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Installing WIFI thermostat to old boiler with only red, black and white wire?

Hi all, what a great website and community this is. Thank you in advance for any advice here.

We bought an old house with steam heat boiler last summer. This is our first winter here. I have a Honeywell T9 thermostat to replace our older one. My boiler has no control board or terminal box, a local boiler guy re-wired it when he couldn't get continuity on the board last Fall when he helped us turn on the system. It's rigged up now that the pilot is always on, and the full flame will kick on and turn off when the thermostat calls for it. I open a water valve every couple days for a few seconds to keep the water level correct in that external glass tube outside the boiler.

The boiler is 40-50 years old and we plan to replace it next year, so we are just trying to get through this winter without spending the money.

My question is:

There are two 24 volt transformers downstairs. One is in use, running directly to "W" terminal of Thermostat. Two wires, black and red, are connected to our current thermostat. There is an extra white wire that is disconnected from the boiler and thermostat terminals. Can this white wire be my needed C-wire? Or do I need a pair of wires to make this connection? Any Ideas?

Current Setup:

Red Wire connects to "R" screw of transformer and "B" terminal of Stat
Blue Wire connects to boiler power switch and "W" terminal of Stat
Single White Wire sitting next to transformer and thermostat, connected to nothing.


  • Zman
    Zman Member Posts: 7,493
    You only need one "C" wire. If you are confident you know which transformer is feeding the t-stat, land your "C" wire on that one.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • wmgeorge
    wmgeorge Member Posts: 222
    What a Rats nest of bare wires, wires going no where and more. You need to find someone who know how to do things correctly before something is burnt out.
    Old retired Commercial HVAC/R guy in Iowa. Master electrician.