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  1. #1
    Join Date
    Jan 2014
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    Default Thermostat reading 10 degrees too high - wiring issue?

    Recently purchased a "fixer upper" and the thermostat was dirty and falling off the wall. I decided to replace it with a Honeywell programmable. The old thermostat had wires for Rc, Rh, W, Y, and G. I put the new Honeywell on with exactly the same connections, and the thermostat reads about 10 degrees higher than actual temperature in the room.

    I checked for dirt blocking the sensor, didn't find any, so I decided to replace the thermostat with a fancy NEST wifi thermostat. Contacted NEST to see if it would work with the Rc, Rh, W, Y, and G wires, and they said yes.
    Put the NEST on exactly as they recommended, but the NEST is reading the same way, about 10 degrees too high. I removed the thermostat from the base to make sure I wired it correctly, and noticed the spot where the Y wire is connected is extremely warm.

    I followed the thermostat wires down to where they connect to my furnace/AC, and saw that there are terminals marked Y, W, G, R, and C.

    I noticed the Y and W wires from the thermostat are both connected to the Y terminal on the transformer. The W terminal on the transformer is not connected to anything.

    The G wire on the thermostat is connected to the G terminal.

    The Rc wire from the thermostat is connected to the C terminal. The Rh wire is connected to the R terminal.

    Is having both Y and W connected to the Y terminal on the transformer be the source of my problem? Should I move the W wire over to the W terminal, or will that mess something up?

  2. #2
    Join Date
    Sep 2012
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    Boston
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    Default Re: Thermostat reading 10 degrees too high - wiring issue?

    is the thermostat located over a heater? is it located on a wall that has a heating pipe running up through the wall right behind it? what's on the other side of the wall that the thermostat is on? a piece of heat? an oven?

  3. #3
    Join Date
    Jan 2008
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    2,203

    Default Re: Thermostat reading 10 degrees too high - wiring issue?

    The way to verify a thermostat's accuracy is to put a known accurate thermometer right next to it where it will encounter the exact same conditions the thermostat does. Even the cheapest thermostat will usually be within a couple degrees unless something is wrong with it. If you're measuring anywhere else, then of course you're going to have a variation, just the same as measuring at the ceiling and floor levels brings different numbers.

    So what does a thermometer do to keep you comfortable? Nothing. So forget what the thermostat is set at- it means nothing- just make the house comfy and note what it says, then worry about undue variations from that number. And like MLB says, it's location matters so if it's poorly sited, move it.

    Phil

  4. #4
    Join Date
    Jan 2014
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    3

    Default Re: Thermostat reading 10 degrees too high - wiring issue?

    Quote Originally Posted by MLB Construction View Post
    is the thermostat located over a heater? is it located on a wall that has a heating pipe running up through the wall right behind it? what's on the other side of the wall that the thermostat is on? a piece of heat? an oven?
    Thermostat is not over a heater or anything. It is on a wall where a duct from the furnace leads up to the second floor, but the thermostat reads high even when the heat is not running. I compared the reading with a thermometer next to it.

    Nobody thinks the wiring maybe an issue? Even when the base of the thermostat was extremely hot to the touch near the Y wire?

  5. #5
    Join Date
    Aug 2007
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    Maryland
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    Default Re: Thermostat reading 10 degrees too high - wiring issue?

    I'm going to say something is obviously wrong if the thermostat is extremely hot. Could be a fire hazard. I would say it's time to get an HVAC tech in to figure it out.

  6. #6
    Join Date
    Jul 2010
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    SoCal
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    Default Re: Thermostat reading 10 degrees too high - wiring issue?

    Quote Originally Posted by ed21 View Post
    I'm going to say something is obviously wrong if the thermostat is extremely hot. Could be a fire hazard. I would say it's time to get an HVAC tech in to figure it out.
    Yes, call a tech. You may have your wires messed up.

  7. #7
    Join Date
    Jan 2008
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    2,203

    Default Re: Thermostat reading 10 degrees too high - wiring issue?

    Thermostats should not 'heat up' except for the ones directly controlling electric baseboard heat- and even those rheostats shouldn't warm up much.

    My guess is that either the thermostat is mis-wired or that someone has replaced the low-voltage transformer in the furnace which supplies thermostat power with one running at a higher voltage during a repair. With it 'self-heating' something is definitely wrong and it will definitely be reading incorrectly!

    Phil

  8. #8
    Join Date
    Dec 2013
    Location
    New Hampshire
    Posts
    27

    Default Re: Thermostat reading 10 degrees too high - wiring issue?

    Heat uses only the W and Rh terminals. Rc Y and G are used for air conditioning and fan-only settings. A warm wire to the Y terminal indicates a malfunction of the air conditioning circuit. Disconnect the wire to the Y, Rc, and G terminal and check the voltage of the transformer. High voltage may have caused a short in the thermostat and relay. The wires from the Rh and Rc should be connected to the transformer not the Y and W wires, but this reversal would only make the fan control inoperative.

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