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Common Wire for Boiler

I have a pretty old American Standard GPMX - 6r boiler that I inherited when I purchased the house. It had a 2 wire thermostat hookup, but I found for my new thermostat, I need a 3-wire. It needs to be attached to the common. I ran the new wire, but I want to know where to find the common terminal at the boiler. Can't find instructions for it. I am pretty lost.

Thanks for you help.

Re: Common Wire for Boiler

I'm not real good on boilers and I'm sure that Nashua Tech will be along, he's the man. In the meantime why do you need a three wire for your new stat? Just because you have a common on your new stat doesn't mean you need one going to the Boiler, if you remember where the wires two wires went at the boiler and at the stat just hook them up accordingly. Like I said Nashua Tech should be along and he'll square you away. Good Luck :)

Re: Common Wire for Boiler

Find out what you have for Heat and then ask us the Questions, first you say Furnace then you say Boiler. If you have a furnace you need four wires, R=Red or hot(voltage), W=white or heat, G=green or blower, B=black or common. I believe you only need two wires for a boiler R and W.

Re: Common Wire for Boiler

As Sten clearly points out, you probably don't need a 3-wire t-stat for your boiler.

For the heating application you describe in your post, you need a 2-wire heating only t-stat; additional wires are used for heating/cooling or programmable applicatiions----check the box and literature that came with your new t-stat to make sure you have the right one.

A multimeter can be used to check which 2 wires open & close when you raise & lower the t-stat setting if you want to use the t-stat without using the 3rd wire.

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