Abstract:
A furnace fan control for providing operation of the fan after the termination of the furnace burner in an adaptive manner to provide for longer fan operations after the burner turns OFF, as the burner ON time increases.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     For many years forced air heating systems wherein a furnace has a fuel burning apparatus heating air and the air is circulated to a space in which the temperature is being controlled by a circulation fan, have used temperature responsive fan controls such as the L4064B Fan and Limit Controller shown in Instruction Sheet 68-0024-1, Rev. 1-85, of Honeywell Inc. In such a system, when a burner operates and the temperature of the furnace or plenum reaches some predetermined temperature, the air circulation fan is energized and remains energized until the operation of the heating device is terminated and the temperature of the air in the plenum drops to some predetermined lower temperature. The temperature for starting up the fan and stopping the fan has some temperature differential which is generally preset in the fan control. With such fan controls, the placement of a temperature responsive element of the fan control in the furnace plenum is quite critical to prevent the fan from operating for a longer time period than necessary and circulating cold air to the space. 
     Other types of fan controls make use of timers which, when the heating device is energized and the plenum air temperature increases, the fan is turned on by a temperature responsive element and the termination of the fan operation takes place after some predetermined time. Still other types of fan controls are completely time controlled in that they turn on the fan a predetermined time after the heating apparatus is energized and turn off the fan a predetermined time after the heating apparatus is turned off. Such time controlled fan controls must be tailored to the particular furnace and are not universally adapted to provide the most satisfactory operation for a furnace. 
     The present invention is concerned with an adaptive time control for the turn off of a furnace circulator fan. The furnace is normally turned on through a conventional room thermostat and at the same time a circulation fan is energized. Depending upon the length of the time of operation of the heating apparatus or burner, upon the termination of the burner, the circulation fan is maintained energized. If the furnace is operated for longer periods of time and a considerable amount of heat is stored in the furnace, after deenergization of the furnace, the circulation fan will be maintained for a longer period of time. 
     Specifically, the invention concerns a timer control which is adapted for use to control the continued operation of the circulation fan after the burner is turned off having a first timer which has a high and a low timing count fill rate and a second timer which has a high fill rate. The first timer is reset upon receiving an input signal and provides an output signal for energizing the circulation fan. Simultaneously with the operation of the circulation fan, the second timer is energized. Upon termination of the burner operation by removal of the input signal to the first timer, the first timer is energized at a high counting fill rate but, depending upon the length of time the burner has been operating, the second timer can adjust the fill rate of the first timer to provide a period of circulation fan operation dependent upon the burner ON time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic of the furnace fan control; 
     FIG. 2 is a graphical representation of the operation of the fan control; 
     FIG. 3 is a circuit diagram of the fan control timers; and 
     FIG. 4 is a power supply circuit for the furnace fan control. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the timer control or furnace fan control 10 is shown connected to a circulator fan 11 which might be an air circulation fan of a temperature conditioning apparatus or furnace having a fuel or gas burner supplying heated air to a space. Fan control 10 receives an input signal at 12 indicative of the operation of the temperature conditioning apparatus or burner when a need for heat is called for by a conventional room thermostat. A power supply 13 receives alternating current voltage from a source of power over circuits 14. Power supply 13 furnishes power over circuits 15 to a timing circuit 20 which controls the operation of a relay 21 connected in the power supply circuit of fan 11. 
     By means of the timing circuit, the operation of the circulator fan, after the burner operation is terminated, is controlled in response to the burner ON time as graphically shown in FIG. 2. As the burner ON time increases in total minutes, the fan operation, after the turn off of the burner, is continued for a time period of minutes as shown by the graphical representation 22. Specifically, at burner operations of up to two minutes, the fan is operated for two minutes shown at 23 after the burner is turned off. When the burner operates for a period of time greater than four minutes, the fan is operated for four minutes, as shown at 24 after the burner is turned off. Between the two minute and four minute operation of the burner, the fan operation is linearly changed between the two minute and the four minute operation as shown at 22. 
     The specific circuit of the preferred embodiment is shown in FIG. 3. Two timer chips or first and second timing circuits 30 and 31 are of a conventional type known as the 4541 timers and available on the commercial market as CD4541 Programmable Timer with Oscillator made by National Semiconductor Inc. and shown in its 1984 CMOS Data Book. Timer chip 30 has an input signal circuit at terminal 6 over circuit 45 which is produced upon burner or temperature conditioning apparatus operation at the input 32 whether it be the thermostat or some circuit closing upon burner operation. The presence of an output signal at 45 resets timer 30 and provides an output at 33 to energize fan relay 21. Contacts 21A of the fan relay then cause energization of circulator fan 11. 
     Timer 30 has a high rate of count to fill the timer in two minutes and a low rate of count to fill the timer in four minutes. The rate of count depends upon the resistance of circuit 34 and the input at 35. When the signal is removed from 45, timer 30 will begin to fill or count and the signal at 33 will remain to keep the circulation fan energized and provide a variable delay until the timer is completely filled. 
     Timer 31 has a counting rate to fill in four minutes. Upon receiving an input when the relay 21 is energized at 40, timer 30 is reset and begins a counting fill. When timer 31 completely fills after four minutes, an output at 35, which is connected to the circuit 34 of timer 30, is provided to change the counting rate of timer 30 by modifying the resistance circuit 34. 
     DESCRIPTION OF THE OPERATION 
     Referring to the graph in FIG. 2, let us assume that the burner ON time is one minute. Upon energization of input 45 to timing circuit 30 in FIG. 3, the timing circuit is reset and circulation fan relay 21 is energized. After the one minute burner operation the input signal at 31 is terminated and the timing circuit is allowed to count and fill at the fast rate to provide for the two minutes of fan operation before the signal at 33 is removed. The second timing circuit has no particular effect on the one minute burner operation. Let us select a burner ON time of five minutes, as shown in FIG. 2. With the resetting of the timing circuit 30 and the energization of the circulation fan relay, operation would be similar to the one minute burner cycle. The energization of the second timing circuit 31 again takes place. As the second timing circuit fills up in four minutes, after four minutes of operation, an output is provided at 35 to modify the resistance of circuit 34 and change the fill rate of timing circuit 30 to a slow fill rate and maintain the energization of fan relay 21 after the removal of input signal 30 at 45 for four minutes, as shown in FIG. 2. To obtain the sloping characteristic 22, assume that a three minute operation of the burner takes place. With a three minute burner operation, the second timer circuit 31 would only be three-fourths filled so that upon termination of input signal 45 by the termination of the burner operation, both timing circuit 30 and second timing circuit 31 continues to fill. The timing circuit 30 is filling at a high rate for the next minute, and timing circuit 31 completing its fill in the next minute. The output at 35 modifies the rate of fill of timing circuit 30 and thus the slower rate of fill would make the second half of fill of timing circuit 30 at a two minute fill period. This then keeps relay 21 energized for a total of three minutes to provide the characteristic of line 22 in FIG. 2.