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Category: HVAC
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Experience:  15yrs in the HVAC industry with certificates and degrees.
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Furnace thermostat will not turn on the heat.

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My furnace won't start when the thermostat calls for heat. The draft inducer motor never stops. I can get the ignitor to start by shutting off the power for a few seconds. When I do that (power off then back on) the ignitor glows, the gas comes on and the furnace runs normally until the next time heat is called for.

Have you watched it when it calls for heat the next time to see if it tries to light again? Does the ingitor come on the second time?
Customer: replied 4 years ago.

I have not tried that. It is running now so I will have to wait until it stops at 72 degrees. And then find somebody to turn the thermostat up while I watch it in the basement.

Yes do that and get back to me.

Customer: replied 4 years ago.

Your tester won’t work because the thermostat has already called for heat and failed before I crank up the temperature to look for a start. My thermostat is set at 72. The temperature went down to 70 when we tried your test by turning it up to 75. It did not start until I turned the power off and then on again. I think what I would have to do is set the thermostat to a low number where the room temperature is much higher and then try your test again. If that is correct, let me know and I will do it that way.

You may have an issue with the thermostat itself. Make sure the batteries are replaced on the thermostat. Also you can take the thermostat off the wall so you can see wires. If it is not starting, twist RH and the W wires together when it does not start or jump the wires at the terminals with a paperclip. See if it comes on. If the furnace starts up just fine I would say you have a bad thermostat.
Customer: replied 4 years ago.

With the room temp over 73 we turned the stat down to 55 to make sure it was off. With me watching the furnace and my assistant turning the temperature of the thermostat up to 85 there was no start or attempted start. I did my usual by turning off the power to get a start, which worked as usual. The thermostat test above I don’t think I will be able to do until tomorrow. Let me know if the above test requires the thermostat test.

Yes I believe you have a thermostat problem. You have to manually jump the wires at the thermostat or at the furnace board. If it starts up you have a thermostat issue.
Customer: replied 4 years ago.

I have changed the batteries in the stat and will try jumping the terminals tomorrow. I will report to you then.

Ok great!
Customer: replied 4 years ago.

Okay will do the jump test tomorrow. Meanwhile think about how the water drain line to the sewer would affect our problem if clogged.

It should not unless you have a condensate sensor that has been activated by the back up in water. Normally it would never come on though. One of the other good techs on here brought up a great point. If the inducer motor is never shutting off, then it will never be able to call for heat again. If that is the case, then the relay on the control board is bad and stuck closed for the inducer motor.

Customer: replied 4 years ago.

My thermostat has terminals RC (red wire) and W (white wire) I assume you wanted me to jump the red and white wire. I did jump the red and white, which had no effect. While holding the jumper in place I tried my power off and back on trick and got a re-start. As soon as it started I pulled the jumper and the furnace shut down. Meanwhile, the inducer motor keeps running. Is the inducer motor relay soldered to the board or is it a plug-in relay?

The inducer relay would be part of the board. If that is happening you will need to replace the board. That for sure is the issue here.
Customer: replied 4 years ago.

You are 100% sure it is the board?

Yes it is the board.
Customer: replied 4 years ago.

Okay, I am on my way to pick up the $165.00 board. Will report back to you after it is installed.

Ok no problem.

Customer: replied 4 years ago.

The board is replaced and everything seems to be working, I will confirm after a few more cycles.


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