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I have an old heat-only gas system with radiators, the boiler is a Willaimson-Thermoflo GWA070NS. I'm trying to install a Nest thermostat but only the R and W wires were in use with my old one. There's no C wire in the wall, but I could easily add one...However, the C terminal already seems to be in use.

(imgur seems to be rotating all these pictures 90 degrees to the left; apologies for that)

C wire terminal

There are two green wires and one blue wire connected to the C terminal. The blue wire goes here, to what I believe is the gas valve:

Blue wire

One green wire goes up through the metal hose and connects here, near the hood; I think this is the circulating pump:

Green wire #1

However, the other green wire is just secured to this screw on this metal box:

Green wire #2

I'm wondering if I can connect the wire to the thermostat to this second green wire (if it's truly unused) or if it's important to have this secured to the metal of the boiler for grounding. And if so, what's my best move for getting power to my thermostat?

Thanks! Edited to add photo of diagram on boiler (didn't even see it before this!) It looks like it 100 percent does need to be attached to that screw. I'm not sure if I can shove another wire into that terminal or if I should cut one of the green wires so I can pigtail it to the extra wire...? Any guidance appreciated.

Boiler diagram

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  • Can you post photos of the wiring diagram posted on the boiler please?
    – ThreePhaseEel
    Nov 29, 2021 at 1:56
  • @threephaseeel Added! Thanks!
    – Sam
    Nov 30, 2021 at 23:12

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According to your diagram the 24V Common is bonded to the chassis and to the Earth wire from the supply.

You can connect your C wire anywhere convenient.

IMO the most convenient place is that screw where one of the green wires is connected to the chassis. Don't disconnect the existing wire from the chassis, use the screw to connect them both. Since the 24V common is bonded to the chassis you could equally connect your stat C wire to any other screw that is screwed to a metal part of the chassis although that would only reduce reliability (and possibly voltage) so just use the screw in your picture.

Slightly less convenient, IMO, would be to cut one of those wires (any one, doesn't matter), strip both cut ends, and use a terminal block, wire nut, or whatever you like to connect all three wires together.

I don't like to mess around with those push-on spade connectors. I find it hard to make good reliable crimps with them (that's just me, I'm sure someone's going to flame me for that), and I recommend you don't "shove another wire into that terminal", if you are referring to the one that already has three. You would need to buy a new, larger one and crimp four wires into it. Instead do something that is easy to maintain using screw, lever, twist, or other connectors.

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  • Hell yeah, adding it to the screw worked, which makes sense in retrospect. Glad the solution was so much simpler than I feared. And thanks for going into those spade connectors; I've never encountered them before and so was nervous about messing with them; good to know that instinct was right. Anyway thanks so much for your help, Nest is up and running properly finally!
    – Sam
    Dec 1, 2021 at 1:30
  • This obviously worked for the OP, but I'm confused about how. I thought the 24V C(ommon) wire was to supply power, but in this case, it's also connected to the ground. Does that mean the entire furnace chassis is hot with 24V DC? Granted, it's highly unlikely that little power would kill someone (or even lead to a light tingle), but how does that work?
    – FreeMan
    Dec 1, 2021 at 15:22
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    24V is AC power between R and C. R is the "hot", C is the "Common". Older stats have R along with Y, W, etc which connect R as a signal to turn something on. But none of those (Y, W, etc) can be used as C without the side effect of turning on part of your HVAC system. In a loose analogy to 120V wiring, think of a thermostat as the switch in a switch loop. "R" is analogous to black, and Y, W, etc analogous to white as they close a circuit that does something. If you want to install a Smart Switch you need a real white wire in the switch box. That is very similar to what "C" is.
    – jay613
    Dec 1, 2021 at 16:06
  • Re the furnace chassis being hot ... 24V won't hurt you. In most furnaces the 24V side is floating (isolated). In OP's furnace, the Common side is bonded to the chassis which SHOULD be grounded. So it's just pinning the 24V AC to a reference. If anything, it's a tiny bit safer than having TWO floating 24V legs.
    – jay613
    Dec 1, 2021 at 16:09

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