Here's my current setup - separate A/C and boiler units. Some of the pictures may be confusing because the wires are differently colored than they're named, but I'm going by my actual thermostat.


More pictures at the air handler: https://imgur.com/a/lbQGVYg

Old thermostat has five wires - RH, RC, G, W, Y. There's a random UNUSED Red wire in the wall which COULD be common wire but for the life of me I CANNOT find the other end of it. It is wrapped in the same wires going to HEATING (that is, RH and W). However, as you can see at the boiler, there is RH and W coming out but no red wire there. Literally no idea where the end of that unused red wire is. It seems to be dead (tested by placing in C terminal in new thermostat).

At the air compressor outside, there's two wires going in - RC and Y. At the boiler there's two wires going in - RH and W. At the air conditioner/blower upstairs, there's 3 wires - RC, G, Y.

This all makes sense to me, all the cooling wires seem to go to cooling systems and all the heating systems to go heating systems. But I have no common wire for the C in my new Honeywell smart thermostat, so here are some options I was considering.

  1. Buy one of these plug-in wall transformers, which I think could be the easiest plug and play solution, except I don't like it because it would be ugly : https://www.amazon.com/gp/product/B07C3J6PQX/
  2. Buy a 120VAC to 24VDC transformer (like 1., but not wall plug in), and put it directly in the wall in a box and run the wires up to the thermostat . May not be to-code
  3. There looks to be a BLUE wire in the fan/blower that seems to be a COMMON wire? If anybody can confirm that's what it is that would be amazing and then maybe I could run the blue wire down to the thermostat
  4. If 3 is true, I presume I can also do the fan wire trick (G and Y terminals jumped, G wire in C terminal) and I won't have to run any wire at all
  5. Any other suggestions?


  • 1
    Can you post a photo showing a different angle of the wiring at the air handler please? Also, is there a spare wire in the cable from the air handler to the thermostat? Apr 14 '21 at 0:32
  • Thank you for helping out !! I've edited my question but here are some more pictures (imgur.com/a/lbQGVYg). Honestly sorry for the poor view - there is a beam right in front of the panel in the attic so it's difficult to get a good view of anything, and the wires are a mess
    – squeegene
    Apr 14 '21 at 2:57
  • 1
    I take it there are no spare wires in the existing cable from the thermostat to the air handler? Is it possible to run a new thermostat cable to the air handler even? Apr 14 '21 at 3:00
  • 1
    Oh, sorry missed your second question. Your assumption is correct, there's no spare wires from the t-stat to the air handler. I think it would be possible but difficult - I think you're referring to option 3 in my post? That's partially why I think option 4 could be a solution for me
    – squeegene
    Apr 14 '21 at 3:03

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