OK I just recently bought a honeywell thermostat controller. When I removed my old thermo (slider switch type with mercury) only 2 wires were connected, but a third wire was in my box still covered (not bare wire). One wire connected was white and the other red the 3rd odd ball was green. What is this green wire? Is the 24v common wire? If it is I would like to wire it to my thermostat to keep my backlit display on constantly... anyone know? It's a new built home one year old.

I read the FAQ I seen a wiring diagram it seems the green wire might be for fan relay would I need to connect this? If so how come it was not connected before?

I am glad you checked the FAQ. Great material. Did the old thermostat and the furnace both have a G terminal? If one didn't, then you couldn't have the fan only function. If you don't want the fan only function, or can't use it, you could use the green wire as common. Just connect it to the common at both ends.

I am glad you checked the FAQ. Great material. Did the old thermostat and the furnace both have a G terminal? If one didn't, then you couldn't have the fan only function. If you don't want the fan only function, or can't use it, you could use the green wire as common. Just connect it to the common at both ends.
Common wire as in that I can use it as a 24v source? Here is a pic of my setup. The first pic is the thermostat I took out all it had was a Red and White connection as indicated by the screws that's what was connected that's it. The 2nd pic is the pic of the thermostat I just bought and it has multiple inputs as you can see now is it possible to use that green wire to connect to the C connection (bottom right hand) in my new thermostat as a 24v source to keep the back lit on constantly?
http://img.photobucket.com/albums/v516/rapunzel_1/IMG_1660.jpg

http://img.photobucket.com/albums/v516/rapunzel_1/IMG_1661.jpg