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2 Wire Thermostat Connection for an EcoBee Wifi Thermostat on an old Weil McLain Model H-4 Boiler

I could not find much information on the Internet dedicated to this topic, so I am sharing what I for others who endeavor into such a task. The objective herein was to replace an old 2 wire mercury thermostat with a modern ecobee3 thermostat, and in doing so provide a new common line for the wifi thermostat utilizing the existing 2 wire design. I made use the FAST-STAT common maker system for this installation. Attached is the wiring diagram, with modifications, as a resource for others in their endeavors.


  • hot_rod
    hot_rod Member Posts: 14,786
    excellent info, thanks for taking the time to figure it out.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
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