Abstract:
A system that allows fresh air to be brought into a furnace as part of the return air circuit. This not only helps to replace stale air within a home, it also provides adequate air supplies to establish enough positive pressure within the dwelling to prevent air infiltration around windows, doors, forced air registers and around room perimeters of rooms along the baseboards. The system consists of a furnace that has a return air supply duct, an intake louver and filter for bringing fresh air into the system, an intake fan mounted to the intake, an intake duct that brings the fresh air from the intake louver to the furnace and a motorized damper, installed in the intake duct, to control the flow or air. These components are controlled by a control panel that works with the thermostat and furnace controls to operate the intake fan and damper as for optimum operation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     This invention relates to air ventilation control systems and particularly to air ventilation control systems that provide fresh makeup air in tightly insulated buildings. 
     2. Description of the Prior Art 
     In the past, air ventilation systems in buildings and homes were simple-one opened and closed windows to adjust the comfort ranges in the building. Today, heating and cooling systems are considerably more complex. This complexity has been exacerbated by the latest building specification that call for the so-called super insulating of homes and other buildings. While the use of super insulating techniques has improved heating and cooling efficiency, it has produced problems. Because they are tightly sealed against drafts, these homes generally contain twice the insulation required in prevailing building codes, along with double or triple glazed windows. The envelope of this type of home is completely encased in a polyethylene air/vapor barrier, and all of its windows and doors are fitted with special seals to prevent the escape of heated inside air and its subsequent replacement by cool outside air. The tight sealing of these structures prevents the natural exchange of stale inside air for fresh outside air, which leads to elevated indoor air pollutant levels. 
     To overcome these problems systems have been developed to bring fresh air into the conditioned space to replace the “stale” air in the space. The problem in doing this is that the fresh air being introduced is usually at a different temperature than that of the inside space. Thus, for cold climates, bringing in fresh air means bringing in colder air that takes energy to heat before it can be used. Similarly, in hot climates the opposite problem exists. To solve this problem, air-to-air heat exchangers and mixing dampers have been developed. 
     U.S. Pat. No. 4,044,947 to Spethmann teaches a control apparatus for controlling an outdoor air damper and a recirculated air damper of an air conditioning system for maintaining a predetermined condition, such as temperature, and a predetermined volume of the air to be discharged to the zones supplied by the air conditioning system. In this manner, separate volume control dampers may be eliminated by requiring the outdoor air and recirculated air mixing dampers to perform both the functions of controlling the condition of the mixture of outdoor air and recirculated air and controlling the volume of the mixture of air supplied to the zones. 
     The control apparatus may also use variable gain amplifiers connected to a temperature sensor, which senses the temperature of the mixed air, either upstream or downstream of the fan, and a second input connected to at least one volume sensor responsive to the volume of air moving through the system. The outputs from the variable gain amplifiers are connected to the appropriate outdoor air damper and/or recirculated air damper for regulating the condition and volume of the mixture of outdoor air and return air supplied to the zones. In this way, upon an increase in the static pressure in the system, the gain of the amplifiers is adjusted to reduce the volume of air supplied to the system by the outdoor air and recirculated air damper. The volume of the system return is regulated by the same type of exhaust and recirculating air dampers. 
     U.S. Pat. No. 4,776,385 to Dean relates to those devices for enabling and disabling heating and cooling equipment and providing supplemental ventilation to an enclosure. An air intake unit is ducted to a central heating and cooling installation. The intake fan unit has an outside temperature thermostat associated therewith and which is wired to a control circuit that also monitors temperature in proximity to a thermostat-equipped exhaust fan unit located in an attic or upper crawl space. Sensing devices are used in association with the plurality of thermostats to selectively enable or disable the heating and cooling equipment and to actuate the intake and exhaust fan units, thereby providing ventilation in the space. 
