I'd like to install a smart thermostat in place of my old mercury coil thermostat. The old thermostat controls both heat and AC – it uses only 4 wires, the new clearly requires 5.
Wiring to the thermostat won't be a problem as it is inches away from the furnace and the 4-wire will be easy to replace with a 5-wire.
I've taken the panel off of my furnace and see there is a transformer and a common wire. There is a notice that says "yellow 24v common wire is for outdoor unit only" Does this mean I can't add a yellow wire to my thermostat?
Inside the unit I see yellow, black, blue, red, green, and white wires. I have blue, white, red, and green going to existing thermostat. can I just loop a new 5-wire brown wire to that yellow?
Edit: Furnace Photos:
It seems there are two brown thermostat wires. The first goes into the wall and has 4 wires and emerges where my thermostat is.
As near as I can tell, We need a common wire. My furnace has a transformer:
There is a warning on the door of this transformer:
A closer look at the rat's nest…
I can't find a wiring diagram anywhere. I know it's a suntherm H02 series heater.
Best Answer
Alright - this installed and working.
It took a little bit of time to figure out what was going on with that crazy wiring without the benefit of a wiring diagram. We reverse engineered the wires based on where they were on the old thermostat.
The thermostat came with a c-wire adapter which basically allowed be to wire the unit using the existing 4-wire cord. I'm pulling the 24v from the transformer and, so far it's not running under-voltage. If that changes then I guess we'll have to install another transformer. So far so good though. Thanks for the comments!