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Thread: Thermostat question.. Heat Ok, Cooling way off...

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    DIY Senior Member Murphy625's Avatar
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    Default Thermostat question.. Heat Ok, Cooling way off...

    I have a Honeywell round thermostat that looks like a manual mercury type but is in fact electronic. I think its a CT87N.
    Anyhow.. when the thermostat is set to heat the house, it turns the heater on at about 2 degrees below set point and off at about 1 deg above the set point.
    However, when it is cooling in the summer, I can set it at 76 degrees and it will still bring the house down to 70 or so.
    The air conditioner turns on and off during the day as if its being controlled by the thermostat but the thermostat seems to want to cool the house down a good 6 to 8 degrees or more past the set point.

    Any ideas? I'm a bit stumped on what it could be.

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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by Murphy625 View Post
    I have a Honeywell round thermostat that looks like a manual mercury type but is in fact electronic. I think its a CT87N.
    Anyhow.. when the thermostat is set to heat the house, it turns the heater on at about 2 degrees below set point and off at about 1 deg above the set point.
    However, when it is cooling in the summer, I can set it at 76 degrees and it will still bring the house down to 70 or so.
    The air conditioner turns on and off during the day as if its being controlled by the thermostat but the thermostat seems to want to cool the house down a good 6 to 8 degrees or more past the set point.

    Any ideas? I'm a bit stumped on what it could be.

    Is you thermostat near your Return air intake ?

    How are you measuring the temp ?
    Theory only works perfect in a vacuum.

    Cyber Security Protection for Windows C:\ > WWW.WinForce.Net

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    DIY Senior Member Murphy625's Avatar
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    Quote Originally Posted by DonL View Post
    Is you thermostat near your Return air intake ?

    How are you measuring the temp ?
    I'm not measuring anything.. just reading what the thermostat says.

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    DIY Senior Member dj2's Avatar
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    Your thermostat could be out of calibration. The hole behind the thermostat could be too large, allowing air in. The thermometer could be faulty. And so on.
    You can try to test it to find out if it's out of calibration and then fix it, or simply replace it with a non-programmable one.
    Considering the low price of a new thermostat, I'd replace it.

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    DIY Senior Member Murphy625's Avatar
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    Quote Originally Posted by dj2 View Post
    Your thermostat could be out of calibration. The hole behind the thermostat could be too large, allowing air in. The thermometer could be faulty. And so on.
    You can try to test it to find out if it's out of calibration and then fix it, or simply replace it with a non-programmable one.
    Considering the low price of a new thermostat, I'd replace it.
    But why would it work just fine when heating? In fact, its only about 1 degree off from a real thermometer when in heating mode. Its only in cooling that it gets out of whack.

    Really strange...

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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by Murphy625 View Post
    But why would it work just fine when heating? In fact, its only about 1 degree off from a real thermometer when in heating mode. Its only in cooling that it gets out of whack.

    Really strange...

    It could be that the precipitator is heating the stat when in AC mode. That could make the set point differential wider.

    What type of AC / Heating system do you have ? Has it always been that way ?

    You should verify that it is wired correctly for your equipment type. (Switch #1 and switch #2 sets the type)


    Good Luck.
    Last edited by DonL; 08-29-2013 at 04:36 AM.
    Theory only works perfect in a vacuum.

    Cyber Security Protection for Windows C:\ > WWW.WinForce.Net

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The location of the thermostat in relation to air outlet (and returns) can make a difference in how well it works as can whether it gets any direct sunlight (neither good). Some thermostats have adjustable hysteresis (difference between on/off), and you may need to adjust that. As well as selection of the proper type of equipment.

    Ductwork setup for heating, where the outlets are low on the wall or in the floor, are lousy for a/c, since it's hard to get the heavier air up to the ceiling...you can get some pretty significant stratification or layering of air where your feet are cold, and your head is hot. In some houses I've been in, they ran the duct up the wall and put a second register higher up. During a/c season, they closed the lower register, and opened the upper one. In heating season, they reversed it. When I added radiant floor heating, I moved all of my registers up closer to the ceiling, and the cool air distribution is much better as is the comfort levels.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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