Question about Honeywell Electronic Programmable Heat/Cool Thermostat - CT3300A100 Heater

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Honeywell Chronotherm III Thermostat does not show temperature

We have a Honeywell Chronotherm III thermostat. The time and the temperature is not showing on the display. I do believe the batteries are working because the display is still showing some of the settings. None of the buttons respond to reset the time. The fan does will blow in the on position. I switched the thermostat with another one in the house, and it appears that the problem is just with this particular unit. Any tips?

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  • 9 Answers

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,

Posted on Dec 12, 2008

  • 252 Answers

SOURCE: I have an old Honeywell thermostat (Chronotherm

you are not geting AC power from the furnace

Posted on Sep 27, 2009

roniecon
  • 6826 Answers

SOURCE: What does -AC mean on a Honeywell programmable

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

Posted on Nov 08, 2009

  • 308 Answers

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

Posted on Jan 23, 2010

SOURCE: Honeywell Chronotherm III Thermostat shows time of

Hello,Message AC means that there is no 24volt power to the thermostat from the unit/s it controls.Please check all switches & curcit breakers to the unit/s as there is no low voltage power to the thremostat from the transformer in the unit/s.Maybe got turned off by mistake.

Posted on Nov 01, 2010

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Honeywell Chronotherm III Thermostat shows time of day-AC. No heat in the 2 zones with this message.


Hello,Message AC means that there is no 24volt power to the thermostat from the unit/s it controls.Please check all switches & curcit breakers to the unit/s as there is no low voltage power to the thremostat from the transformer in the unit/s.Maybe got turned off by mistake.

Nov 01, 2010 | Honeywell Electronic Programmable...

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Honeywell Chronotherm III Lowest temperature setting is 45F Any way of getting it lower thanks


If you are successful in setting it lower , you will end up with frozen pipes and large plumbing bills. Since the internal walls of a building can be as much as 20 degrees lower, that the room.
And the location of the thermostat, may be even warmer, that that of the areas where the plumbing is.
Now what is your reason for wanting the thermostat operate at a lower temperature?

Jan 31, 2010 | Honeywell Heaters

2 Answers

I have a very old manual Chronotherm thermostat.


No. The clock is just the set back timer. Pull the battery and use the stat manually.

Jan 26, 2010 | Honeywell Electronic Programmable...

1 Answer

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

Jan 22, 2010 | Heaters

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