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:

Wiring inside of furnace. It seems there are two brown thermostat wires. The first goes into the wall and has 4 wires and emerges where my thermostat is.

Thermostat hookup point

As near as I can tell, We need a common wire. My furnace has a transformer:

Transformer Wiring

There is a warning on the door of this transformer:

Thermostat caution sticker

A closer look at the rat's nest... Inside the furnace

I can't find a wiring diagram anywhere. I know it's a suntherm H02 series heater.

  • 1
    Hello, and welcome to Home Improvement. Would you post a picture of the furnace electricals, and even better the wiring diagram? And, you should probably take our tour so you'll know how best to participate here. Commented Dec 16, 2019 at 23:52
  • Yes, can you post a photo of the wiring diagram? That sounds very odd that there'd be some sort of issue caused by routing the C wire to the t-stat, unless the transformer in your furnace is much wimpier than the 40VA units typically used... Commented Dec 17, 2019 at 0:00
  • Also, this seems a little contradictory, because the outdoor unit typically has a yellow wire to signal cooling and a common wire. The yellow is the switched wire from the thermostat, and the common completes the signaling circuit. Your existing thermostat might be using blue for cooling.
    – JPhi1618
    Commented Dec 17, 2019 at 16:14

1 Answer 1


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!

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