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Kenton Guidry
Kenton Guidry, HVAC Technician
Category: HVAC
Satisfied Customers: 966
Experience:  12 years install/service experience.
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Aux heat on thermostat when heater shuts off

Resolved Question:

Heater keeps shutting off and thermostat says 'aux heat'. What do I do?

Submitted: 7 years ago.
Category: HVAC
Expert:  Kenton Guidry replied 7 years ago.

Do you have a heat pump with a back up heat source?
What is the temperature outside?

Customer: replied 7 years ago.

I do not have a heat pump. As far as I know, just a central heat and air system.. It was working fine. I went out of town for new years came back and it is freezing inside. Outside temp is 28 degrees.

I shut off circuit breaker, turned gas controller off and on and turned back on circuit breaker..Blower comes on and about 45 seconds later igniter goes off. But burner only stays on for 15 or 20 mins.

Expert:  Kenton Guidry replied 7 years ago.

Do you have a flashing light anywhere on the furnace?
Can you tell me the condition of your air filter?

Customer: replied 7 years ago.

Yes, the circuit board has two flashing lights on it. Didn't check the filter.

Expert:  Kenton Guidry replied 7 years ago.

Please look on each side of both panels located on the furnace for an error description for the flashing light you have.
Also take a look at the filter.

Customer: replied 7 years ago.

Filter good limit switch open is light code.

Expert:  Kenton Guidry replied 7 years ago.

There are a few possible causes for this.
One would be a dirty filter.
Another would be the limit switch is faulty.
If the blower failed to come on, it would also cause this.
Does the blower motor come on when in the heating mode?

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Customer: replied 7 years ago.

Yes, blower came on after reset, before ignition.

Expert:  Kenton Guidry replied 7 years ago.

This is what I would like to do.
Turn the system to 'off'.
Then turn the fan to 'on'.
Let the furnace cool down.
Then after three minutes or so turn the fan to 'auto', and the system to 'heat'.
Watch the start up at the furnace.
Let me know what happens.

Customer: replied 7 years ago.

It started as normal. Will give a few minutes and see if it stays on.

Expert:  Kenton Guidry replied 7 years ago.

Ok. Let me know.

Customer: replied 7 years ago.

So far so good. This is the longest it has stayed on so far. If it does shut off again is there a next step?

Expert:  Kenton Guidry replied 7 years ago.

If the furnace shuts off after this, I would recommend looking into the high limit switch being the problem.
If the blower motor is running fine, the filter is good, then I would go with the high limit.

Customer: replied 7 years ago.

Is that something that needs to be cleaned or changed?

Expert:  Kenton Guidry replied 7 years ago.
It is a part that is changed.
Customer: replied 7 years ago.

Okay. Thanks for your help..

Expert:  Kenton Guidry replied 7 years ago.
You are very welcome.

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