number of stages in operation may vary in accordance with the call for heating or cooling.  
         [0036]     Alternatively, for example, as mentioned previously, if the continuous air flowrate programmed by the controller  48  is less than a selected air flowrate, the greater of the two will prevail. Still further, if a continuous fan operating mode is not selected, but a turnoff delay is selected, and a compressor cycles off during the pre-run delay, system operation should go to the blower turnoff delay percent of the pre-run air flowrate (namely, 50% of 50% or 25% of full air flowrate). Under the same conditions, if the compressor cycles off during a short run delay, the controller  48  should adjust the motor operating speed to provide the blower turnoff delay percent of the short run air flow percent (for example, 50% of 80% airflow or 40% of full air flowrate). If an air flowrate percentage less than a predetermined minimum is calculated by the controller  48 , the control signal provided to the motor control unit  37  should call for the minimum air flowrate percentage, such as thirty-five percent of full airflow.  
         [0037]     Still further in accordance with the invention, in operating modes wherein the heat exchangers  38  and  39  are heat pump type heat exchangers and the system is operating in the heating mode for heating the space  22 , the air flow should be limited to not less than thirty-five percent of full air flow while operating on a single stage of operation. If one or more auxiliary heating stages are being called in addition to the heat pump stage, no adjustment to air flow need be made.  
         [0038]      FIG. 9  is a table showing preferable air flowrates as a percent of total capacity of the motor driven fan  36  and the percent PWM signal required to obtain that setting for different operating conditions in accordance with the invention. The microcontroller  48  is also operable to receive a signal from the humidity sensor  26  to reduce air flow so that the heat exchangers  38 ,  39  or  40  have a better opportunity to remove moisture from the air flowstream in a cooling mode of operation. In such a mode the compressor “on” cycle time may be extended by two minutes during each cycle and the selected blower turnoff delay may be defeated and replaced by a routine wherein the airflow is reduced by twenty percent of whatever setting is in effect when the room temperature is within two degrees of the cooling setpoint or below the setpoint.  
         [0039]     The HVAC system  18  may be operated in a mode when a so-called wet heat type heating source is furnishing heat via the heat exchangers  38 ,  39  or  40 . With so-called wet heat being applied via one or more of heat exchangers  38 ,  39  or  40 , air flow is modulated based on load value and no duty cycling is provided at terminal W 1 .  
         [0040]     Those skilled in the art will appreciate that a single or multistage HVAC system in accordance with the invention enjoys improved operation with respect to controlling forced airflow through an enclosed space being conditioned by the system. By allowing selection of the percent of maximum airflow throughput by the system, noise levels within the conditioned space may be reduced, sensed temperature of air circulating within the space may be modified to eliminate the adverse effects of temperature stratification of the air, and moisture removal may be selectively enhanced. The system may use less power than prior art systems and improved filtration of airflow may be obtained by controlling the air flowrate. Still further, improved comfort resulting from air circulated through the indoor heat exchanger(s) of a heat pump type system may be enjoyed. The system components described herein may be obtained using commercially available components and known manufacturing methods, and the microcontroller  48  may be programmed using conventional programming practices.  
         [0041]     Although preferred embodiments of the invention have been described in detail herein, those skilled in the art will recognize that various substitutions and modifications may be made without departing from the scope and spirit of the appended claims.