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80% furnace troubleshoot, power available to furnace, no heat
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When troubleshooting, understand you are working with potentially lethal voltages and a highly flammable gas. If you do not have the ability to do these operations safely, do not attempt them.
No burner ignition click here
Burner stays on for a short time (4 to 7 seconds) Check here
Failure light codes Check here
Pressure switch problems click here
Inducer or combustion fan problems click here
High temperature problems click here
Control board (IFC, brain board) problems
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On the left are IFC controls. The IFC is a control that is essentially the brain of the furnace. The controls on these furnaces are designed to blink a code to assist in troubleshooting.
The code will not always tell you exactly what is wrong, but will tell you the area in which to look. When the furnace has a call for heat, the IFC begins a sequence of operation in order to safely start and operate the furnace. If at any time the IFC is not satisfied, the unit shuts down. |
In order to troubleshoot a furnace, you need to know how it is supposed to work. If you don't know how it should sequence, you can't locate where the sequencing stops and the failure begins. Below is the sequence of most modern IFC controlled furnaces.
80% furnace sequencing
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.
If you determine the IFC has failed.
The IFC is specific to the furnace brand and model. It is always best to replace with a factory part.
If the factory part is not available or must be specially ordered, check out this video on a universal replacement part.
If the factory part is not available or must be specially ordered, check out this video on a universal replacement part.