This programmable thermostat is suitable for use with hot air systems with no fan control and millivolt systems. You may program up to four settings per day, with each day being programmed differently if desired. This thermostat has two operating modes, including automatic with temporary bypass and manual. It will retain the settings in memory even during a power outage, eliminating the need to re-program the thermostat. Suitable for 2 or 3 wire applications. 3.8" (97 mm) wide x 4.9" (125.5 mm) high x 1.0" (26 mm) deep.