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Thread: 2nd Gen Nest running AC compressor when trying to heat

  1. #13
    Junior Member

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    @stang - thanks for your reply. Some updates...thanks to Lowes, I exchanged the Nest thermostat for a new one and installed it yesterday morning. I've noticed no issues so far. Although now I'm a little hesitant to relax knowing this issue can resurface down the road. Interestingly, I have an 8 year old carrier 58MXA LP furnace with a year old Trane XL15i (I'm no expert, but the carrier sounds to be similar to mine?). Before I installed the replacement Nest, I investigated my thermostat wiring. It can get any more basic. 5 wires from thermostat to 58MX (W,Y,G,C,Rh). The Trane AC is tied into the carrier control board at the C and Y terminals. I have a Honeywell humidifier tied in as well to C and the HUM terminal. I had 27 VAC on the red wire at the thermostat base. Could +3V be causing Nest plate failure? If so, then how can I ever relax not knowing what the h*ll what I'll find when I get home? The AC trying to run in 20 deg outside temps?

    Subsequently a friend at work told me he suspects his first gen Nest burned out his AC compressor this summer by constantly cycling it off and on.

    I'm also going to try and record the battery levels on the Nest over the next few days. It shouldn't be power stealing. I have verified that C is installed on the board. If I didn't have C, my old digital thermostat wouldn't work, right?

  2. #14
    HVAC Thermostat Specialist

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    Your old thermostat may or may not have worked without C. It depends on the thermostat. Some digital thermostats use batteries, some use furnace power, and some are power stealing.

    What you CAN do to guarantee that the A/C does not run with heat is to turn off the a/c breaker for the heating season. However, a $250 thermostat should NOT be having these types of issues!

  3. #15
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    I just registered for this forum after experiencing the same issue described in this thread today. I have a Carrier A/C system and a Utica gas boiler for heat. Around lunch time I kept wondering why I was being blown on by cold air, looked outside, and my compressor was running. Called Nest support. They had me switch the C and the G wires among other things and basically determined there was a problem with the unit. He was not specific, but said something about the connections maybe shorting out. They are overnighting me a new one and including a paid return package. Does not instill confidence in the unit though.

  4. #16
    Junior Member

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    I've been a week on my replacement and haven't noticed any issues. I've been keeping track of the battery voltage to see if it steadily drops (thinking it might be trying to power steal over the Y terminal even though I have C terminal). Its been fluctuating up and down by a few tenths of a volt. Post back to let us know what happens with your replacement. I'm still not comfortable knowing other people (see post above) had theirs fail after 7 months.

  5. #17
    Junior Member

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    I'm glad I posted to this forum because I have a timeline to reference.

    So my second 2nd generation Nest thermostat just went haywire on me tonight, almost 3 weeks to the day. It turned on my AC compressor in 36 degree outdoor temps. I cut the breaker to my AC unit and it started making a terrible humming sound at the unit outside. I flipped the outdoor disconnect on it and then it started cycling my furnace on and off quickly. It's going back to Lowe's (thank god for their good exchange policy). No more. Done with this piece of junk. Considering returning the one on my heat pump unit too. Why take the risk on $8000 worth of HVAC equipment to 'supposedly' save a few bucks in heating and cooling? No doubt it's a sexy device. I want it to work.

    Not willing to be their testing team on thousands of dollars of HVAC equipment in my house. Just very fortunate it happened while I was at home and not put of town for a weekend. Hope this helps others.

  6. #18
    Junior Member

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    Just an update. Bought a 2nd generation Nest at Lowes on advice from Nest support that the newer models are more tolerant of voltage fluctuations and installed with heat being the only request and no cooling needs. Heat call came on and immediately the a/c compressor began to cycle. Brand new sealed unit. Amazing. Glad I bought this one at Lowes and not directly from Nest (don't have to wait a month for a refund check) it is going back today for a refund.

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