     U.S. Pat. No. 4,735,130 discloses a fresh air make-up apparatus for a furnace located in an enclosed building that comprises an elongated diffuser housing positioned adjacent the furnace, with the housing having an inlet in communication with fresh outside air and a pair of outlets, one being connected to the cold air return of a forced air furnace at a point substantially upstream of the heat exchanger, and the other outlet leading to the room. The outlet leading to the cold air return provides fresh make-up air for the building, while the opening leading to the room provides both a means for diffusing and preheating the fresh air supplied to the cold air return and a means for providing fresh air diffused with warm room air to the combustion chamber of the furnace. The apparatus also includes a removable filter for removing contaminants from the fresh air. The apparatus can include a heat exchanger for preheating the fresh air with warm air from the warm air duct of the furnace. This warm air is then exhausted outside the building to provide a continuous air exchange within the building. Although this system is simple, it merely adds air to the intake of a furnace. Without any systematic conditioning or operating parameters. 
     U.S. Pat. No. 5,082,173 to Poehlman et al. teaches a dynamic environmental control system for controlling the environment in a super insulated tightly sealed home which uses a heat recovery ventilator (HRV) connected to a forced air heating system for ventilation. The system performs a number of functions according to predetermined priorities. Firstly, for health reasons, the system will initiate an air exchange if HRV running times have been inadequate to ensure a minimum fresh air content and at the same time to dilute air pollutants which build up over time in an unventilated home. In addition, in order to minimize the effects of high humidity outputs caused by cooking, bathing, washing or drying, the system permits the user to initiate a timed HRV run to rid the home of the anticipated humidity. Such operations are special events that are activated over a background monitoring of inside temperature gradients that can be equalized by the system activating a forced air system whenever a preset maximum value is exceeded. The system may also function in one of its aspects as a seven day four temperature setback thermostat and is programmed to initiate an HRV run to remove humidity from a home if the humidity level is not within a desired range or if an inside dew point violation is likely; provided that the inside and outside moisture levels are favorable for removing humidity from the home. However, it is limited in that it is designed for use during the heating season when a heater and an indoor ventilation system are in use and when the windows are closed to seal in the environment in the home. The device is also restricted by climatic conditions in that it can only be used effectively in relatively dry climates where the moisture content of outside air is normally lower than the desired moisture content of inside air. For this reason the system is less suitable for use in areas that experience a regular extended rainy season of several weeks or more during the heating season. Moreover, the system uses complex electronic circuits that control the system. Such circuits are difficult to program and maintain. 
     U.S. Pat. No. 5,881,806 to Rudd discloses a system that is a fan recycling control system for using the existing air distribution fan and ducts of an air conditioning system, having heating and/or cooling and/or humidifying and/or dehumidifying operating modes. The system is designed for the periodic distributing of ventilation air and mixing of air throughout the interior air space served by the air conditioning system during periods in which the air distribution fan is not operating due to lack of a positive signal from the thermostat for heating or cooling, or the humidistat for humidifying or dehumidifying, or constant fan modes. The fan recycling control provides a means for operating the air distribution fan for a first selectable time period after a second selectable time period from the end of the last operation of the air distribution fan, where the last operation of the air distribution fan could have been due to a positive signal from the thermostat or humidistat for heating or cooling or humidifying or dehumidifying or constant fan modes of the air conditioning system, or due to fan operation initiated by the fan recycling control. The system includes an outside air damper recycling control having a means to open a motorized outside air damper, for the purpose of drawing in ventilation air, each time the air distribution fan operates, and for as long as the air distribution fan continues to operate, having a means to cycle, periodically close then open, the outside air damper based on selectable time periods since it was last opened or closed, respectively, and having a means to cause the motorized outside air damper to close at the end of each operation of the air distribution fan. Although this system has many of the desired features, it has a complex set of controls to operate the system. This makes setting and operating the unit more difficult. For example, the control panel has three timers: one for fan off time, one for fan on time, and one for the damper cycle time. If any of these times are mis-set or not set, the system cannot function properly. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The instant invention overcomes these difficulties. It is a system that allows fresh air to be brought into a furnace as part of the return air circuit. This not only helps to replace stale air within a home, it also prevents air infiltration, around windows, doors, floor registers, and wall base perimeters. This occurs through normal use of bathroom and kitchen ventilation units, which cause periods of negative pressure with the dwelling. The hardware of the basic system consists of a furnace that has a return air supply duct, an intake louver and filter through which fresh air enters into the system, an intake fan mounted to the intake, an intake duct that brings the fresh air from the intake louver to the furnace and a motorized damper, installed in the intake duct, to control the flow of air. These components are controlled by a control panel that works in conjunction with the thermostat and furnace controls to operate the intake fan and damper as needed and desired for optimum operation. 
