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need to wire a simple thermostat

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Robert_H
Robert_H Member Posts: 323
in Thermostats and Controls

We service a furnace and the customer is in her nineties and is really afraid of digital thermostats and defiantly thermostats with batteries. She and her care giver asked us to install a HW "Round" that they had picked up. I installed it and it worked for a bit. But after a few minuets, it quit which I thought might happen. We had previously installed the simplest battery thermostat we could find Honeywell TH1110E1000/U but she just wants a simple non battery thermostat.

I picked a HW CT31A which is a mechanical thermostat but it does not have the RC connection which is needed for this furnace/air handler.

Almost every elderly person I run into wants a simple Honeywell Round thermostat but HW shifted the rounds to power robing design and it really pisses me of that it is often incompatible and they won't step up to the plate and provide what people want. But I digress.

The Furnace/AC is a Boyerton LBD85 with a coil. this diagram shows a T87F thermostat which I believe is a mechanical, mercury, model.

Document_2026-05-04_220001.jpg

I read that a resister can be install between W and C on the control board but when I called HW tech support they said that was a custom modification and they could not advise it.

Any advice on how I can better support this customer would be appreciated

Robert

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 27,453

    Get a T87F from eBay. They aren't common, but they are available.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Robert_H
  • ratio
    ratio Member Posts: 4,250

    The RC wire is for a separate transformer in the condenser, when that was an add-on to a furnace. It's been a while since that was a thing, most everything is all fed from the xfrmr in the furnace nowadays. If there's no separate transformer to energize the contactor in the condenser, no RC wire is needed. If there is, an interposing relay (I suggest a RIB U1C) can be used, energized from the Y call from the thermostat, to isolate the two transformers.

    Robert_H
  • Robert_H
    Robert_H Member Posts: 323
    https://forum.heatinghelp.com/discussion/comment/1895839#Comment_1895839

    and at $17 that's a relatively inexpensive solution

  • Robert_H
    Robert_H Member Posts: 323

    Actualy I ran across those on EBAY last night and am considering getting a few for these circumstances.

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 12,368

    Easy Peasy

    Screenshot 2026-05-05 at 7.51.42 PM.png

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • HVACNUT
    HVACNUT Member Posts: 7,489

    If there's a spare wire, just power the TH1100E with 24 volts. No batteries needed.

    If no exra wire, do they REALLY need G for fan only? Jump Y an G at the furnace fan center relay, NOT at the thermostat, and use the extra wire previously on the G terminal to connect to Common. The heat side doesn't use G for the fan as it goes through the fan/limit control for a different speed.

    EdTheHeaterMan
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 12,368

    This is obviously an oil burner situation. The RIBU1C is a great fix for thermostat problems. And @HVACNUT has a great fix for getting an additional wire for the C terminal if needed.

    image.png

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • mattmia2
    mattmia2 Member Posts: 17,010

    looks like exactly what you want:

    https://www.supplyhouse.com/Honeywell-Home-T87N1000-Round-Non-Programmable-1H-1C-Mechanical-Thermostat

    round mechanical, can support separate heat and cool transformers if you remove a jumper

  • mattmia2
    mattmia2 Member Posts: 17,010

    er, reading the fine print the honewell has a battery that might require attention every couple decades

    this wr/emerson doesn't

    https://www.supplyhouse.com/White-Rodgers-1C26-101-Heat-Cool-Snap-Acting-thermostat-in-white-vertical-mount-11459000-p

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 12,368
    https://forum.heatinghelp.com/discussion/comment/1895899#Comment_1895899

    when I use that Honeywell T87N thermostat I sometimes take the fan switch off of the control subbase and break the lever so little old ladies don't touch the wrong switch and call you with a problem. A pair of side cutters does the job nicely.

    Just an old trick from an old man in a wheelchair.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

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