I bought a Sensi WiFi thermostat and I have been trying to install it. I can't quite figure out how this wiring should work. Here is a link to pictures of the boiler and the current thermostat. The red wire from the thermostat is connected to Y on the boiler terminal and the black wire connected to G. The white wire wasn't connected to anything so I tried connecting it to the C terminal to be a common wire. When going through the sensi installation guide it says that having a Y and G isn't a valid configuration. I don't see what is wrong with the setup. Can anybody help me correct this and get my thermostat working please?

Thermostat: enter image description here

Boiler plate: enter image description here

  • imgur.com/a/2stcyom link didn't stay in post for pictures, here it is Jan 27, 2019 at 16:42
  • Can you post pictures of the nameplate and/or wiring diagram for your boiler please? Jan 27, 2019 at 17:18
  • I hope this is what you mean m.imgur.com/baGUeyY I'll try to see if the wiring diagram is around somewhere. I just recently bought the house. Jan 27, 2019 at 17:57
  • How many wires are in the existing thermostat cable, and is adding an additional 24VAC transformer + relay to the system an option? Jan 30, 2019 at 0:45
  • Also, I take it your system is heating-only, correct? Jan 30, 2019 at 0:46

1 Answer 1


You'll need a relay here

Your boiler, in addition to using nonstandard wire nomenclature, does not provide 24VAC power directly to the thermostat. Instead, power to the thermostat circuit is supplied through the parallel combination of a control relay contact and the NC contact on the combustion pressure switch. This means that when the thermostat stops calling for heat, 24VAC power to the 'stat is cut off until the pressure switch opens again. This is fine for a mechanical thermostat like your current one, but a smart thermostat like the Sensi will get its brain turned to mush every time the boiler kills its power supply.

As a result of this infelicity of the HE Series III's control wiring scheme, you will need to fit a HVAC relay (often sold as a "fan relay" -- these will have a 24VAC coil and at least one NO contact on them) to your system to isolate the thermostat from the boiler's thermostat circuit. The relay needs to be mounted in a suitable place in the boiler's wiring compartment, and is wired as follows, although you may need to crimp 1/4" female quick-connect/tab terminals onto the wires going to the relay to make the connections to it, depending on the relay you get of course:

  • One relay coil terminal to the white wire from the thermostat cable
  • The other relay coil terminal connected to a jumper nutted with the black wire from the thermostat cable and another jumper that goes off to the C terminal on your boiler
  • One relay contact terminal (the COM terminal) jumpered to the Y terminal on your boiler
  • The other relay contact terminal (the NO terminal) jumpered to the G terminal on your boiler
  • And the red wire from the thermostat cable connected through to the R terminal on your boiler

Once this is done, you can then wire the Sensi as follows:

  • White thermostat wire goes to W (to drive the relay coil with 24VAC on a call for heat)
  • Red thermostat wire goes to R (to power the thermostat, using the boiler's transformer)
  • Black thermostat wire goes to C (to provide the 24VAC return for thermostat power)

This way, the Sensi will operate the new relay on a call for heat, closing the thermostat circuit as a result, while having constant 24VAC power available to it, so it doesn't lose its brain every time it stops calling for heat.

  • I tried wiring everything up exactly as you wrote and it ended up frying the transformer Mar 2, 2019 at 20:28
  • @DannyWilloughby -- strange...what did you use for a relay? Mar 2, 2019 at 21:10
  • This one. supplyhouse.com/… Mar 4, 2019 at 1:20
  • ....very strange indeed, this one's got me baffled, that relay should work. Mar 4, 2019 at 1:26
  • @DannyWilloughby The only thing I can think of is that the Sensi is pulling too much power from the boiler -- in which case, you'll need to use the isolation relay, but with a separate transformer... Mar 4, 2019 at 1:30

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.