     Specifically, the system is designed to control air recirculation and to introduce filtered fresh air into residential units at various times and for specific durations selected in a 24-hour period. 
     The system also: 
     a) shares control with the thermostat of a residential forced air furnaces; 
     b) initiates air recirculation at the times selected and for durations specified by the user; 
     c) operates in conjunction with or independent of heating or cooling cycles; 
     d) operates dampers and blowers to admit and control a flow of filtered fresh air through a duct tied into the return air plenum ahead of the furnace filter, to mix with the air circulated during periods of heating, cooling and recirculation; 
     e) operates heaters, in fresh air supply ducts, to pre-heat the incoming air during periods of extreme cold weather; 
     f) controls the blowers, in air-to-air heat recovery units, and associated damper networks-used for energy savings (regardless of climate) during make-up air supply, whether used in a residence with a forced-air heating/cooling system or one with a stand-alone recirculation/make-up air system; 
     g) can operate with unit controlled voltages of various magnitudes (e.g., 120 volts and below, either AC or DC, as dictated by voltage requirements of the components used); 
     h) can use differential pressure to establish the need for air intake; 
     i) can monitor for the presence of gases (e.g., carbon monoxide) to initiate air intake; and 
     k) can shut-down air intake and recirculation upon detection of smoke using either an in-duct mounted smoke detector or other smoke or heat detection devices equipped with the necessary switch or relay contacts to disconnect the power to the control circuits. Unlike the prior art systems, which solve one problem, this system is designed to work in a variety of ways to ensure that the flow of air into a home is maintained to provide the optimum level of comfort and safety. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a mechanical schematic showing the components of the stand-alone system embodiment. 
     FIG. 2 is an electrical schematic of the stand-alone embodiment of the control system. 
     FIG. 3 is an electrical schematic of the 120 VAC portion of a second embodiment of the control system in which a furnace or air conditioning unit is incorporated into the system. 
     FIG. 4 is an electrical schematic of the 24 VAC portion of a variation of the control system of FIG.  3 . 
     FIG. 5 is an electrical schematic of another variation of the 120 VAC portion of the control system of FIG.  3 . 
     FIG. 6 is an electrical schematic of another variation of the 24 VAC portion of the control system of FIG.  3 . 
     FIG. 7 is an electrical schematic of a second optional circuit configuration of the 120 VAC portion of the control system of FIG.  5 . 
     FIG. 8 is an electrical schematic of a third optional circuit configuration of the 120 VAC portion of the control system of FIG.  5 . 
     FIG. 9 is an electrical schematic of a fourth optional circuit configuration of the 120 VAC portion of the control system of FIG.  5 . 
     FIG. 10 is a front view of the system control panel box. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawing figures, FIG. 1 is a mechanical schematic showing the components of a stand-alone air refreshing system. This diagram shows all of the parts of the system used in this embodiment. This system is intended for use where no forced air heating system is in place. It allows an operator to introduce fresh air into a building at regular intervals to break up stagnation and to remove stale air, as discussed above. 
     Referring to FIG. 1, the various hardware components are shown. These include an air intake fan  10 , and an air exhaust fan  11 . These components are ganged together to create an air-to-air heat exchanger. 
     As shown in the figure, air moves through a series of ducts  12 . The specific airflows are discussed in detail below. To establish and control these airflows, several input filter stations and dampers are provided. 
