I have a Weil-Mclain 150000 BTU steam boiler, and have only 2 wires connected to my thermostat, but would like to add a new Honeywell WiFi thermostat. I have no C wire, no A/C, or any other type of heating system in my home besides a water boiler.

Is it possible to install this thermostat and how?

  • 2
    How many wires are in the cable at the thermostat? Have you contacted Honeywell support? Do you have the schematic for the boiler?
    – Tester101
    Oct 25, 2014 at 19:48

1 Answer 1


Yes - but you will need to run a new thermostat wire. With your setup, the thermostat wire you run will need three conductors. However, consider installing one with five conductors or more for a few extra dollars - helping to future-proof your wiring.

Old 'two-wire' thermostats worked like a switch. The house got cold, the thermostat made a connection, 24VAC was sent to the boiler until the warm house broke the connection.

Your new thermostat needs constant 24VAC (that's where the C wire comes in) as supplied by your transformer, and uses the third wire to communicate with the boiler heating relay. Thermostat uses the 24VAC to operate, and when there is a call for heat, it sends a 24VAC signal to the boiler.

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