Am trying to install a programmable thermostat. I seem to have found the letter designations on the old thermostat for 'W', 'G', and 'Y'. The only two terminals left are 'RH' and 'RC' which were connected to the same wire on the old thermostat. The manual for the new thermostat is looking for a 24v connection and a common connection. Were the 'RH' and 'RC' connections on the old thermostat the common or the 24v. I suspect that the wire was the common connection and there is no wire for the 24v which would power up the new thermostat which would make sense since the thermostat can be powered by 2 'AA' batteries. Can anyone confirm this before I hook this baby up? Thanks.

Well, I took my best shot and hooked the thermostat up and the house didn't burn down and the thermostat works fine. I studied a schematic (block diagram) of the old White Rodger's thermostat and hooked up the red wire to the 24 volt connection on the new thermostat and left the common terminal without a wire on the new thermostat. Seems that the common connection is done in the heating and airconditioning units themselves by tying one end of all the switching relays together so when the thermostat sends the signal(s) with the W, G, Y terminals on the other side of the relays, power goes to the respective units and the proper things happen. The thermostat works fine with heat, air conditioning and fan only operation as it should. The thermostat has a backlighting feature where you can select a backlight color (for example: red when the heat is on, blue when the A/C is on and white when the fan is on alone - 64 colors to choose from). Unfortunately, I haven't figured that out yet but with the overnight lows reaching minus 27 degrees celsius (about minus 8 degrees fahrenheit), the backlight is cool but the heat is more important right now and like they say 'if it not broken, don't ry to fix it). Note: when Googling for something, punch in the model number of the thermostat instead of going to the manufacture's homepage or link. That's how I found my White Rodgers wiring detail because it didn't show up on their site. Ah! Nice and comfy with programmable heat.