     A recirculation air intake filter  13  provides the normal recalculating air to the system from the building. Under the “on” mode of operation, this input is fed exclusively to the blower motor for redistribution into the building. As shown, the air input can also be split as discussed below. Note also that all of the air filters shown have a grill element and a filter element; for the filter  13 , these are labeled as the grill  13   a  and a filter element  13   b.    
     Next, a recirculation air filter  14  is used to put air into the building from the blower fan. This is composed of a grill  14   a  and filter element  14   b . 
     A fresh air intake air filter  15 , with grill  15   a  and filter element  15   b  is shown on the input side of the air intake fan.  10 . This filter cleans the incoming air. Note that it is designed to be positioned high on an exterior wall of a building. 
     To control the flow of air through the system several motorized dampers are used. A first damper  16  is placed in the duct after the air intake fan  10  but before the main junction with the output of the main blower fan. This damper, therefore, controls the flow of intake air into the system. A second damper  17  is positioned as shown, in the by pass duct that goes to the exhaust fan  11 . A third damper  18  is positioned at the outlet grate of the exhaust duct that comes from the air exhaust fan  11 . Note that this duct is designed to exhaust at a low spot on an exterior wall of the building. A fourth damper  19  can be installed to dump air into a space, such as a garage, to help provide additional conditioning as desired. This damper also attaches to the duct leading from the air exhaust fan  11 . 
     The main recirculation blower fan  20  is shown in the center of the main duct of the system. 
     FIG. 1 shows the ability to operate the system in two basic modes. The first mode is the standard operating mode commonly used today. In this mode, the main fan  20  pulls air from the interior of the building through the return air duct  13 , where it then flows back into the building through the filter and grill of the output grill  14 . In this case, the dampers  16 ,  17 ,  18  and  19  are closed. 
     In the second mode of operation, the new air intake system is used. In this case, dampers  16 , 17 ,  18 , and, if desired  19  are opened. In this system, the main fan  20  pulls air in through the return air duct  13 . In addition, the air intake fan  10  pulls in outside air, which is ducted to the downstream side of the main fan  20  for mixing and distribution. Simultaneously, a quantity of the return air is ducted to the air exhaust fan  11 . This heated air passes through the heat exchanger to pre-warm or pre-cool the incoming air. After passing through the heat exchanger, this air either is exhausted to the outside, or can be dumped into a less conditioned space, as desired. Airflows coming from the return air duct and air intake fan are mixed and then passed into the building through the filter and grill  14 . 
     An induct smoke detector  21  can be placed in the ductwork on the intake side as shown. In addition, a carbon monoxide (CO) detector  22 , which is not part of the ductwork system, can be installed and its contacts placed in the control circuit (as discussed below). Thus, as shown on FIG. 2, the CO detector has a set of contacts that overrides the timing portion of the circuit to cause fresh air to be introduced into the building. 
     FIG. 2 shows an elementary electric schematic diagram for the system shown in FIG.  1 . As discussed below, there are several ways to control the system to meet the needs of many different operating situations. 
     For all of the wiring diagrams, the following conventions apply. At the top of the diagram is the input to the circuit. The designations L 1  and L 2 , indicate a source voltage. In all cases, this voltage is specified as 120 VAC. Of course, other voltages can be used, with the appropriate adjustments to the circuit being made. L 1  is deemed the “hot” leg of the circuit. The identifier L 2  is the neutral or common. 
     At the top of the circuit is a set of switch contacts from the in-duct smoke detector. If it detects smoke, these contacts open and all of the system is shut down. Next, is a timer circuit. This consists of a master on off switch  22 , a timer  23  that has a timing element (discussed below) and an associated set of contacts  24 . The timing element is connected to L 2 , as shown. Again, details of the timing element are discussed below. The timing contacts  24  connect to a number of circuit elements as follows: First is a “Run” indicator light  25 , which indicates that an operating or “run” sequence is occurring. It is mounted on the face of the control panel (see FIG.  9 ). Next, three relays  26 ,  27  and  28  are connected in parallel. Note that relays  27  and  28  have isolation switches  27 -A and  28 -A respectively to remove these elements from the circuit as desired. Additionally, relay  28  has a set of contacts  27 - 1  (the first number designates the relay, the second number the contact number within the relay) from relay  27  in the line. In this way, relay  28  will not operate if relay  27  is not operating (such as if the relay isolating switch  27 -A is open). 
