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

Discussion in 'HVAC Heating & Cooling' started by Murphy625, Aug 28, 2013.

  1. Murphy625

    Murphy625 Member

    Messages:
    149
    Location:
    Michigan
    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.
  2. DonL

    DonL Jack of all trades

    Messages:
    3,810
    Location:
    Houston, TX

    Is you thermostat near your Return air intake ?

    How are you measuring the temp ?
  3. Murphy625

    Murphy625 Member

    Messages:
    149
    Location:
    Michigan
    I'm not measuring anything.. just reading what the thermostat says.
  4. dj2

    dj2 Member

    Messages:
    398
    Location:
    California
    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.
  5. Murphy625

    Murphy625 Member

    Messages:
    149
    Location:
    Michigan
    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...
  6. DonL

    DonL Jack of all trades

    Messages:
    3,810
    Location:
    Houston, TX

    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: Aug 29, 2013
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,798
    Location:
    New England
    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.
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