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Honeywell thermostat for steam heat

Jeff_4 Member Posts: 15
operates with a honeywell round thermostat. My question is why does the temp of the house go 2 to 2.5 degrees below what the therm is set for in order for the boiler to go on.. For example, if the therm is set at 70, the house temp drops to about 68 degrees before the boiler begins....It happens no matter what the house temp is set for. Is this normal for a millivolt..or a steam boiler? The boiler shuts off correctly once the house reaches the right temp. ...and the thermostat is only 2 years old and designed for a millivolt system!


  • BillW@honeywell
    BillW@honeywell Member Posts: 1,099

    The "round" millivolt thermostat is a TS87. It is a mechanical control, designed in the late 1940's. It has a coiled bi-metal as a sensing element, and a heat anticipator which must be set to match the gas or oil control on your boiler or furnace.+ or - 2 degree performance is typical for these old mechanical designs. I would suggest you select a contractor from the "Find a Contractor" feature on this web page, and ask for a new T8002C1000, our new battery powered, programmable thermostat that will work with your millivolt steam system, and your central cooling system,if you have one. It is an "EnergyStar" rated product, simple to program and featuring a lighted display, 5-2 day programming and +-1 degree accuracy. This thermostat is not available at retail stores. For more information, visit www.honeywell.com/yourhome. Thanks!
  • I am assuming the Thermostat is a TS-86

    with a fixed anticipator for Powerpile systems. Before I would replace the thermostat I would have someone who knows how to:

    1. Make sure the backplate is level.

    2. Make sure the thermostat is calibrated (a lost art).

    3. Have someone take a complete set of Millivolt readings on your system to make sure that the gas valve pull in is correct. Also make sure that there is not a high millvolt drop across one of your controls. This needs to be done by a professional gas technician who understands Powerpile Systems.

    If that does not work then install a new TS-86A-1371

    I am not familiar with the TS-87 that Bill Wolfe from Honeywell mentioned. It might be something new they have come out with???
  • BillW@honeywell
    BillW@honeywell Member Posts: 1,099

    A senior moment or typo crept in and was caught by the ever vigilant Mr. Tim. You are indeed correct, the number is a TS86A1371. BTW,Tim, the new T8002C1000 is worth taking a look at...we now have electronic accuracy for millivolt systems! Thanks for the correction!
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