Honeywell offers thermostats for both low voltage (24 volts) and line voltage (120 or 240 volts) systems. The most common type of voltage from a central heating and/or cooling system is 24 volts (24Vac). The old thermostat may say somewhere on the thermostat that it is for 24 volt (24Vac) or maximum 30 volts (30Vac). If you see 120 Vac or 240 Vac on the thermostat, a low voltage thermostat will NOT work with your system. You will need a line voltage thermostat. This type of voltage is common with electric baseboard heating. Regardless of the voltage, Honeywell strongly suggests that you turn off the power to the system at the furnace, at the fuse, or at the circuit breaker panel before installing or replacing a thermostat. If you are unsure about your voltage, please contact a local heating and cooling contractor in your area for assistance.