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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

  • What model is the furnace? – ThreePhaseEel Oct 29 '17 at 4:58
  • Why is pressure sensor hard to bypass? On my 1984 furnace you just put an alligator clip between the two pressure sensor leads, and the furnace thinks the exhaust fan is always running. A carbon monoxide detector in the living room will see us through the winter; until I can reasonably try applying a fix with a new sensor. – Wayfaring Stranger Dec 19 '17 at 0:35
  • Some control boards look for a change to make sure pressure / flow sensors have not become stuck. The control board may be looking for a transition on the flow/pressure switch and if not a safety shutdown prior to a over fire issue that could damage the fire box. – Ed Beal Feb 19 '18 at 15:07
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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.

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When a furnace cycles too much as you have described in your post the problem is usually the setting of the "heat anticipator". This is a setting on your thermostat that controls the on time of the burners and controls the cycling time. Pull the cover off of your thermostat and look for a small lever and pointer with settings from about .02 to 1.2. Now you need the recommended setting of this anticipator. If you have paper work from the furnace, look for the setting in that paper work or call the manufacturer of the furnace for the setting. There are other ways to determine the setting by checking the amperage draw of the 2 heat wires that are attached to the thermostat. If nothing else works call a licensed HVAC company and have them adjust the setting for you. They could also perform a "furnace service" while at your home. If you "GOOGLE" the term "heat anticipator setting" you can find numerous writings and U-TUBE videos of how to perform the anticipator adjustment and setting. Once this is done, if you are still having cycling problems, you can check the other things that have been recommended in other comments and posts.

  • One more suggestion, and this comes from servicing large commercial units. Sometimes the flame sensor works erratically and it's ability to send the proper signal to the control panel changes. Everything will check out okay but the unit continually short cycles. The answer was to replace the flame sensor with a part specified by the manufacturer and not a generic sensor. I have only seen this happen a few times but it does happen. – d.george Oct 31 '17 at 9:46

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