"- ac" message on thermostat


Old 02-01-13, 08:39 AM
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"- ac" message on thermostat

My propane furnace stopped working properly today. It is cycling on and off, unable to hold the temperature it is set for, but it is able to hold a lower temperature. It runs for a bit and then cuts out. The Honewell thermostat shows the message: - AC everytime it cuts out.

I tried turning heat on and off on the thermostat. I tried the fan in both the on and auto positions. I also flipped the breaker for the furnace off and back on.

I read on another site that the - AC message means that the thermostat isn't getting power from the furnace. If it works intermittently and shows that message intermittently, is it losing power and then getting it back? What are the most likely causes of this problem?

The furnace is pretty old. I replaced the draft fan a couple of years ago, so that is probably okay.


Last edited by stickshift; 02-01-13 at 09:59 AM. Reason: augmented title
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Old 02-01-13, 08:48 AM
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Some of the older Honeywell T'stats will show AC when there is a furnace failure.

A make and model number of your heating unit would be an excellent start. The model number can be found just behind main access panel on front of unit.
Old 02-01-13, 01:55 PM
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Guessing doesn't cut it as a diagnostic method.

If you have the instruction manual for the thermostat, you should be able to look up what the AC display is indicating.

If you don't have the manual, try Googling the makle and model of the thermostat to see if you can find the instruction manual on line.

If you do, I'd make a copy of it for future reference and look up what the AC message is for.
Old 02-01-13, 02:29 PM
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Some honeywell t-stats (the chronotherm III is one) take their power directly from the heating control circuit. -AC on those stats does indicate that there's no power.

Furnaces have high limit switches which in some cases cut off power to the heating control circuit.

Your high limit switch might be tripping. Insufficient airflow is the most likely cause.

Observe what happens at the furnace when it stops working.
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