Furnace shuts down intermittently when calling for heat
I have a thermo pride gas furnace that will shut down when the thermostat is calling for heat, it might run for a day fine, might only run half an hour. I reset the furnace and still have the same problems. I tried pushing in the pressure switch with a screw driver while its calling for heat, the inducer kicked on and it cycled fine, and its running now. What is stopping the sequence. Intermittently? Any ideas? thanks
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Re: furnace shuts down intermittently when calling for...
The flame sensor is not working properly, it is not allowing the ignitor to ignite, and this is making the system shut down, the flame sensor has obtain a build up of carbon, you will need to remove the flame sensor, and take a little piece of sand paper to remove the black carbon from the flame sensor stem, clean until the stem is shinning, replace the flame sensor and turn the furnance back on this will correct the problem.
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An intermittent problem can take patience and time. Process of elimination too. If you are unfamiliar with servicing and repairing a gas furnace, you may want to leave that chore to a professional. Now, what exactly is coming back on after the set temp is reached? Blower only or is it firing the furnace also. My experience says it's just the blower. This is caused by... Furnace overheating, then shutting down, then bringing blower on to cool it down. Probably not your scenario. It's possible the control board is acting up. Highly possible that you have a high temp limit with a loose connection, or needs to be replaced. Schematic will call it HS or LS, something like that and it should have 2-6 of em. Possible that the G wire from the t-stat is shorting to R, and bringing the fan on. Try posting more info? Model & serial number. Fan cycling or furnace? Have you checked new t-stat installer configuration? Hope this helps! And work safe
Turn of the system at the thermostat. If you turn off the blower, assuming you mean at the tstat, there's no need to turn off the pilot. If blower is running and won't stop, even if turned off at tstat, turn off power by any means necessary, and pilot does not need to be turned off. A "standing pilot" is made to stay lit at all times, running or not.
How many degrees before it reaches desired temp, does it shut off? A thermostat is designed to shut the furnace off a couple/ few degrees before it reaches set temp. It has what is called a heat anticipater, which shuts the burner down, and lets the blower use the remaining heat from the heat exchanger to bring it up to desired temp making it more efficient. If it is more than a few degrees, replace the thermostat. You can jump out the red and white wires to confirm the faulty thermostat. If by doing this, the furnace never shuts down, until you separate the red and white wires. That confirms a faulty thermostat. Hope this helps someone.
On the call for heat from the thermostat the inducer should come on first, a pressure switch proves that the inducer is moving enough air up the stack, closes and energizes the ignition and burners come on. 30 to 60 seconds later the blower comes on. Everything should stay on as long as the thermostat is still calling for heat. When the thermostat is satisfied the burner and inducer shut off and the fan will run to remove the heat from the heat exchanger and then shut off. Cycle complete. Some inducers have a post purge so when the thermostat is satisfied the burner shuts off, the inducer runs for 30 seconds or so and then shuts off. I am thinking that your furnace may be running on the limit switch and when the burner is off but the inducer stays on, maybe the limit is shutting of the inducer. Check your air filter and make sure all your heat registers are open and that the return air grill is not blocked.
Could be a defective thermostat in the plenum.
When a gas furnace shuts down the fan(blower) will run until the plenum cools to the safe range. Usually 1 and 1/2 minutes.
Intermittent operation is a defective thermostat or relay.