I've had that particular problem with an old thermostat before. No wifi or anything, but when the batteries would run out, it would get stuck into AC mode, no

I've had that particular problem with an old thermostat before. No wifi or anything, but when the batteries would run out, it would get stuck into AC mode, no matter the previous settings. Getting back from vacation was like a meat locker. It seems weird that the "failure" mode of these things seems to be make the plants think it's winter.