Wiring noob here. Can someone give me step-by-step instructions for where and how I should attach a C (common) wire to a Triangle Tube Prestige Excellence 110 boiler for a ecobee3 thermostat?

I think the gray box on top (pictured below) with the blue, brown, red and white wires is the 24VAC transformer, but I'm not sure.

Also I'm not sure where or how I should connect a C wire.

photo of wiring

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2 Answers 2


Nowhere convenient, and nowhere that is clearly documented but it's a reasonable guess that the white wire connected to terminal X10-7 is the 24V common. You would have to splice in to it as there does not appear to be anywhere else available to connect to it. None of the convenient terminals on the orange terminal blocks is 24V Common.

Do some testing first to make sure that pair of white/red wires there is the 24V supply and then to ensure the red one is used as "+" (bearing in mind it's AC). If so, I'd cut the black cable exactly in half, insert a new terminal block there and add my C wire to it.

Note the other black cable and a lot of the wiring is 120V. Be cautious.

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I think the 24v terminal is the orange one on the bottom right labeled with the ports 7 to 19. you should be able to confirm this by seeing only small (18 or 20 gauge) wires entering into these ports and a red and white wire in location 7 and 8. I am installing a Nest thermostat and if I need a Common wire, I will add some comments when I figure this out too! I sent triangle tube a request for this information. Scott

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