Most programmable thermostats can be set to change their heating or cooling based on the time of day or day of the week, and some can change the heating system between different stages. However, they typically do not detect weather trends or weigh other factors.
As of 2015, smart thermostats rely on various detection technology to determine if the house is occupied. For users with unpredictable schedules, these thermostats can provide heating and cooling when needed without requiring constant tweaking.
Other thermostats can track performance and energy usage. While users still need to adjust heating and cooling schedules manually, this information provides valuable feedback for finding the best way to optimize energy use. Users might, for example, find that dropping their in-home temperature too far while people are away results in higher energy usage.
Some commercial systems, however, are able to provide virtually limitless programmability. In these systems, the thermostat is directly connected to a computer, which can be programmed to react to all measured parameters. This, combined with extensive logging and the ability to download online information, gives operators the ability to set any profile they wish. While these custom systems are expensive to install, those who own large buildings can save a considerable amount of money with even slight efficiency increases.