I'm working on adding a smart thermostat, and as many older setups have it, my current thermostat doesn't have a 'C' wire.

However, if I pull the wires further out from the wall, the feed from my A/C air handler has 'extra' wires.

I would guess the blue colored wire may be C, but I'm not sure, and I'm not sure it even has power. I would like to check that it has 24+V, but I'm rusty on my electrical diagnostics. With my multi-meter (set on stun, I mean Volts) would I use Red to the blue wire and Black/common to ... some other wire? If so, which one?

Update: I added an image of the wiring at the air handler. I noticed that the white and blue wires are disconnected there. If I knew where I needed to, I could connect the blue wire.

If the white wire was connected, the Ecobee I'm connecting has an adapter that would make it work without the C.

This image is of the existing thermostat bracket. thermostat bracket

The air handler, C-wire disconnected air handler wiring

2 Answers 2


The blue wire is unconnected (floating) at the moment

I can see from your photograph of the wiring at the air handler that the blue wire in the thermostat cable is simply disconnected at that end.

Finding your C wire (using the air conditioner trick)

See how the yellow wire from the thermostat is wire nutted to a red wire going off into a second brown cable? That second brown cable is connected to your air conditioner's outdoor unit, and the white wire in that cable must be connected to your C wire in order for you to have air conditioning.

So, nutting the blue wire from your thermostat-cable into where the white wire from the air-conditioner cable is joined into (after stripping the end of the blue wire of course) will make your blue wire the C wire you're looking for.

  • How can I check the voltage to make sure? Commented Nov 30, 2018 at 22:34
  • That was it. I was able to check the voltage from that to the RC (Red out to the thermostat.) The EcoBee is running now! Commented Dec 1, 2018 at 0:49

The secondary of the transformer has two outputs: R and C.

The thermostat wants to power itself on 24V between R and C.

If you were to accidentally connect the thermostat's C terminal to R, absolutely nothing would happen. Because the thermostat would be trying to power itself between R and R, which is 0 volts.

As such, guessing which transformer terminal is not particularly hazardous, but stay away from the 120/240V mains side of the transformer!

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