changing to programmable thermostat


Old 12-13-05, 09:18 PM
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changing to programmable thermostat

I just bought my first house and one of my first small projects is to change the old mercury switch thermostat to a programmable one.

I pulled off the face and there are 4 wires, none of them labled and I have no idea what goes where. I have a gas furnace and central air, so obviously 2 of the wires to go those (which ones I have no clue), but what about the other ones?

Any advice on how I can properly identify all the wires?

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Old 12-14-05, 05:25 AM
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You will have to go based on where they are/were connected to the original thermostat, or go look at the other end of the wires and see where they are connected at the furnace.
Old 12-14-05, 07:52 AM
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There should be terminal designations on the old thermostat....look for letters such as R,RC,W,G etc. next to each wire. Your new thermostat will show a diagram for four wires, and will tell your where to put the wire that was "R" etc. If nothing is found, you will have to trace the wires back to the furnace to identify the function of each wire.
Old 12-14-05, 08:45 AM
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The directions on most programmable thermostats are usually excellent. They realize that nobody will buy them if they can't figure out how to connect them. The directions I've seen seem to cover a dozen or more different situations you may find on your old thermostat, and tell you how to connect it up in a lot of different cases. Wire color, along with the markings on the old thermostat (usually just a single letter) where those wires connect to the old thermostat, are big clues.

If you're still having trouble, those HVAC guys over in the plumbing, air and heat forums deal with thermostats a lot more often than electricians do. Choose the particular forum that matches the kind of furnace you have.
Old 12-14-05, 09:04 AM
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Usually, RED is power from the transformer, WHITE is for heat, YELLOW is for Air Conditioning, and GREEN is for manual FAN. However, it could be hooked up almost any way so you'll probably have to look at where the wires terminate in the furnace.
Old 12-14-05, 10:12 AM
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Thanks for the help, I wanna say one of the wires is a light brown/tan color and that one really threw me.

I'll give it another look and I guess I'll have to trace them back because there are no letter designations on the old thermostat.

Thanks again for the advice.
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