Question about wiring a CT87B Honeywell thermostat

Discussion in 'HVAC Heating & Cooling' started by leejosepho, Dec 11, 2012.

  1. leejosepho

    leejosepho DIY scratch-pad engineer

    I knew our furnace was not cycling properly, so I decided to move our thermostat to a different wall while also replacing the old T87F Honeywell...and then I discovered what I think is a wiring problem that essentially bypasses the anticipator that had not been set correctly anyway.

    note: The old T87F is presently mounted on a newer base plate nearly identical to the plate for my new CT87B.

    For a 4-wire heating/cooling system like we have, the instructions say to power Rh and Rc...but my picture shows how the current plate is wired differently. So before I follow the directions, does anybody know a reason I should not?

    Last edited: Dec 11, 2012
  2. big2bird

    big2bird IBEW Electrician

    Anaheim, Ca.
    Correct. Hook it up per directions. Red is power. RH and RC should be jumpered. W is heat. Yellow is cool. Green is fan, IF, they used the normal color code. Verify BOTH ends.
  3. leejosepho

    leejosepho DIY scratch-pad engineer

    I did not verify both ends, but I did follow the directions and it all works well. I am not certain the anticipator had been bypassed by the previous connections, but I do know the furnace now acts like it should. In the past, it would occasionally begin a cycle and then stop abruptly and restart again, and then sometimes also follow a regular cycle with a brief one. But again, now it all works as it should.

    After studying the different wiring diagrams that came with the new thermostat, the previous base that had been on the wall appears to be capable of handling heat and AC along with a heat pump even though the markings on the terminals differ from what the diagram shows for a CT87J setup. Overall, it looks to me like the actual thermostat is the same for all variations and then the model number changes in keeping with the base being used.
    Last edited: Dec 13, 2012
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