AC turns on when on Heat Mode unexpectedly

[kdub]

I installed a Nest Learning Thermostat, 3rd generation, on 6/13/2024. It worked fine. Then in October I switched to Heat Mode. It ran in Heat Mode for 6 days in October and everyday in November. I have Gas Heat with Forced Air. I use wires Y1, G, Rc, W1, C.

My issue is that twice now, my AC unit turned on while the thermostat was in Heat Mode. The only way I could shut off the AC was to reboot the thermostat. The first time was in October during the afternoon. The second time was 11/26 at 4 AM.

I used a Wyze Thermostat for the previous 3 years without any issue. And I am so concerned the Nest Thermostat will destroy my AC by running it in frigid weather, I have reinstalled the Wyze Thermostat. But I would rather use the Nest Learning Thermostat.

Does anyone have any advice on what could be wrong?

[Pamela2908]

I’m have an issue with the same 3rd generation thermostat. Mine wont heat at all no matter what I set it to. I’ve tried reboot and resetting several times. It stays 62 in my home. Unbearable! 

[virnab]

Hi Pamela2908,

Thanks for posting in our community.

I’m sorry to hear that you’re having a similar inconvenience with your Nest Learning Thermostat (3rd gen) as kdub is reporting. I can see in your case it is not heating at all, no matter what you set it to. Thank you for letting us know that you have also rebooted and reset your Nest Thermostat. No worries; it is my pleasure to assist you with this inconvenience.

To provide you with better assistance, please answer the following questions:

• Does the thermostat screen indicate that it’s actively calling for heating?
• What type of HVAC system do you have?
• When did the heating last work?
• Did heating ever work with the Nest Learning Thermostat (3rd gen)?
• Did you turn the HVAC system off?
• Have you recently experienced a brownout or blackout?
• Did you select the correct fuel type?
• Does the furnace activate while the fan doesn’t?
• When was the last time you changed the furnace's air filter?
• Have you experienced extremely cold weather lately?
• Is this the first notable temperature drop of the year in the area?

Besides this information, I would appreciate it if you could check the power readouts and share with me a picture of the results. Here are the instructions to get the power readouts:

1. Go to Settings
2. Then select “Technical Info.”
3. Select the option “Power.”

The information that I need from the screen you’re going to get is the following:

• Battery
• Voc
• Vin
• Lin

Thanks for your kind attention; I will be looking forward to your response.

Best regards,

Virna

[virnab]

Hi kdub,

Thanks for reaching out to our community.

I’m sorry to hear that your Nest Learning Thermostat (3rd gen) stopped working fine by October 2024, when you switched to heat mode, and it ran in heat mode for 6 days in October and every day in November. Now your AC unit turned on while the thermostat was in heat mode, and the only way you could shut off the AC was to reboot the thermostat. Thank you for letting us know that you have rebooted your Nest thermostat in October, and the second time was November 26th, 2024. I appreciate you sharing all this information to help you use your Nest Learning Thermostat again. No worries; help is here.

Before I start further troubleshooting, let me ask you a few questions:

• Are there any error messages? If so, what are they exactly?
• What kind of HVAC system do you have?
• When was the last maintenance check for your HVAC system?
• Could you please send me a picture of your energy history to verify the features? You can refer to this article to check the energy history: [How to] Check Energy History for unexpected thermostat behavior.
• Do you notice any intervals or specific patterns between these strange behaviors?
• Does the issue happen on heating only, cooling only, or both?
• Have you ever received only cooling when calling for heat?

Besides this information, I will also appreciate you sharing a picture of your current wiring installation.

Thanks for your kind attention; I will be looking forward to your kind response.

Best regards,

Virna

[kdub]

Virna,
I had a phone call with Google Support and went through many of those questions. I was told the reference number for that call was: 7-9458000037185.

There were no error message.
I have Gas Heat with Forced Air.
The heating works fine. That is not an issue.
No blackout or brownout issues.
Yes, I selected the correct fuel type.
No, the furnace does not activate without the fan.
I change the filters every 3 months.
No extreme cold weather.
And no, the issues did not happen on the first notable temperature drops.

I no longer have the thermostat installed, but the wires used are Y1, G, Rc, W1, C.

I would be willing to reinstall it to get those power readouts. But to what end? I have a $300 thermostat that I cannot trust. My Wyze thermostat has worked for years without this issue. I have reinstalled that.

[virnab]

Hi kdub,

Thanks for kindly responding to my questions. I appreciate you letting us know that you were assisted through a phone call with Google Support and providing us the case number, because it allows me to provide you with better assistance.

My team would like to know more about this behavior. When you get a chance, please fill out this form. Remember to include the link to this post. Let me know when you’re finished. From there, we'll contact you via email.

Let me know when you're finished. Your confirmation is crucial to ensuring that I can follow up on your case effectively.

Best regards,

Virna