Is this wiring scenario dangerous? Is it likely produce sparks or damage my HVAC system?

Please read the description and see the image below.


I didn't have a C-wire for my Emerson Sensi smart thermostat so I got an AC adapter. The adapter has two wires. Instructions say to put one of them in the C slot and the other into either RC or RH.

However my system (gas furnace and central AC) requires both the Blue RC and the Red RH wire to function properly. I tried putting the second wire into Y2 and W2 to see if that would work, no luck.

The working combination was to place one wire into the C slot as seen below, and the second one I twisted together with the RH wire. Works well so far.

Let me know if you think this could be a problem, and what else I might try to fix this.

From my reading, it seems like I could probably add a C-wire without this silly AC adapter, but it's an apartment with a cramped furnace closet and I would like to avoid running new cables through the wall.


  • Hello, and welcome to Home Improvement. Sounds OK to me, but let's see if one of our pros has more to say. And, you should probably take our tour so you'll know you'll know the details of contributing here. Jun 29, 2020 at 16:56
  • Could you share the document that advises you to use an AC adapter?
    – jwh20
    Jun 29, 2020 at 17:04
  • @jhw20 No official document. That's just the general recommendation out there if you google "install smart thermostat no c-wire" For example from: smartthermostatguide.com/what-if-i-dont-have-a-c-wire This thermostat is hooked up only to heat in a low voltage system. The addition of a fourth wire, either with an adapter or new wires, could act as the C-wire if properly wired to the furnace. Jun 29, 2020 at 17:26
  • @jwh20 this is the exact adapter I'm using. It's a hack for sure amazon.com/gp/product/… Jun 29, 2020 at 17:28
  • Can you post photos of the wiring at the furnace/air handler ends of the picture? Also, are you OK with changing things out for a different way of making a C wire? Jun 30, 2020 at 0:28

1 Answer 1


Well you are using low voltage infact finger safe voltages unless your adapter was a welder in a previous use I would not be concerned if everything is working, I have used adapters to power thermostats when it was cheaper than pulling new wire when RH & RC and fan were all good with my adapter (or transformer).

  • Great! thanks for this info Jul 3, 2020 at 17:56
  • If you find answers or comments helpful upvote or accept green check mark that will help others find information with a similar question.
    – Ed Beal
    Jul 4, 2020 at 22:28

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.