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  1. #1
    Join Date
    Aug 2008
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    12

    Question about AC Thermostat

    I have a Honeywell Chronotherm III thermostat that is the original in the house (1993). I have a question about its operation that I hope someone can shed light on.

    It seems that the thermostat will keep the AC running for a long time even after the thermostat says that the desired temperature has been reached.

    For example, it is set on 77 right now, and has been on 77 for a couple of hours, and the AC has been running the whole time. I understand that the thermostat shows an average over a period of time, but how long should it be on a set temperature before shutting off the AC?

    It does eventually turn off later in the evening and then appears to start a normal "cycle" of off and on.

    I read in the manual that this thermostat also senses the wall temperature. Could this be skewing the temp readings and keeping the AC running longer than it should?

  2. #2
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
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    3,368
    The A/C can and will run when the setpoint temperature is displayed; it's job is to maintain that temperature.

  3. #3
    Join Date
    Aug 2003
    Location
    Lincoln, Nebraska
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    1,051
    As said if the thermostat is not going below 77 then it is doing its job. The long run times could be normal or could be attributed to somthing else.

    When was the last time the unit was serviced?
    Its a good Life!

  4. #4
    Join Date
    Aug 2008
    Posts
    12

    Re: Question about AC Thermostat

    The unit was serviced in May, so it is charged, etc...

    The unit is underpowered, and we have other issues (Attic ventilation, etc...), so it is not running at peak efficiency.

    I was just wondering why it would run for hours on end when reading the set temp. Is the thermometer in these things sensitive enough to know that shutting off for 5 minutes would not allow it to keep the temp? Does the sensed temp have to drop before it will shut off?

    Thanks

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,742
    When it reads 75, the actual temp might be 75.5.
    So it keeps it running until 74.6

    Here's the big question.

    Is it keeping you comfortable.
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  6. #6
    Join Date
    Jul 2008
    Location
    Fairless hills
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    12
    is this them old mecury stats?

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,742
    No, the ChronothermIII is one of the early model programable digitals.

    A bit of a PIA to program.
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  8. #8
    Join Date
    Apr 2005
    Location
    Tampa, Florida
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    1,634
    Quote Originally Posted by brandoh73 View Post
    I have a Honeywell Chronotherm III thermostat that is the original in the house (1993). I have a question about its operation that I hope someone can shed light on.

    It seems that the thermostat will keep the AC running for a long time even after the thermostat says that the desired temperature has been reached.

    For example, it is set on 77 right now, and has been on 77 for a couple of hours, and the AC has been running the whole time. I understand that the thermostat shows an average over a period of time, but how long should it be on a set temperature before shutting off the AC?

    Sounds like typical, normal Honeywell digital 'stat operation. They measure temperature in 1/16th of a degree and use a little bit of calculus to determine the best runtime possible. They also lie on the display. If it's 77.5F and your setpoint is 77, it's going to show 77. Same is true if it's 76.5F, it'll still show 77F. It's looking at those hundredths of a degree and using its formula to determine when it should shut down. In general, assuming it's using default settings, the shortest it will try to run the unit is approx 20 minutes. Lots of variables involve here, so it could easily run 5 mins and shut down or it could run for 3 hrs then shut down.

    The cheaper thermostats use simple setpoints...Honeywell's decided to use math instead. I like Honeywell.

  9. #9
    Join Date
    Aug 2008
    Posts
    12
    Thanks for the info everyone. I kind of figured that it may be reading in tenths and just reporting full degreed. At night, it seems to run for 15 min, then shut off for 5, and as the night goes on, it will run for fewer 15 min intervals.

    As for the comfortable question, It does an OK job, but I could use more insulation in the attic (im in Dallas, TX), so that the cool air will stay in the house longer. Im looking into this now.

    Any suggestions on actually upgrading or changing thermostats? I would like a newer one like the Honeywell touch screens or the Rite-Temps. Any opinions on which one is better? (In general, not a specific model, I am looking at the ones for around $100 at Home Depot).
    Thanks

  10. #10
    Join Date
    Aug 2008
    Posts
    12

    Re: Question about AC Thermostat

    Thanks for the info everyone. I kind of figured that it may be reading in tenths and just reporting full degreed. At night, it seems to run for 15 min, then shut off for 5, and as the night goes on, it will run for fewer 15 min intervals.

    As for the comfortable question, It does an OK job, but I could use more insulation in the attic (im in Dallas, TX), so that the cool air will stay in the house longer. Im looking into this now.

    Any suggestions on actually upgrading or changing thermostats? I would like a newer one like the Honeywell touch screens or the Rite-Temps. Any opinions on which one is better? (In general, not a specific model, I am looking at the ones for around $100 at Home Depot).
    Thanks

  11. #11
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,368
    The VisionPRO TH8000 series is a nice thermostat, but I doubt they'll have them at Home Depot.

  12. #12
    Join Date
    Apr 2005
    Location
    Tampa, Florida
    Posts
    1,634
    That's easy -- avoid the RiteTemps. There's a reason you'll only find those in the home improvement stores and not sold by contractors. Similar for the Honeywell products found at said stores. They're a totally different product than what the pros sell.

    Personally, the Honeywell IAQ if you have a variable-speed unit or don't want to ever hear the thermostat click. Otherwise one of Honeywell's VisionPRO series (TH8xxx).

  13. #13
    Join Date
    Sep 2002
    Location
    I don't know
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    2,903
    The chronotherm III is an excellent t-stat; if it's functioning properly, keep it.

    Though they're easy to program, the number of buttons may make them seem intimidating to (ignorant) homeowners.

    They're a totally different product than what the pros sell.
    From what I've read and Honeywell's literature they're stripped down pro versions with shorter warranties. (Still good products)
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

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