If you recently installed a smart thermostat and find that it randomly turns off the air conditioning or heating, it’s probably not broken. It’s just using a «smart» feature, which, frankly, isn’t very smart.

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Most smart thermostats have a feature that can turn off automatically when you’re away from home, saving you money on your energy bills with no effort on your part.

This feature can, depending on the thermostat model, use two different triggers to determine if you’re at home: a motion sensor on the thermostat unit itself, and GPS on your phone (also called geofencing).


On the Nest thermostat, these features are called Auto-Exit and Departure/Departure Assist, respectively. The former uses only the Nest Thermostat’s motion sensor to determine if someone is home or not, while Home/Away Assist uses a combination of your phone’s motion sensor and GPS. If you only have auto-off turned on, the Nest only uses the motion sensor to determine if you’re at home or not, and this can cause some problems if your Nest thinks you’re not at home when you really are.

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If you are at home but spend most of your time in a room where your smart thermostat is not present, after a while the thermostat will think that you are not at home and will automatically turn off until motion is detected. So if you spend a lot of time in another room and notice that the air conditioner doesn’t turn on when it should, it’s probably because you haven’t walked past your thermostat in a while and it thinks you’re not home.


This is frustrating, of course, but there are a few things you can do to fix it. First, you can turn on the thermostat’s geofencing feature so that your smart thermostat uses your phone’s GPS to see if you’re at home or not. Alternatively, you can simply disable all auto-off features and manually change the temperature when you leave and when you return home. (In the case of Nest, this means disabling Auto-Away and Home/Away Assist — remember they should separate features.) This is less than ideal, but there are some users who prefer to at least some manual control of your smart thermostat, even though it can do everything for you.

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To enable geofencing on your Nest thermostat, you just need to turn on Home Assistance so that the thermostat uses your phone’s GPS to determine where you are. On Ecobee3, you can use geofencing with the IFTTT recipe.

To disable all nest motion detection features, simply disable Auto-Away and Home/Away Assist in settings. On Ecobee3, disable Smart Home/Away in settings.

Whatever you decide to do, you at least now know what is the likely cause when your thermostat decides to take a long break even when you are at home. Of course, it’s also a good idea not to rule out a possible HVAC failure if your thermostat isn’t heating or cooling your home to the optimum temperature.

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