Different (bad) dual-fuel behavior started Today (Nov 20, 2025) - support is useless

[lprimak]

I have dual-fuel setup, with heat pump with no aux electric heating and a natural gas-fired furnace for colder temperatures.

I had the Nest (3rd gen) for about 9 years and it always behaved the same. Heat pump would run when the outside temperature is above the lockout temperature that's configured (30 degrees) for me.

Heap pump would run as long as necessary and the furnace would not come on until desired indoor temperature is achieved.

Today, something different started happening. At an outside temperature of 50 degrees, the furnace started to come on at about 10 to 15 minutes after heat pump came on. First regular "heating" message appear, then "alt heat" after about 10 to 15 minutes, which used to only happen when the outside temp would fall below 30 degrees (lockout temperature)

I tried resetting the thermostat, and support was very much useless.

In my area, electricity is much cheaper than gas, and I want to have the heat pump work as long as possible. This is why I purposefully made the lockout temp so low.

Does anybody have the same experience? What gives? How can I fix it?

For now, I have disconnected W1/2 wires that control the furnace.

[eeonthecape]

Same for our two zone heat pump systems with backup gas furnaces up propane furnaces. It’s going to cost a lot of extra money on our propane bill. I’m hoping replacing the thermostats with something non-Nest will fix the issue. Even worse, today, our upstairs zone is cycling both the heat pumps and the gas furnaces back and forth on the same heating call which could be extremely dangerous for a gas furnace using propane as it needs to run long enough to burn the gas it sues and dissipate the generated heat out of its heat core. The upstairs gas furnace just came on and ran for 45 seconds and then the heat pump started running again. This is a seriously dangerous issue that must be fixed immediately. 

[jcordle]

I am having similar and other bizarre behavior which also started on 11/20/2025 from the thermostat. Walking by thermostat while heat pump is heating the display comes on and the heat pump shuts off and the gas furnace comes on.

[PLandon · Accepted solution]

I was having same problem.  Has resolved since yesterday around 5PM eastern time.  You?

[jcordle]

Actually I chatted with support and they walked me through steps in the settings  menu to restart the unit without losing my settings and it has worked fine so far.

[lprimak]

I have tried the "reboot" as well but it didn't work. However, some people reported the problem resolving. I am going to wait a couple of days and reconnect the W1/W2 wires and see if it has resolved or not. Stay tuned.

[lprimak]

The problem seems to have been resolved for me as well. Thank you