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I’m having issues with my furnace sometimes not kicking on.

I hear the furnace start, But then it will not fire.

However, If I go down to the basement and flip the power switch off and on the furnace starts and runs great. It happens at least a few times a week.

Is the problem in the thermostat or the furnace?

Where should I start troubleshooting?

It is an older Natural gas, forced air furnace that works great when it is functioning properly. Installed in 1995, I think its a Honeywell.

  • 3
    @Alaskaman: The "not" you added is incorrect. The furnace does, in fact, operate after resetting the power. – user2357112 supports Monica Dec 20 '18 at 0:07
  • Scott B, "I hear the furnace start, But then it will not fire" Sounds like it is not working. i edited the title is that more what you mean to say? – Alaska Man Dec 20 '18 at 19:23
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Your furnace controller board is very likely indicating a trouble situation and shutting itself down. This is the equivalent of a car turning on its check engine light and not wanting to move. Shutting the main power off clears the error code and allows it to work temporarily but doesn’t fix what’s actually at fault.

First, change out your furnace filter. Many error codes are simply because the homeowner never changed the filter. Check the manual to see if there’s a filter timer reset button.

If the shutdowns still happen then the next time the furnace shuts down, keep the power on to the furnace so that you can read the error code. Your furnace should have a visible status led that is blinking (otherwise carefully take the cover off your furnace, avoiding any live wires, and look at the controller board). Count how many times the status led is blinking each cycle (sometimes how quickly it is flashing is also important). Reference your service manual to diagnose the error code. You may have a faulty sensor, sticky motor, bad controller board, too many closed dampers, a blocked air return, incomplete combustion, cracked heat exchanger, blocked condensate line, etc etc. some of these things are life-critical situations. The blickenlights will tell you.

Ps. Since this is a gas furnace, make sure your carbon monoxide detectors are working and don’t continue to operate it in this unstable and potentially dangerous state. It could be leaking carbon monoxide into your home which leads to permanent nighty-nights.

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    Agree with everything above. Usually on the inside of the furnace cover is a diagram of the electrical schematic and a table listing what the fault codes are. Like @RoboKaren states, count how many times the LED is flashing and then check the table to see what the error is. – EL MOJO Dec 19 '18 at 22:12
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    Also, good to stress that opening the cover of the furnace with the power on should be done with the utmost care. There will be live wires inside and the furnace could potentially start an ignition sequence at any point. If you can look through any openings to see the LEDs without removing the cover, so much the better. I'd at least make sure the thermostat was set in a way that it would not call for heat if opening live. – J... Dec 19 '18 at 23:18
  • Thank you all for your very quick response. I will replace the filter this weekend and have a look and report back. thanks again – Scott B Dec 20 '18 at 14:20
  • @ScottB ok. I wouldn’t use your furnace until then. And if you do do work on it yourself, have an offsite friend check in by phone with you every so often to make sure you haven’t gone nighty-night. – RoboKaren Dec 20 '18 at 17:17
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    @RoboKaren I had similar symptoms and it was the capacitor failing to start the fan. – Yolo Perdiem Dec 25 '18 at 18:53

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