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  1. #1

    External temperature error

    I have a dual-fuel system, heat pump with gas aux. I have noticed that the external temperature as displayed on the thermostat rises several degrees when the unit runs (5 or more degrees). I am wondering if this could cause the unit to sometimes switch back and forth between heat pump and gas when it is really cold enough to use gas exclusively.

    I assume the problem is that there is some waste heat near the unit while it is operating, and the temperature at the probe rises. Is this normal?

    I am thinking of remounting the probe a few feet away from the unit.

  2. #2
    Join Date
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    Did you install this sensor yourself?

  3. #3
    Join Date
    Sep 2005
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    Texas
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    2,793
    where do you live? if your gas rates are cheaper I wouldnt even use the HP for heating

  4. #4
    Quote Originally Posted by bobb25 View Post
    Did you install this sensor yourself?
    No, I did not install the sensor (nor any other part of the system). This is a new system installed as part of a remodeling project.

  5. #5
    Quote Originally Posted by air2spare View Post
    where do you live? if your gas rates are cheaper I wouldnt even use the HP for heating
    Tennessee. We have reasonable electric rates (TVA). Gas is pricey.

  6. #6
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    If it was installed according to directions, then get the installer back to check it out. I have had problems with Honeywell sensors.

  7. #7
    Quote Originally Posted by bobb25 View Post
    If it was installed according to directions, then get the installer back to check it out. I have had problems with Honeywell sensors.
    He has checked it once. It was originally stuck up under the base of the unit, dangling just above the dirt. He moved it up to the side of the unit and used nylon wire ties to strap it to the electrical supply conduit.

    The thermostat is a Honeywell, so I guess the sensor is as well. What kind of problems have you had with them? If the unit is not running, the outside temperature displayed on the thermostat is withing a degree or two of another outdoor thermometer I have. It's just when the unit is running that the temperature starts to rise.

  8. #8
    Join Date
    Jan 2004
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    Lancaster PA
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    Did he run a seperate wire for the sensor. Or use the same bundle that he used to wire up the OD unit.

    Directions say to run a seperate wire for the sensor.
    Also, if he has it straped to the line voltage wire running the the OD unit, it can also interfer with it.
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  9. #9
    Join Date
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    The problem I was having, is the sensor would not read correctly for the customer. Everytime I got there and ohmed it out, everything was fine. I even changed sensors and same thing. Spent a lot of time on the phone with Honeywell. They told me to run shielded thermostat wire up to the thermostat and make sure it wasn't touching any line voltage. Did all that, and never figured it out. I was back at least 10 times.

    Your problem sounds like somethig is throwing the sensor off when it's running. If it isn't air outside, see if the sensor wires are touching any line voltage wires. If I remember right, the directions also give you maximum length of run.

  10. #10
    Join Date
    Aug 2007
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    Northern VA 38 degrees N by 76 degrees W
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    Quote Originally Posted by CaptainKangaroo View Post
    He has checked it once. It was originally stuck up under the base of the unit, dangling just above the dirt. He moved it up to the side of the unit and used nylon wire ties to strap it to the electrical supply conduit.

    The thermostat is a Honeywell, so I guess the sensor is as well. What kind of problems have you had with them? If the unit is not running, the outside temperature displayed on the thermostat is withing a degree or two of another outdoor thermometer I have. It's just when the unit is running that the temperature starts to rise.
    Two major problems, Thermistor wire not not isolated and run in seperate cable, and poor location. You have at least one.

  11. #11
    Quote Originally Posted by beenthere View Post
    Did he run a seperate wire for the sensor. Or use the same bundle that he used to wire up the OD unit.

    Directions say to run a seperate wire for the sensor.
    Also, if he has it straped to the line voltage wire running the the OD unit, it can also interfer with it.
    I don't know if it is a seperate wire or not; I'll check that.

    It doesn't act like electrical interference to me (could be wrong, of course), since the temperature slowly rises after the unit comes on, then slowly drops after the unit shuts off. I would think interference would make it jump around while it was running, then settle down immediately after it shut off. Then again, maybe the firmware in the thermostat is averaging out the spikes over time? Surely not...

    Thanks to all for the good info.

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