     In series with these elements are three sets of contacts. First, are the contacts from the CO detector, second a timer bypass switch  29  and third is the room thermostat  30 . All of these contacts are normally open. If any or all should close, the timer is bypassed and the circulation system operates without the timer. Normally, the CO detector switch  22 - 1  and the timer bypass switch  29  are not operated regularly. The room thermostat, however, operates according to ambient air conditions. The operation of the system using the thermostat is discussed below. 
     The elements shown below those discussed above are the fan motors and damper motors. These are controlled by relay contacts shown in the circuit. The main fan  20  is controlled by a normally open contact  26 - 1 . Below that is the air intake fan  10 , which is controlled by normally open contact  27 - 2 . Below that is damper  16 , which is controlled by normally open contact  27 - 3 . Below that is the air exhaust fan  11 , which is controlled by normally open contact  28 - 1 . Below that is damper  17 , which is controlled by normally open contact  28 - 2 . Below that are dampers  18  and  19 , which are controlled both by a normally open contact  28 - 3  and by a selector switch  31 . This switch is used to select which damper ( 18  or  19 ) to operate, which causes air to be dumped either outside (through damper  18 ) or to a secondary space, such as a garage (damper  19 ). 
     Normal operation of the system just described is as follows: 
     The room thermostat and the main fan  20  can be selected to run continuously or can be set to operate when the room thermostat is activated. Otherwise, the system operates by the timer. Assuming the entire system is activated as shown in FIG. 2, when the timer is activated, the intake and exhaust fans and all of the dampers (for dampers  18  and  19  only one is selected at a time) are open and operational. In the preferred embodiment, the timer is set to operate the system for 15 minutes (adjustable plus or minus) and to leave it off for 60 minutes (adjustable plus or minus). Note that such timers are readily available commercially in a variety of models. In the circuit of FIG. 2, if the thermostat activates, the timer is bypassed and the system operates until the thermostat opens. Thus, if the thermostat activates during the 15-minute timing cycle, no difference is noted. If the thermostat activates during the 60-minute quiet time, the system will operate independently of the timer. The advantage of this system is that it allows the fans to operate without the thermostat activating. 
     FIG. 3 shows a schematic for a second embodiment of the invention. In this embodiment, the system is integrated with a forced air furnace or air conditioner. In this embodiment, there is no air-to-air-heat exchanger, although one can be added, as discussed below. The primary purpose of this system is to provide a source of clean, fresh air to mix with recirculated air in a forced air heating or cooling system. It also works to refresh the air in a building by operating independently from the heating cycles of the furnace as desired. Unlike the stand-alone system discussed above, this system uses the furnace blower motor to circulate air through the building. It has an intake air filter, an intake air fan, and a duct that moves the air to the furnace. Within the duct, there is a damper to control the flow or air. As in the embodiment above, an in-duct smoke detector may be placed within the duct as a safety measure. The specifics of this system, designated as the “integrated system” are as follows: 
     Referring now to FIGS. 3 and 4, the integrated system electrical schematics are shown. FIG. 3 shows the 102 VAC system. As before, L 1  and L 2  signify the 120 VAC line inputs. The first component on L 1  is a contact  31  from an in-duct smoke detector. This contact is normally closed. In the event of smoke detection, it opens the circuit and shuts down the system. The 24-hour timer  32  is placed in the line as shown. As before, this timer has a timing component  33  and a contact component  34 . 
     Below the timer, in sequence, are the timer bypass switch  35 , the contacts  36  from the CO detector, and a contact from a differential pressure switch  37  (discussed below). These switches and contacts are connected to a running light  38  and a relay  39 . 
