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AC T-Stat Won't Accept New Lower Setpoint

Honeywell RTH221B1021. Installed 2012. The other day I decided to lower the setpoint from 80 to 78 degrees. T-stat still maintains 80. I checked the batteries which are OK. Can't figure this out unless this t-stat has some kind of faulty resistor etc. Never had a problem before. I will try other T-stats I have lying around if this thread doesn't come up with some solution that I'm missing. I see it has three wires.


  • ratioratio Member Posts: 2,496
    A heat pump in cooling mode, or straight AC? Is the outside unit operating (blowing warm/hot air out the top)?

    If the unit is sized properly, it might not be able to catch up for quite a while if it's 80° inside.

  • Jamie HallJamie Hall Member Posts: 12,952
    Well, thermostats do fail -- but not usually so they refuse to change setpoint.

    So -- I presume you have checked the obvious? That, in fact it does turn off the AC when the temperature drops to 80, rather than keeping going to 78? Place a reliable thermometer right next to it, to verify that the display isn't lying to you.

    Then run a second quick test: turn the setpoint up -- say to 85, assuming it's warm enough for the building to reach that. Does it still lock on and hold 80, or does it allow the temperature to rise before kicking on? And if it rises, rise to what?

    Assuming it passes that, now a third test: from 85, which it now holding, wait until the thermostat shuts off the AC, and then drop the set point to something much lower -- say 75. After a short pause, the AC should kick on and keep going until it gets down to 75 -- and then shut off. But watch out for this test: it's possible that the AC system won't shut off, but that the temperature doesn't get down to the set point. That's not the thermostat's problem; that tells you that the AC doesn't have the capacity to get down there. And if the AC does shut off, but the thermostat display still indicates it's above the set point, make sure that it is the thermostat that has shut off the AC, rather than the AC itself.

    And tell us what has happened...
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • Big Ed_4Big Ed_4 Member Posts: 1,359
    Is the thermostat set to "Hold" to bypass the program . If not the set point you set is temporary .. the program set point could be set to 80* at different periods
    I have enough experience to know , that I dont know it all
  • ChrisJChrisJ Member Posts: 11,073
    Is it that the system can't lower it to 78 or is it the tstat actually cycling at 80?
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • D107D107 Member Posts: 1,686
    edited July 27
    @ratio @Jamie Hall @Big Ed_4 @ChrisJ Just got home and checked it out. Wasn't on Hold but it's been so long since I'd re-set this I had forgotten that this has four different period settings. So I changed all four set points, but first time it still went off at 80, but interestingly the 'cooling' notice was blinking, indicating the t-stat was calling for cooling but AC unit was off.

    Then I turned unit off and re-started, now it seems to be cooling down on its way to 78. Will see if this maintains. If not I'll go through all of Jamie's list.

    (This is straight AC, 3 ton 1989 unit, usually no problem bringing it down to 72 even at 90 outside temp.)

    This T-stat is one that will always show the set temp as the actual temp--which I like. So if it's set to 78, when it gets to 78.5 or whatever the tolerance is it still shows 78 then AC kicks in, and when it gets down to whatever the differential is 78? 77.5? it will still show 78.

    Thanks for all the trouble-shooting tips--I'll copy them and save them for the future. Since I'd just re-set the other three periods to 78, perhaps it needed a little time to re-sync with the unit to put that into effect. Note to Self: Always read the directions again!
  • HVACNUTHVACNUT Member Posts: 3,414
    The cooling blinking is likely a 5 minute anti short cycle time delay.
  • D107D107 Member Posts: 1,686
    @HVACNUT Yes, but odd that the unit was running, THEN stopped with the blinking light. Usually if you try to turn on unit within the five minute delay it won't start to begin with. But regardless of that anomaly I think you're probably right.
  • Big Ed_4Big Ed_4 Member Posts: 1,359
    Change the batteries ..
    I have enough experience to know , that I dont know it all
  • WellnessWellness Member Posts: 101
    edited July 27
    Don't know if this is the case with the Honeywell, but some thermostats (especially in "auto" mode) won't accept an AC setpoint that is below or within a few degrees of a heat setpoint that may already be pre-programmed into the device. So maybe check the heat setpoints.
  • mattmia2mattmia2 Member Posts: 1,438
    It is also configurable as a "deadband" setting on some t-stats. Make sure someone didn't set that to like 10 degrees.
  • D107D107 Member Posts: 1,686
    @Wellness @mattmia2 Thanks for the great tips. It has now held the 78 target temp for 12 hours and working well, so in this case it looks like my error was not setting all four time periods to 78 degrees since I'd forgotten it was programmable.
  • Big Ed_4Big Ed_4 Member Posts: 1,359
    Normal error ...
    I have enough experience to know , that I dont know it all
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