I have a Philco 80,000 btu two stage with variable speed blower, high efficiency furnace with a two wire thermostat, on/off. The furnace 6.5 years old and when the thermostat calls for heat the furnace cycles normally but only for 5 minutes then it shuts down and starts the whole process over again until the thermostat is satisfied. I have bypassed all the sensors except the pressure sensors as they are a little harder to bypass. I have wired across the thermostat wires too.
I have checked the flame sensor by pulling the wire off and it still senses a flame for about 2 seconds then shuts off.I have tried the fan on different speeds and even run the furnace with the lower door off. It's an updraft model. I think it has been like this from new as I thought it was the way a 2 stage worked but then I talked to a few people and they said no it should keep running until it reaches set temp. LED green/red/yellow. Green and red led never flicker So there you have it.
I hope some one can help me fix this thing as next would be the circuit board and more money, for now it will just keep on cycling.
Thank You for any help.
Paul
Furnace keeps cycling on and off until it’s up to set Temp
blowerfurnace
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Best Answer
this could be caused by a few things. Does the main gas valve loose power for that time that it cycles off. If it continues to receiver 24-volts during the entire cycle, the problem is in the main control valve. I has to be renewed. If the power is interrupted, some thing is breaking the signal. The transformer could be overheating and opening a coil momentarily. Jumper the red to the white on the board then find out where you are loosing the signal by jumpering across every component with the meter on a low ohms scale. I have seen boards that will open intermittently. A tarnished flame sensor will do this. buff it good with scratchy pad of fine sand paper. Good Luck.