Nest Thermostat Randomly Switches to AC While Set to Heat

[eidemilm]

Hello,

A few months ago, I installed a Nest learning thermostat (note:  Learning has been disabled and all schedules cleared) to replace a Honeywell.  It functions well for a week or two and then will randomly start cooling despite still being set to heat and still  showing "heating" on the display.  To fix it, I hold the dial in for ~10 seconds to restart the thermostat.  Here are the specifics:

Heat source:  Electric.  Heat type:  Forced air.  Separate AC condenser, not a heat pump.

Honeywell wiring (wire color - connection label):

1. Red - RC with a jumper to R
2. White - W
3. Yellow - Y
4. Green - G
5. Blue - B

Nest wiring:

1. Red - RC (Power)
2. White - W1 (Heat)
3. Yellow - Y1 (Cool)
4. Green - G (Fan)
5. Blue - C (24V Common)

I have to believe this issue is caused by something to do with the wiring.  I had an HVAC tech glance at the connections and he said everything looked correctly connected.  Any guidance is much appreciated.

Thanks,
eidemilm

[Markjosephp]

Hello eidemilm,

It seems like you've taken some thoughtful steps in troubleshooting the issue with your Nest Learning Thermostat. The fact that you've disabled learning and cleared schedules suggests that the problem may be related to the wiring or a compatibility issue. Since you've already had an HVAC tech review the connections and everything appears to be correctly connected, here are a few additional suggestions:

1. Ensure that the thermostat is compatible with your electric forced-air heating system. While the wiring seems correct, double-check the compatibility of the Nest with your specific HVAC system.
2. Make sure your thermostat's firmware is up-to-date. Sometimes, issues like this can be resolved through software updates that address compatibility or performance issues.
3. In addition to manually restarting the thermostat by holding the dial, try power-cycling the entire system. Turn off the power to the HVAC system and thermostat for a few minutes, then turn it back on. This can sometimes reset any electronic glitches.
4. Check the configuration in the Equipment settings in the Nest app or on the thermostat itself. Confirm that the heating and cooling settings are configured correctly and that there are no conflicting or unusual configurations.
5. Even though the HVAC tech said the connections looked correct, it might be worth double-checking the wiring. Ensure that each wire is securely connected to the corresponding terminal and that there are no loose connections. Share with us your old thermostat and Nest Thermostat's wiring setup.

Let us know how it goes.

Warm regards,
Mark

[eidemilm]

Hi Mark,

Thanks for the thorough insights.  I tried to upload an image of the wiring from the old and new thermostats, but received a message observing, "You do not have permission to upload images.".  I will pursue your other recommendations in the coming few weeks and will let you know how things work out.  The issue hasn't returned for the past two weeks...It's likely just lulling me into complacency before it returns.  🙂

Thanks again.  Enjoy the upcoming New Year's holiday.

Matt

[Markjosephp]

Hey Matt,

You're very welcome for the insights, and I'm glad to hear that the issue hasn't resurfaced in the past two weeks. It's a positive sign, but I completely understand your cautious optimism—sometimes those pesky issues have a way of surprising us.

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If the problem persists or if you have any further questions, don't hesitate to reach out. Enjoy the upcoming New Year's holiday as well! Here's to a smooth transition into the new year and a comfortable, issue-free home. 

Warm regards,
Mark

[eidemilm]

Hi Mark,
Our nest thermostat decided to cool our home while we were out of town for the holidays.  We received a mobile app alert that the home was 45 degrees.  I ended up having an HVAC tech come out that day to diagnose the issue.  He determined that our Nest was defective (activating both heating and cooling at the same time) and disconnected the yellow wire as a short-term fix while we were away.  I contacted Google and they sent a replacement Nest, which I have since installed; I will send the defective unit back this weekend.  While this experience was frustrating, Google's customer care was pretty easy to work with and I'm satisfied with the result.
Thanks again for your guidance.
Matt

[Markjosephp]

Hi Matt,

Thanks for the update, and I'm sorry to hear about the unexpected thermostat issue during your holiday trip, but I'm glad to see that you were able to address it promptly. It must have been quite frustrating to come home to a chilly house. It's great that you were proactive in reaching out to an HVAC tech for a quick fix, and even better that we stepped in to provide a replacement.

If you ever have more questions or need further assistance, feel free to reach out by starting a new thread. Wishing you a comfortable and hassle-free home environment.

Warm regards,
Mark