Im just curious if the “c” terminal on the plug in relay is where im suppose to connect my “c” wire on the boiler side. I have 18/5 wire ran already. Just need to find out where to terminate. To the “c” terminal or do i add it in somewhere else. The red wire is connectsd to my “R” on thermostat. Which is on the “G” terminal of the transformer. And the white wire is on the “w” port of the thermostat. But on the “r” terminal on the boiler relay. Where do i add the “c” wire to run a wifi thermostat??enter image description here

  • 1
    The C terminal should be your common, however, you'll likely also have to swap R and G. You'll want R to go to R, and W from the thermostat to go to G. Otherwise the thermostat will only get power when it's calling for heat. R is one side of the transformer secondary coil, and C is the other side. G is connected to one side of the relay coil, while the other side is connected to C. So when the thermostat calls for heat, it closes the contact, allowing power to flow from R to G, through the relay coil to C.
    – Tester101
    Apr 5, 2018 at 13:10
  • I can just pull the red and white wires out and just swap them?? Or will i end up changing the configurations up? And changing how the soldered wires to eat of them react? Do i need to swap those along with the red and white wire? Also my thermostat terminals are W that calls for the heat and R that brings power. So with none of them being terminated to “w” on the relay im confused
    – Kirk
    Apr 5, 2018 at 19:33
  • The W terminal on the relay is isolated, and is used to control optional equipment. The G terminal on the relay is connected to one side of the coil, and is used to tell the boiler that the thermostat is cold. Y is also an isolated terminal, used to control A/C units.
    – Tester101
    Apr 7, 2018 at 0:23

1 Answer 1


Here's what your system currently looks like.

Simple thermostat wiring

As you can see, when the thermostat calls for heat (closes the switch), the coil in the relay is energized. This closes the relay contacts, and the boiler does what it does.

If you're adding a fancy new thermostat, you'll need to connect the C terminal. The thermostat will then be powered by the transformer, using the R and C terminals. However, since in your situation, the R terminal from the transformer is actually going to the W terminal on the thermostat. The thermostat will only have power at R, when it's calling for heat (the switch is closed).

Thermostat only powered when calling for heat

To make this work, you'll have to change the wiring around a bit. In the end, it should look like this.

Thermostat wired to boiler transformer and relay

  • Awesome thank you so much. I definitley grasped the concept. With the drawings. And i wired it up. Just swapped and labeled the two wires in the thermostat terminals and added the “c”. Everything fired up and is working great. Appreciate your time and help!!
    – Kirk
    Apr 9, 2018 at 22:36
  • Old question but I came here with a similar issue on a GSA-150 steam boiler. Swapping the wires made me a little nervous but worked perfectly! Everything runs correctly now and no need to buy (and wire) one of those power adapter things Google sells Jan 7, 2022 at 20:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.