An AC-only machine cools a storage facility: no heat. The 30 year old thermostat is to be replaced with a wifi thermostat, which requires a C-wire to power the electronics.

There are 4 wires:

  • Red (28V)
  • White (?)
  • Green (fan)
  • Yellow (Cooling)

enter image description here

Convention indicates white is to for calling heat: that being said there is no heater.

  1. Why is it connected to the thermostat, if the machine is AC only?

The potential difference between the white and the red is 28V:

  1. Can said potential difference power the wifi thermostat (can WHITE serve as the C-Wire)?


I traced the thermostat wiring back to the machine and snapped a photo:

enter image description here

A second identical wire connects the indoor unit with the outdoor unit. It seems that both blue wires are connected to the machine's blue leads. Now I am wondering if I can rely on the convention of blue to serve as the C-wire.

  • Do you have access to the air handler where the other side of this wire is most likely connected? You really need to see where that white wire goes to make sure you're not accidentally supplying power through it to an unknown accessory. I suspect there is also 28V between the red and yellow wire, but you can't power a thermostat like that. – JPhi1618 May 6 '19 at 21:06
  • @JPhi1618 Your questions have prompted me to open the machine and snap a photo, which is now in the original post. – gatorback May 6 '19 at 21:51
  • That picture doesn't show everything. The main thing you need to look at is where the two wires from the 24v transformer go. That thick red wire is probably one wire from the transformer. It probably also has a white wire coming off of it. The other wire on the transformer is the Common or C wire. That might go to the white bundle, in which case, the white wire is your C wire. You can't rely on blue being common on a system that old - it might be, but also might not. – JPhi1618 May 7 '19 at 4:28
  • Looks like the wiring diagram is posted behind the wire bundle. If you can make heads or tails of that, you can likely find the common side of the transformer as @JPhi1618 suggests – Matt Jul 22 '19 at 3:50

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