I am trying to set up a nest. My system was a simple heat only R/W wire set up.

I attached the green wire to the c terminal on my furnace, but no power is being fed from it, or at least the Nest is not turning on.

I see the existing red and white wires go to another brick, but it doesn't have anything labeled C.

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  • 1
    How did you test the voltage exactly? You need to test between the C and R terminals. Mar 19, 2020 at 14:55
  • And keep in mind that it's ~24v, so an outlet tester expecting 120v may not indicate voltage.
    – isherwood
    Mar 19, 2020 at 14:57
  • Ahh I see. Well, I connected all the wires to the nest and it did not turn on. I also touched the bare copper green wire expecting to feel a gentle buzz.
    – AdamG
    Mar 19, 2020 at 15:05
  • 1
    From What I read about the Nest tstat, you don't usually need a C wire (common). Can you simply connect the Nest like the old tstat? And testing for power by "feel" is not a recommended approach! Mar 19, 2020 at 15:31
  • You do need a c wire or it can perform inconsistently or take a while to charge and start-up after a power failure.
    – AdamG
    Mar 19, 2020 at 16:15

1 Answer 1


The problem I see is that your 24vac transformer isn't really doing anything in your system. It might be there for an accessory that you don't have, but it's not hooked up. If it was doing anything, there would be a wire connected to the R and C terminals, and that is not the case. Use a multimeter to measure between the R and C terminals to see if it is outputting 24v - if not the transformer might not be getting power at all.

In a typical HVAC system, the thermostat acts as a series of switches that connects the R wire to Y, W, and G to activate 24v relays in the system that turn on the compressor, heat, and fan respectively. Assuming the thin white and red wires in your pictures go to the thermostat, that's not the case for you. Your heater just needs the white and red wires to be connected, but they don't carry voltage. It operates like a "millivolt" system and expects the (mechanical) thermostat to only provide a contact closure to connect red and white when heat is needed.

This is not strictly compatible with the Nest thermostat, but it can be made to work with the help of a relay. In the diagram, the connections on the nest would be: Red - R, blue - C, and black - W (meant to be the white wire, but I used black).

Relay diagram

You have all the items on that diagram except for the relay. You will need to find a "normally open" relay with a 24vac coil. It is a pretty common item if you know what you're looking for and here is an example. (I just noticed, one of the reviews on that relay also talk about using the nest with an old millivolt heater.)

The diagram for that relay is:

Relay diagram

Where terminals 1 and 3 are for the coil, and 2 and 4 are normally opened (note the diagram is flipped compared to the one above).

  • I also found this article describing a similar install, but he didn't have a 24v transformer in place so he used a Honeywell box that contains a transformer and a relay in one package.
    – JPhi1618
    Mar 19, 2020 at 21:05

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