     Below those components are a contact  39 - 1  from relay  39  and a contact  50 - 1  from relay  50 . Either of these contacts operates an air intake fan  40 . The air intake fan  40  has a speed control switch  41  in the circuit as shown. Finally, at the bottom of the circuit is a damper control switch  42  that controls the damper motor  43 . The switch  42  has two modes: open, in which the damper is open continuously, or “off”, in which the damper remains closed, or auto, in which the damper is controlled by the contacts  39 - 1  and  50 - 1 , which operate in conjunction with the air intake fan  40 . 
     FIG. 4 is the 24 VAC circuit for this system. Beginning at the left of the figure, at the 24 VAC input terminal, the circuit begins with fuse  44 , followed by a normally closed contact  45  of an in-duct smoke detector. The circuit extends on into the furnace control to a temperature limit switch  46 , which is connected to the common terminal of the room thermostat switch  47 . From the normally open contact of the room thermostat, connection is made back into the furnace through the stack over temperature switch  48  and the power vent relay  49  to the common side of the 24 VAC supply. 
     The circuit branches after the room thermostat, where it feeds relay  50 , which then connects to the common terminal. As discussed above, the contacts  50 - 1  for relay  50  are shown in FIG.  3 . 
     The circuit branches before the temperature limit switch  46  to go to the “on” side of the thermostat&#39;s fan control switch  51 , then to contact  39 - 2  of relay  39  (FIG.  3 ), which is in parallel with the furnace blower control switch  52 . The outputs of these connect to the indoor blower relay  53 , located in the furnace. This describes the system with the thermostat&#39;s fan control switch set to “on” position, in which case the blower runs continuously but for the timed interventions from the control panel shown in FIG.  3 . 
     The other pole of the room temperature fan switch  51  is labeled auto. In this mode, the blower runs only during heating cycles, as its control is operated only when the thermostat calls for heat. The room temperature thermostat fan switch  64  is a switch located in many thermostats. It allows the user to set the circulating fan in the furnace to run continuously (in the “on” position”) or cyclically, when the furnace operates (in the “auto” position). 
     What follows is a description of system operation: the control system has an air intake fan speed control that adjusts the fan speed so that the amount of filtered outside fresh air being introduced to the system is as desired. In the preferred embodiment, the control has an adjustable range from OFF to HIGH and down to LOW. 
     Even if the Air Intake Fan is shut OFF; as long as the system is under control of the system, the furnace blower will come on and the damper will still open at times preset for air circulation to admit makeup air to counter possible inside air deficiencies. The damper also operates as usual during heating cycles. 
     System Shutdown 
     If for any reason it is desired to completely eliminate system control of the heating/ventilating system and return full furnace control exclusively to the Room Thermostat: 
     1. Switch the Furnace Blower Control switch to Thermostat. 
     2. Shut off the circuit breaker for the unit in the Power Distribution Panel. 
     3. Set the Room Thermostat Fan (RTF) switch for the desired mode of operation: 
     On=furnace blower runs continuously providing constant inside air circulation. 
     Auto=furnace blower runs only during heating cycles. 
     Note: with the system shut down, filtered, fresh, makeup air no longer enters to mix with the return air during air circulation or heating cycles to compensate for air deficiencies. 
     This describes the basic, preferred, design for the combined, intake system and furnace (cooler) system. The remaining circuit diagrams show variations of this basic circuit. These are discussed in turn below: 
     FIGS. 5 and 6 show the system discussed above with the following changes. First, the 120 VAC damper motor  43  is replaced with a 24 VAC damper motor  60 , which is supplied by a transformer  61  from the 120 VAC circuit. The second change is that the contacts  31  and  45  from the in-duct smoke detector have been replaced by jumpers  63  in cases where no in-duct smoke detector is used. Note also that this figure eliminates the CO detector  36  and the differential pressure switch  37  (see FIG.  3 ). Of course, both of these components can be added to any of the circuits shown, either alone or in combination, as desired. 
     FIG. 7 shows a variation of the circuit of FIG.  5 . In this circuit, the 24 VAC damper motor  60  is replaced with a 120 VAC damper motor  43 . This also eliminates the need for the transformer  61  of FIG.  5 . FIG. 7 is otherwise identical to that of FIG.  5 . 
