I've purchased an Ecobee 3 Lite smart thermostat which requires 3.5 VA to operate but I'm using an Aube RC840T-240 electric heating relay with 24 V transformer which only provides 1.2 VA.

Can I purchase a separate 120V (to connect to a different circuit or outlet) to 24 V transformer and use that to provide enough power for the thermostat? If so, how would I wire my thermostat? (The Aube relay/transformer provides R, W, and C wires so what would I connect from the Aube and from the external transformer?)

1 Answer 1


This is A-OK

Wire R and W from the relay box to Rh and W/W1 on the Ecobee, while leaving the relay's C terminal unused. Then wire the secondary of the separate transformer to Rc and C on the Ecobee. The Ecobee will figure all the rest out by itself, running off of the separate transformer on Rc while switching power from Rh to W to control the heat.

P.S. any standard 24VAC HVAC transformer will do here.

  • Thank you so much! Can you explain why I need both R from the relay box and R from the new 24V transformer? Aren't they both 24V sources?
    – SofaKng
    Jan 19, 2017 at 1:43
  • 1
    They are going back to different places, and electricity needs to be sent back the way it came. Jan 19, 2017 at 1:57

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