This Lux programmable thermostat uses an internal disposable battery for its primary power. Alternatively, power can be supplied using a C wire (24 VAC). In this case, if a battery is used, the battery will serve as backup power. A "C" wire is optional (see manual for exceptions). It will work with a 2 wire system. A convenient home/away feature allows for quick setpoint adjustments. There is no vacation feature on this model.