About Programmable Thermostats

Benefits
Heating and cooling losses from a home (or building) become greater as the difference in temperature increases. A programmable thermostat allows reduction of these losses by allowing the temperature difference to be reduced at times when the reduced amount of heating or cooling would not be objectionable.

Digital thermostats provide versatility
They commonly allow setting temperatures for two, four, or six periods each day, and rather than being limited to a single “hotter” temperature and a single “colder” temperature, digital thermostats usually allow each period to be set to a unique temperature. Digital thermostats usually allow the user to override the programmed temperature for the period, automatically resuming programmed temperatures when the next period begins which allows flexibility to the homeowner.

More expensive models use a built-in PID controller ( proportional-integral-derivative controller)

So that the thermostat learns how the system will react to its commands. Programming the morning temperature to be 68 F at 7:00 AM, for instance, makes sure that at that time the temperature will be 68 F. A standard programmable thermostat would simply start working toward 68° F at 7:00 AM. The PID controller decides at what time the system should be activated in order to reach the desired temperature at the desired time. It knows this by remembering the past behavior of the room, and the current temperature of the room. It also makes sure that the temperature is very stable so that the comfort level is increased.