How does a thermostat work?

Older mechanical thermostats use a bimetallic strip, made of two metals bonded together that expand by different amounts when heated. As the temperature changes, the strip bends, making or breaking an electrical contact that turns the heating or cooling on or off. Modern digital thermostats instead use an electronic sensor such as a thermistor, whose electrical resistance changes with temperature, and a small computer compares the reading with the set point. To avoid switching the system on and off every few seconds, thermostats allow a small range around the target, known as a deadband or hysteresis. Smart thermostats add Wi-Fi, schedules and sensors that can learn a household's routine to save energy.