     FIG. 8 is a minor variation of FIG.  7 . In addition to changing the 24 VAC damper motor to a 120 VAC damper motor, FIG. 8 shows the jumper  63  for the in-duct smoke detector has been replaced with contacts  31  for the in-duct smoke detector. 
     The final variation of this circuit is shown in FIG.  9 . Here, the damper motor and air intake fan have been replaced by a heat recovery ventilator  70 . This heat recovery ventilator is similar to that shown in FIG.  1 . In this example, the heat recovery ventilator  70  has an air intake fan  71 , an air exhaust fan  72 , an air intake fan control switch  73  and an air exhaust fan control switch  74 . These switches are designed to turn these fans on or off. The fan speed control is eliminated from this circuit. A timer bypass switch  75  has been added as shown. Note also that the bypass switch is ganged with components  35  and  35 - a . In this way, if the bypass is opened, the timer is disconnected and the relay  39  is continuously activated. All other aspects of the system remain the same as shown in FIGS. 5 and 6. 
     Operation 
     The system places a set of timed contacts in the furnace blower control circuit so that inside air automatically circulates at selected times, in sequences and for durations programmed in the timer, while it adds variable amounts of filtered fresh air that mixes into the system before furnace return air filters. When the furnace is operated by the room thermostat with the room thermostat fan switch in the “auto” position, outside air is admitted only during the heating cycles and air circulation does not continue once the demand for heat has been satisfied. When the room thermostat fan switch is “on” (as shown), the automatic recirculation of inside air takes place as programmed in the system control panel. Thus, the blower fan operates at the times specified and set into the timer. If the room thermostat calls for heat during a time that air is being circulated, the furnace blower and continues air circulation for the set-time remaining. If the thermostat calls for heat when the timer is quiescent, the timer is overridden and the blower operates. If the thermostat shuts down while the timer is still in the quiescent period, the blower stops operating. 
     FIG. 10 is a detail of the front panel controls of the control panel  80 . These controls are as follows. At the top is the Furnace Blower Control Switch (e.g.,  52  on FIG.  4 ). This switch selects two different controlling units: first is the timer, which places the furnace Indoor blower (e.g.,  40 ) under control of the timer in this position, as long as the room thermostat fan switch is set to “on”, programmed air circulation takes place in accordance with the timer settings. The room thermostat controls heating in conjunction with the timed blower operations (as discussed above). 
     With the switch set to the thermostat position, the room thermostat takes full control of furnace for all functions, e.g., burner operation during calls for heat, etc. With room thermostat fan switch control of furnace for all functions, e.g., burner operation during calls for heat, etc. With room thermostat fan switch  51  set to “on”, the furnace blower would run continuously but for intervention by the system&#39;s timer which limits introduction of filtered outside air to set periods and during heating cycles. With room thermostat fan switch  51  set to “auto”, the furnace blower  40  runs only during the heating cycle, with filtered outside air added only during the cycle. 
     The bypass switch  35  is used to over-ride the system control unit. If the switch  35  is in the “on timer” (normal) position, the timer is used to operation of the damper, air intake fan and the furnace blower. In the “all run” position, the damper opens, and the air intake fan and furnace blower run continuously (but only if the room thermostat fan switch  51  switch is also in the “on” position). 
     The damper switch  42  can set the damper to be off, open, in which case it is open all the time, or on “auto”, where it opens and closes by timer operation. 
     The air intake fan speed control  41  allows the user to manually set the fan speed. As discussed above, the speed control can be set to operate on low speed or “off” when outside ambient air temperatures are low and higher when outside air temperatures are warm. 
     Finally, the indicator light  35  illuminates when an air introduction/circulation cycle is in progress. 
     The present disclosure should not be construed in any limited sense other than that limited by the scope of the claims having regard to the teachings herein and the prior art being apparent with the preferred form of the invention disclosed herein and which reveals details of structure of a preferred form necessary for a better understanding of the invention and may be subject to change by skilled persons within the scope of the invention without departing from the concept thereof.