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Thread: Nest not syncing HVAC fan and A/C, coil is a brick of ice!

  1. #1
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    Nest not syncing HVAC fan and A/C, coil is a brick of ice!

    I've had my Nest 3rd Gen for 3 months. It heated during the Winter and cooled properly in the Summer until 2 weeks ago when the Nest started shutting off my HVAC fan, but leaving the A/C on so the HVAC coil froze over badly.

    I also notice that the target temp and room temp were never the same unlike my previous old school Honeywell stat, which I had to reinstall after the freeze until I figure out what's going wrong with my Nest and get a new one.

    Also, I've read a lot of posts by other owners having their target temp and actual room temp in disparity. This never happens with the Honeywell BTW. When the target temp is reached the Honeywell shuts down my HVAC as expected and it restarts when the room temp get too hot or cold.

    The Nest seems to be more focused on the time the fan is running rather than actual room temp. So I don't know how to set the fan preference like 15 mins? (default), 30 mins?, etc.

    Thanks for your help.
    Last edited by shmerls; 07-21-2019 at 06:33 PM.

  2. #2
    Senior Member

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    With Nest verify you have a C wire connected. You probably have a bad contactor in the outside unit.

  3. #3
    Junior Member

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    My HVAC has 4 wires that I disconnected from my Honeywell TSTAT:

    Attachment 1707

    And connected to my Nest:

    Attachment 1708

    As you can see there is not C wire among the 4 wires. This 4 wire set up without a C wire worked
    without any problems since I got the Nest in the Winter - running the Heat for a couple
    months and then switching the Nest over to A/C until which worked fine until one day when the A/C
    froze over, which is when I disconnected the Nest (and returned it to Amazon for full credit) and
    reinstalled my original Honeywell, which has been working fine ever since also without a C wire.

    Now I want to get a new Nest and reinstall it and try again without a C Wire and see if the fan will keep
    running in sync with the A/C simultaneously, but before doing so, I wanted to get some comments here
    like yours before I do the Nest re-purchase and re-install.

    The only issue I see is I don't see a C wire that you mention, coming from my HVAC, so don't have one to
    install like last time:

    Click image for larger version. 

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    Can I trouble you, or anyone, for your thoughts on this C wireless set up? I really appreciate it!

    NOTE: Double click my pictures to enlarge them

    Many Thanks,
    Last edited by shmerls; 07-26-2019 at 09:50 PM.

  4. #4
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    Last edited by shmerls; 07-26-2019 at 09:52 PM.

  5. #5
    Senior Member

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    What you can do is re-purpose the green wire for a C. Move it from G to C at both the thermostat and air handler. Then put a jumper to connect Y and G at the air handler. You lose independent control of the fan but most people don't use it anyway.
    Last edited by Stuff; 08-04-2019 at 05:49 AM.

  6. #6

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    Stuff Y to G at the air handler correct?

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