I'm looking to replace my old thermostat with a wifi one. Looking at the wiring on my HVAC, I can see red and white wires connected (matching what's connected to the back of the existing thermostat). This is what I'd expect, but on the wiring diagram the R terminal is labelled 'Common'. This is strange, as I'd expect the C terminal is the common.

Braemar HVAC Wiring

Is the red wire actually a 'Common' terminal, or is this the standard R/W setup that I'd expect for heating?

The cable that is shown runs from the HVAC to the thermostat. As well as the red and white wires, there are also blue and black wires.

I was planning to connect the blue wire to the C terminal on the HVAC, and then attach the red, white & blue wires to the new thermostat. Is this correct, or am I missing something?


  • How do the wires connect to the thermostat?
    – Tester101
    Commented May 8, 2017 at 11:08

1 Answer 1


This is indeed the standard R/W setup for a heat-only system. You have it correct that you can attach the blue wire to the C terminal on the board and then use it as a C wire for your new thermostat.

If you want to make sure -- you can use a voltmeter to measure 24VAC between R and the Ground terminal between the fuse and the 6 pin connector.

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