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)
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:
One green wire goes up through the metal hose and connects here, near the hood; I think this is the circulating pump:
However, the other green wire is just secured to this screw on this metal box:
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.