How to connect C-Wire to Weil-McLain Boiler CGa Series 3

fei459552100

Hi everyone,

I'm trying to upgrade my old thermostat connected to my Weil-McLain CGA to a smart thermostat that requires a "C" wire. The cable from my old thermostat has 4 wires, but I need to find where I can connect the "C" wire to the boiler. The Low Water Cut Off has 4 wires, and one is already connected to the C Common terminal. Can I just connect my thermostat’s C wire to the same terminal? (Green pen in drawing) Thanks!

Aquastat is Honeywell and Low Water Cut Off is GuardDog. My issue is very similar to this thread. However, my existing wiring at the boiler is pretty different than the other post. So I'm not sure if I can also just connect the "C" wire (Thermostat) with both White wire (LWCO) and Black wire (Boiler). Another person also recommended to use 4th wire as Rc from thermostat and connect it to Red wire at the boiler (dashed lines)? Should I follow the suggestion?

Thank you!!!

Below is my boiler's schematic wiring diagram. https://www.weil-mclain.com/sites/default/files/field-file/CGa Series 3 Boiler Manual 0520.pdf

EBEBRATT-Ed

it looks like you have a "C" terminal on the terminal strip. Or the C wire on your gas valve.

I am not understanding your little sketch. You could possibly get it off the LWCO but I think you have easier options

HVACNUT

According to the diagram, the first wire on the left at the tstat connection is the common. You can splice into that  Everything else should stay the same. 

fei459552100

Thanks for the response. Currently that wire you circled is already connected to White wire from LWCO. Can I still connect the common wire from thermostat to it?

HVACNUT

fei459552100 said:

Thanks for the response. Currently that wire you circled is already connected to White wire from LWCO. Can I still connect the common wire from thermostat to it?

Double check to make sure the white wire on the LWCO is Common. If so, yeah.

Erin Holohan Haskell

@fei459552100, welcome to Heating Help! I've merged your two posts into one here so it's not a duplicate post. Thanks.

fei459552100

I just connected thermostat c wire to black common wire from boiler, the smart thermostat is lighting up and showing the temperature. However, when I set the temperature, it's not heating up? Do you know why? I didn't swap Rh with W, do I need to swap these two wires?