I have an old battery operated thermostat right now with two wires (18/2) attached to it running to a boiler. No AC, fan, etc.-- just a boiler. I can easily replace the wire with 18/5 one, however I'm unsure how to later attach the wires inside the boiler and to then Honeywell wifi thermostat.

Wires inside the boiler:

enter image description here

The thermosat is a Honeywell RTH8580WF.

  • You can see standard color markings on the terminal plate (W, G, etc.). I'd make those connections and then connect your new thermostat according to the instructions provided.
    – isherwood
    Nov 21 '17 at 14:11

Difficult to guess but here's what it looks like:

  • The top left post 1, Thick yellow wire is boiler
  • The top middle post 2, labeled G activates both the fan (thin black wire) and the boiler (thin red wire jumping 1 and 2)
  • The bottom left post 4, Thick white wire is neutral C
  • The bottom right post 5, Thick black wire is hot R

There is no fan delay control so when the thermostat shorts the red and white, it energises both fan and boiler.

With 18/5 wire you'll connect

  • White to top center post: Call for heat W
  • Red to bottom right post: Hot R
  • Blue to bottom left post: Neutral C
  • Green unconnected: fan control G
  • Yellow unconnected: call for cool Y

Important: The RTH8580WF is made for 24VAC systems. If the boiler is using line voltage (120V) it will destroy the thermostat.

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