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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.

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    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. – Daniel Griscom Dec 16 '19 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... – ThreePhaseEel Dec 17 '19 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 Dec 17 '19 at 16:14
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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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