I want to upgrade my old mercury thermostat that I currently have to a newer programmable thermostat. This little DIY project is for my first late 1920's home that I recently purchased. I currently only have heat hooked, and use window A/C's, however I have no problem running a 18/5 Cable down to the boiler instead of a 18/3.

My question is how to I the C wire from the boiler transformer.

Please see pictures attached from the manual for the boiler (I have no idea why they even have 3 listed). Currently the two cables that run from the thermostat are hooked up to T-T terminals (on the top of the figures that run from TH-2).

Would all I need to do is basically run a third cable to the empty C terminal in N-H-C-W-B terminals on there? Figure 1 from Manual Figure 2 from Manual Figure 3 from Maunual

UPDATE: The physical boiler has a Honeywell AT150D1001 transformer attached to it[next to the emergency cut-off switch]; which seems to be supplying the 24V to the system.(See picture below). My question now becomes do I use the white wire or the dark brown wire as the C wire that I run to the thermostat? Actual Wiring On Boiler

  • Were you able to run the C wire to boiler? I have the same model as your and would like to add the C to run the NEST. Thank you Leon
    – Leon
    Commented Nov 30, 2017 at 22:52

3 Answers 3


The device labeled "PS-802 ELWCO", is a low water cut-off. The "C" terminal there, is not the C you're looking for.

I believe the white wire from the transformer, is what you'll want to connect your C wire to. I'm not 100% confident though, since I'm not that familiar with this system. You may be able to contact the manufacturer for confirmation.

  • According to the diagrams, it is the white wire you want -- look at the connector labeling at the bottom of the diagram. Commented Jan 26, 2015 at 23:59
  • Note that the TH-2 T-T posts are ambiguous, it's not obvious which is the R feed and W signal. If they're wired wrong, the thermostat won't power up.
    – Stavr00
    Commented Dec 1, 2017 at 15:04

I just pulled this off. You can't wire the thermostat to the T-T transformer, because when it calls for heat, the thermostat will turn off.

I bought a 24VAC transformer from Home Depot ($16) and screwed it to the boiler chassis just above the T-T transformer, and wired the 120VAC inside the T-T junction box.

Ran a new 18/5 wire from the thermostat to the boiler.

Rh = T-T Right - Red wire

W = T-T Left - White wire

C = New transformer - Blue wire

Rc = New transformer - Yellow wire


Ok, so I have this same boiler that I wanted to add a Nest to and I spent a little time looking into the wiring of the boiler a little more carefully.

I am slightly afraid that the Nest can bypass the pressure switch and low water cutoff if wired to the T-T terminals directly. I had an issue where the Rh wire was connected to the BROWN T wire coming off the pressure switch and the W1 wire connected to the ORANGE T wire. When the pressure switch cut off the power (as it is supposed to), the Nest complained that there is was no power going to Rh. For some reason when this happened, our furnace kicked on and stayed on, which if you look at the wiring diagram shouldn't happen. It seemed as if the Nest supplied power to the W1 wire from its battery - essentially bypassing the pressure switch and low water cutoff - which could be scary.

I think the best solution for a Utica or similar boiler with a (T-T) connection would be to purchase a simple 24VAC relay (http://a.co/d/2RS0NHC). Connect Rh to R terminal on the 24V transformer (you can use tap into the same transformer that is on the side of the furnace), C to the common (C) terminal on the transformer, & W1 to the relay control coil pole (1 on the linked relay) and then back from the other relay control pole (3) to the transformer's common (C). Then hook up the T terminals to each side of a the normally open poles (2&4).

This way the nest will essentially work like a dummy switch and you don't have to worry if it does anything weird. It will always have power and won't bypass any of the essential cut off functions of the boiler.

Edit: Found this little video on Youtube which pretty much explains exactly what I described here. It's not a Utica boiler, but it has basically the same type of T-T terminals for a thermostat and describes how to install an isolation relay for a smart thermostat. - https://youtu.be/f0v3FeDZBeY

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