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90+% gas furnace sequence of operation
1. On a call for heat (thermostat closes), a signal is sent to the IFC and sequencing begins.
2. The IFC electronically looks at the pressure switch.
A. If the pressure switch is open (power cannot pass through), the IFC starts the inducer.
B. If the pressure switch is closed (power passes through), sequencing stops and IFC goes into lockout mode. Lockout code will displayed on the IFC by blinking lights on the module. (key to codes should be listed on the furnace panel).
C. Most IFCs will attempt to try again 10 to 15 minutes later.
3. Once the inducer starts, the pressure switch should close.
A. If the pressure switch does not close, the inducer will continue to run for 1 to 2 minutes then will lockout and try again 10 to 15 minutes later.
B. If the pressure switch closes, the prepurge will begin.
4. The IFC will look at the limit switches.
A. If any of the limit switches are open, the furnace will lockout, the circulating fan will come on and a lockout code will be displayed.
B. If the limit switches are closed, the the warmup time for the HSI begins. A yellow glow will come from one side of the burner box.
5. The HSI warms for from 10 to 45 seconds.
6. The gas valve opens for 4 to 7 seconds to allow gas to pass into the burners. This is called the "trial for ignition"
7. If the burners ignite, the flame is proven, usually by flame rectification, during this time. The flame is sensed by a flame rod on the opposite side of the burners as the yellow glow. There should be a blue light in the burner box.
8. If the burners fail to ignite, the furnace shuts off the gas supply, goes through a purge cycle and attempts to light the burners again.
9. The furnace tries a total of 3 times, then if flame is not established goes into hard lockout. A lockout code will be displayed on the IFC.
10. After 1 hour, the furnace will start the entire sequence again to try to fire off.
11. If the flame is proved, a timer is started to delay the start of the circulating fan for 30 to 40 seconds.
12. When the call for heat is over, the burner extinguishes. The circulating fan continues to run for 2 to 3 minutes to clear heat from the heat exchanger.
13. This sequence is generic. Some furnaces will vary in their actual sequence, but will be substantially the same.
To see some error codes for common furnaces click here
2. The IFC electronically looks at the pressure switch.
A. If the pressure switch is open (power cannot pass through), the IFC starts the inducer.
B. If the pressure switch is closed (power passes through), sequencing stops and IFC goes into lockout mode. Lockout code will displayed on the IFC by blinking lights on the module. (key to codes should be listed on the furnace panel).
C. Most IFCs will attempt to try again 10 to 15 minutes later.
3. Once the inducer starts, the pressure switch should close.
A. If the pressure switch does not close, the inducer will continue to run for 1 to 2 minutes then will lockout and try again 10 to 15 minutes later.
B. If the pressure switch closes, the prepurge will begin.
4. The IFC will look at the limit switches.
A. If any of the limit switches are open, the furnace will lockout, the circulating fan will come on and a lockout code will be displayed.
B. If the limit switches are closed, the the warmup time for the HSI begins. A yellow glow will come from one side of the burner box.
5. The HSI warms for from 10 to 45 seconds.
6. The gas valve opens for 4 to 7 seconds to allow gas to pass into the burners. This is called the "trial for ignition"
7. If the burners ignite, the flame is proven, usually by flame rectification, during this time. The flame is sensed by a flame rod on the opposite side of the burners as the yellow glow. There should be a blue light in the burner box.
8. If the burners fail to ignite, the furnace shuts off the gas supply, goes through a purge cycle and attempts to light the burners again.
9. The furnace tries a total of 3 times, then if flame is not established goes into hard lockout. A lockout code will be displayed on the IFC.
10. After 1 hour, the furnace will start the entire sequence again to try to fire off.
11. If the flame is proved, a timer is started to delay the start of the circulating fan for 30 to 40 seconds.
12. When the call for heat is over, the burner extinguishes. The circulating fan continues to run for 2 to 3 minutes to clear heat from the heat exchanger.
13. This sequence is generic. Some furnaces will vary in their actual sequence, but will be substantially the same.
To see some error codes for common furnaces click here
90+% efficient furnace troubleshoot and diagnosis, If your furnace has signal light on but has no heat
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This is an IFCThis control has a light on the lower left. It will blink a code to tell you the status of the control and furnace.
There is a key to the code blinks on the front of the control. |
This is a different style IFCThis control will also blink a code. The code key for this control will be on one of the panels.
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