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Old 07-09-2006, 12:04 PM   #1
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Thermostat Wiring Problem

I have a Honeywell CT31A thermostat that I bought because it was compatible with my wiring which is 1 red and 1 white. I have a Coleman Presidential III model #7970-856 heater/AC. I wired the red to the "R" terminal and the white to the "W" terminal. My heater had worked fine in the winter, but my AC won't come on. The only way I can run it is if I click the switch on the AC to cool. It stays on and the only way to turn it off is to click the heat switch. I followed the directions for installing the thermostat, so what could be wrong? If anyone can help me I would appreciate it! :o


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Old 07-09-2006, 11:36 PM   #2
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Two wires indicates Heat only thermostat setup. Heat/AC setup use four or more wires, Pwr, heat control, Ac control, fan control.:confused:


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Old 08-15-2006, 09:22 AM   #3
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Red-white setup is heat only. When closed it means a call for heat. My thermostats are Ritetemp 8030C, but I believe that wiring convention is same throughout.

Red is a sort of shared wire, and can also be used in conjunction with 'Common' (blue in my case) to bring 24VAC power to the thermostat so it does not use up batteries. Thermostats also often have separate RC and RH terminals(connected by a jumper wire), in case heating and cooling units are separate and there is a red wire coming in from each.

Heat is controlled by W (white) wire, which is closed to red when thermostat calls for heat.
Cooling is controlled by Y (yellow) wire, which closes to red when thermostat calls for cool.
And fan is controlled by the G (green) wire.
There are other, more complex setups with secondary heating/cooling units etc.

If your unit is heat/cool, it should at least have R, W and Y terminals, likely G as well. The problem it seems, is the wiring - is wiring older than the HVAC unit? If so, you need to upgrade it. I recommend 18/5 thermostat wire. I recently bought a 250' spool of Honeywell 18/5 on ebay for about $50.

Looks like you'll need a more advanced thermostat as well.
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