Currently my central a/c unit is hooked up to a Honeywell CT3500 Programmable Thermostat. I've noticed that the thermostat can handle heat as well as cooling. My furnace is hooked-up to an older manual thermostat. It is a simple 2 wire set-up - red & white. Could the furnace be hooked-up to the programmable thermostat which the a/c is currently using? The R & W slots on the prorammable thermostat are being used by the a/c, but is it just a matter of adding the R & W wire from the furnace to the slots also? I just purchased the home so I would like to find this out before I go out and by another programmable thermostat if I really don't need to. Also, I noticed when I was checking the wiring on the two thermostats, there was a blue wire that wasn't being used for the a/c. The unit seems to be working fine, but I just though I'd ask as the wire is just sitting in the wall and not connected like the other 4 wires. Thanks in advance for the help.

labman

Yes, that is the standard set up. Go to the sticky at the top of the H&AC forum and read the section on thermostat wires. While you are at it, I would dump the one transformer. Just connect the red wire from the one you keep to RH and RC both at the thermostat. Then the other wire from that transformer needs to be connected to the blue wires and com terminals of everything.

acetc

I agree with Labman, drop one transformer as he said , the one in the furnace will be the one to drop or discontinue because it will be the smallest, you may want to have a service tech to do this control wiring to keep from wiping out the new thermostat, but what you propose (one stat ) is the best way to go.