Nest Thermostat WITHOUT heat pump puts forced air gas furnace into defrost mode.

[zunkman]

I keep my thermostat on Heat/Cool mode all year.  Today, the setpoints were at 72/78.  It's currently 38 degrees outside my house.  

I noticed around 6 PM that my Air Conditioning was on.  It was 78 degrees in the house.  I checked the thermostat, and it still showed 72/78.  I went down to check the codes on the furnace, and it showed DFT, which means Defrost.  (In Defrost mode, the A/C runs along with the heat to defrost an icy heat pump.) I did some research and it turns out that Defrost mode should only be used on Heat Pumps when it is chilly and very humid.  But again, I don't have a heat pump.  I changed the Mode on the thermostat to Heat Only, and rebooted the thermostat as well.  For the last 4 hours, nothing has turned on since my house is still cooling off (naturally) from 78 to 72 degrees.

I don't know if the Mode change or the Reboot will do any good, but I'm curious to know if anyone else had an issue similar to this.  Thanks!!

[RenzJ]

Hi zunkman,

Thanks for bringing this up here in the Google Nest community. Let's look deeper into this. Gather power reading of the Nest Thermostat:

For the Nest Learning Thermostat and Nest Thermostat E:

1. Press the thermostat ring to bring up the Quick View menu.
2. Turn the ring to Settings and press to select.
3. Turn the ring to Technical Info and press to select.

For the Nest Thermostat 2020:

1. Tap the touch bar to bring up the Menu view.
2. Slide your finger down to Settings, then tap to select.
3. Slide your finger to Equipment > Power Info.

In addition to this, send us a photo of your current wiring and setpoint while you are in heat mode. Also, if I may ask, are you getting any kind of error message on the Nest Thermostat?

I look forward to your response.

Best,

Renz

[zunkman]

Actually, I rebooted the thermostat and it hasn't happened again.  I think something just got kerfuffled. 

But, for the sake of information, no error messages.  Battery 3.86, VOC and VIN both 35.36. 

I can't send a photo of wiring and setpoint while in heat mode, since I always keep the thermostat in "heat/cool" mode.  The problem occurred while in heat/cool mode, so debugging while in heat only mode would be a lesson in futility.

[RenzJ]

Hello zunkman,

Thanks for the update. Let’s continue observing the Nest thermostat and see if the same issue occurs. If the issue happens again, we’ll need to get a photo of your current wiring. To attach the image, you can go to this link: Google Nest Community Forum. Hit reply on the thread, and you'll see a camera icon or three horizontal lines to expand the toolbar, where you can click the camera icon and send your image.

Keep us posted.

Best,

Renz