The S1-THEC11P5Y is a 5-day/ 2-day programmable thermostat with 1-stage heat and 1-stage cool that offers basic operation of HVAC equipment. It is compatible with most equipment including millivolt systems and it operates on either 24V system power or 2 AA batteries accessed through the front battery compartment. 5-day/ 2-day programmable thermostat with fahrenheit or celsius display. 1-stage heat, 1-stage cool for use with gas/electric, heat pump. Works with two AA batteries/ 24-volt system power. Large, easy to read backlit display. Has a service filter indicator.