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CT87T Honeywell Thermostat

jschlimm Member, Email Confirmation Posts: 1
edited January 2021 in Thermostats and Controls
Purchased new Honeywell thermostat to replace existing 30 yr. old thermostat (manu. unknown). Existing wiring is red/white/green. New thermostat requires red/white/yellow. System is Well Mclain gas boiler for baseboard heat. Heating only system - no cooling. Question is can I use the existing wiring (I sure hope so), and if so, what do I do with the green wire?


  • Jamie Hall
    Jamie Hall Member Posts: 23,270
    Electricity is colour-blind, so it's a matter of figuring out what wire actually goes where. Chances are pretty good (no guarantee) that the red and white are just that, and directly control the boiler. Look down in the boiler room and see what the green wire is connected to -- odds are it is a common. Then see what the new thermostat thinks the yellow wire should be connected to. Or... if you labelled the wires when you took them off the old thermostat...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,506
    Is your new thermostat a CT87K? It only requires 2 wires. Check where they land on the boiler control. Probably only Red and White.
    To what/where was the green wire connected?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,817
    I also would like to know the model number/brand (or a picture) of the old thermostat.

    If it is more than 30 years old, like maybe 60 Yrs old, then the 3rd wire may be a holding circuit as in the series 10 thermostats labeled R W B.

    Then you only need R and W on the new thermostat.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics