Honeywell chronotherm III thermostat won’t turn on A/C unit. The display keeps flashing “-AC”, but the manual doesn’t explain this signal

SOURCE: -AC appears in the display of chronotherm III

I have had the same problem- twice. It was a similar but different problem each time. The -AC means the ac control voltage isnt making it to the thermostat. In my case both times it was a switch in the control circuit that wasn't closing properly. The easiest was the front cover switch. If the front cover is removed from the furnace or has vibrated loose, the switch will open, preventing the furnace from running. The other instance I ran into was a flow switch on the exhaust blower which ensures the there is flow OUT the chimney. The switch wasn't functioning, so the furnace wouldn't start. The blower would come on for a short time, but the burners wouldn't light.

Hope this helps,

SOURCE: I have an old Honeywell thermostat (Chronotherm

you are not geting AC power from the furnace

Ronny Bennett Sr.

• 6988 Answers
• Posted on Nov 08, 2009

SOURCE: What does -AC mean on a Honeywell programmable

If it is just a heater,it means no power.

SOURCE: my honeywell chronotherm III says -AC any ideas

The problem is your 24v power source. I am unsure as to why you put "other than power source or batteries". If you did indeed check that your thermostat is receiving 24v, then I would say that you have a bad thermostat. Hope this helps

SOURCE: A/C won't come on, replaced

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