Need help wiring humidifier into 24VAC heat circuit.

My furnace is an old single stage heat pump. There's no control board or "HUM" leads to connect to. All I got is what's on the panel in the photo. My furnace is a Trane TWV030B140A1 blower with aux heat element.

Please correct me if I'm wrong on any of this. My understanding is that typically it should be wired to "W" (heat) and "C" (common) so water flows to humidifier only on a heat call.

My issue: I don't have a "C" wire.

So what do I do in this case? Is the "B" wire the return instead of "C"?

Here's what I've determined so far if this helps:

  • "R" hot (measures ~28VAC. Should this be 24VAC?)
  • "G" fan (measures 25.5VAC when blower on)
  • "Y" compressor (measures 25.5VAC when compressor on)
  • "W1" and "W2" tied together. Assuming this is aux heat tied to emergency heat since only single stage heater. Measures 25.5VAC when heat on. Should this be 24VAC?
  • Manual says "O" is reversing valve
  • Don't know what "B" is

Any help is appreciated!

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  • common is whereever you're putting the othe probe of your voltmeter when you are mking those measurements.
    – Jasen
    Commented Aug 28, 2018 at 23:43

1 Answer 1


Find the other compressor wire, and that's your C

There will be three wires going to the outdoor unit in your setup -- one comes from the Y wire from the thermostat (call for compressor), another comes from the O wire from the thermostat (reversing valve), and the third and remaining wire is the return path for both those signals, so it must land on the C terminal, or whatever your system calls it (likely B, but can't tell for sure from your photo).

  • Great idea. I took a closer look and there are 5 wires going to the compressor: R, B, O, Y, Black. The R, B, O, Y on the compressor wire are tied to the R, B, O, Y on the thermostat wires in the photo. The Black on the compressor is paired with the W1/W2 on the panel. Does that help determine if it's B? Maybe I need to pop the cover off the compressor and check to see where they're going on that side to figure out if B is the return?
    – nick
    Commented Mar 26, 2018 at 15:59
  • Jumping the gun here. It seems by process of elimination the common on my system has to be the B wire. Just to be sure, is there any other thing that B could be used for on a system like this? Also when heat is on, I measure 25.5VAC across W1 (heat) and B. Same across W2 (heat), G (fan), Y (compressor) with B also. Is it normal to be this far from 24VAC?
    – nick
    Commented Mar 26, 2018 at 18:40
  • @nick -- the other thing B could be a designation for is the reversing valve, but O is already used for that on your system, so B's the common for sure. And yes, it's normal for your system voltage to be a bit off like that Commented Mar 26, 2018 at 23:55
  • I did confirm that O is the reversing valve by toggling my system between heat and cool and checking the voltage across O to see it energize the valve in cool mode and de-energize in heat mode so now I'm also convinced that B is my common. Thanks a lot for your help!
    – nick
    Commented Mar 27, 2018 at 15:04
  • @nick -- we thank people with upvotes around here :> Commented Mar 27, 2018 at 22:30

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