I have a 220V electric baseboard heating system which is controlled by a 24 VAC 2 wire thermostat (RH, W). There is a 220V to 24VAC relay in the circuit from the thermostat to the heating system.

I plan to replace the thermostat with a WiFi enabled smart thermostat so I can control it remotely. The Smart thermostat (Emerson Sensi) needs a C wire (return ?) so the thermostat can be powered.

My choices are 1. Use an external 110V to 24VAC adapter from an outlet to power the thermostat (connect to C and R terminals on the thermostat) 2. Replace the existing relay with a transformer that has a common wire and pull a 3 wire thermostat cable through the floor & wall.

I plan to do option 1 now as it's zero risk in the middle of winter and follow up with Option 2 in the summer. I'm not sure which transformer I need to get for the 3 wire (RH, W, C) application. Any ideas ?

Also, the current thermostat has a backlight feature operated by a button to light up the dial - so, it is able to use the power from the RH wire to close the circuit. Can I simply jump the RH connector to the C connector in the thermostat ? I have a feeling this will cause a short.

Thanks, V

  • What's the contact rating on the existing relay? (Or how much power do your baseboards pull, for that matter?) Commented Jan 31, 2016 at 7:08
  • 25 Amps @ 240VAC. 18 Feet of baseboard length (6 + 8 + 4)
    – Vasuvius
    Commented Jan 31, 2016 at 12:31
  • Would this work ? AUBE RC840T-240 ON/OFF SWITCHING ELECTRIC HEATING RELAY WITH BUILT-IN 24 V TRANSFORMER . [link]smarthome.com/…
    – Vasuvius
    Commented Jan 31, 2016 at 17:53
  • You want a standalone relay @Vasuvius -- I'll link one in my answer. Commented Jan 31, 2016 at 19:26

2 Answers 2


My plan worked perfectly and I called Emerson Sensi customer service to check that it will not cause any issues and the confirmed. The 2 wires to the 24VAC relay that control the baseboard get connected to the Rh and W terminals on the thermostat. I cut the jumper between Rh and Rc to ensure I wouldn't cause a short by adding another power source I used an external 24VAC transformer, connected to Rc and C to provide power to the thermostat. In essence this is not a parallel circuit situation, but 2 independent circuits. One circuit from the external transformer provides power to the thermostat. The other circuit controls heat activation.


First off -- option 1 will be...awkward. You are essentially connecting two transformers in not-quite-parallel here, and as a result, it may not work at all. I would recommend against it -- simply replace the thermostat in the summertime.

As to a replacement transformer -- you can get a 240VAC to 24VAC control transformer as a standard part from an industrial or electrical supply house, as they are commonly used in industrial work. You'll also need a separate 24VAC relay with 25A contacts; again, an industrial, electrical, or HVAC supply house will have this.

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