I want to replace my thermostat (Honeywell th1100dv1000) for an Ecobee 4 but it needs a C-Wire. I own a Slant Fin Monitron Electric boiler currently running with only two wires (R and W). I have a third unused wire.

The boiler manual can be found here

A team of 2 electricians came to my house this week but they were not able to connect it. (I had other jobs for them as well so did not pay for nothing although they spent 2 hours trying to figure it out. They gave me the name of another electrician that has more experience with boilers, but I am hesitant to pay again for someone to tell me that it's not going to work)

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They tried with the C-Wire connected this way but, but when the thermostat calls for heat, it then reboots.

I then contacted Slant Fin to know if it was possible to connect a C-Wire to the boiler and they responded "You will have to provide the C wire with its own power from another source. So, a separate transformer will be required."

It seems Here that another user found a way to connect a C-Wire.

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Here is the picture of my boiler setup. It looks the same to me.

Would I be able to connect my Ecobee 4 Thermostat ? I am not well versed in electric works and kind of unsure of which wire I should tap into for the C-Wire. I am also french speaking, and here in Quebec (Canada), it is mandatory for electric work to be done by a certified electrician.

If someone was able to explain to me step by step what I should look into, and if possible, by editing pictures, so I can show the electrician how to wire it correctly, that would be greatly appreciated.

I can provide more pictures if needed.

Thanks in advance and sorry for my weird English phrasing!

1 Answer 1


Your boiler does not have a 24V transformer that is designed/documented to provide power to external devices. It does have a 24V transformer, but it's "internal", there is no interface to it and no documentation about it. Page 5 of the manual clearly shows that, and shows an added external transformer used where a 24V source is needed (in that case for zone valves).

In the other question that you linked, the answer describes how to reverse engineer the boiler to figure out which wires are the transformer secondary, and then which one is being used as "common". The answer isn't comprehensive, it provides a road map.

You shouldn't follow that answer. It's not the right thing to do. And if you are unable to follow the slightly ambiguous instructions there, you really should not do it.

You have a couple of options:

  1. Some thermostats are able to "leech" power off the T-T terminals without an R or C wire. I don't know if the Ecobee can do this. Find out from the manual and/or their help desk.
  2. Install an additional transformer. You'll need a fourth thermostat wire. See if the existing cable has one that's been cut inside the jacket, or else you need to replace the cable. Then use Ecobee's Rc and C terminals to deliver power to it from the new transformer. T/T goes to R/W, and your new transformer goes to Rc/C.
  • I'd like to point out that replacing a thermostat wire with something that has more conductors is simple and cheap (and if he has to have an electrician do it anyway, won't take them but a few minutes). That shouldn't discourage him. The transformer also seems like it won't be absurdly expensive, though I'm having trouble finding the right part. Most of them are in the range of $20-50. The electrician's time will be far more expensive. @GregGau
    – John O
    Mar 24, 2022 at 19:29

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