Originally Posted by mstone
Here is an example: Let's say it is a hot day and the clever thermostat turns on the A/C. You decide to go outside and mow the yard for 30 minutes. Nest detects that you are gone so it shuts off the A/C. When you come back inside hot and sweaty, there is no A/C or does it somehow automatically know you will be back inside in 30 minutes and lets the A/C stay on?
The Nest isn't that aggressive in terms of the auto-away feature. Usually it's an average of two hours before it turns the house off. But it really depends on your observed habits. Nest published a white paper last year when the came to market based on their research of potential customer habits. They summized that there were two basic kinds of users...the ones who went to work and left the house for long periods of times, and the ones who work from home. One of the reasons Nest has motion sensors is to figure out what kind of user you are. If I left the house for work every morning, the Nest would realize this after a while and more aggressively terminate HVAC function earlier. In my case since I work from home, it's much less agressive.
For example, I travel on business from time to time. During those days, I notice that it usually takes my Nests about 3 1/2 hours to kick into auto-away once my wife and kids have left in the morning. But if I took a new job where I worked away from home all the time, it would notice this after a while and adjust accordingly. And of course, it pays attention to your habits evenings and weekends.
Finally, it's smart enough to understand that it's not supposed to shut off HVAC function during the middle of the night when you are sleeping, except if you've set it up for that. All of the bedrooms in my house (save the guest bedroom) are on the second floor handled by a specific HVAC/Nest combo. So during the summer there isn't a need to keep the ground floor as cool as the upstairs. I set the temperature for 80 degrees after 10 PM. The Nest understands that this is a temperature designed to save energy during the summer months and even acknowledges it on the display (reading "80 degrees for the night"). If the temperature were to rise above 80 degrees, the AC would activate even though no one has walked in front of the motion sensor in the middle of the night.