By maintaining the highest or lowest required temperatures for four or five hours a day instead of 24 hours, a programmable thermostat can pay for itself in energy saved within four years. Programmable thermostats have features with which you may be unfamiliar. The newest generation of residential thermostat technologies is based on microprocessors and thermistor sensors. They store six or more temperature settings a day. They adjust heating or air conditioning turn-on times as the outside temperature changes.
Most programmable thermostats have liquid crystal temperature displays.
Some have back-up battery packs that eliminate the need to reprogram the time or clock in case of a power failure. These thermostats work with most conventional heating and cooling systems, except heat pumps. EM controls have limited flexibility and can store only the same settings for each day, although at least one manufacturer has a model with separate settings for each day of the week. EM thermostats are best suited for people with regular schedules. Digital thermostats are identified by their LED or LCD digital readout and data entry pads or buttons.