Low-voltageLine-voltageMillivolt
Each of these can be used with one or more types of furnace or other heating/cooling system. If you have any doubt about which type of thermostat is suitable for your furnace, refer to the furnace owner’s manual or contact the furnace manufacturer. Thermostats may also come with documentation that indicates what type of furnace they can be used with.
Characteristics
Number of thermostat wires: two or three in systems without air conditioning; four or five with air conditioning; can have seven or more wires with heat pump systemsVoltage: 24 volts AC (usually)
They are commonly used with:
Conventional gas forced-air furnaces, including standing pilot and electronic ignition types Electric forced-air furnaces Single-stage and multi-stage heat pumps Gas-fired, oil-fired, and electric hot water boilers for radiant heat or baseboard hydronic systems Electric central air conditioning systems
One way to know if you have a line-voltage thermostat is to look at the wires. The wire leads will be quite thick, 12-gauge or 14-gauge wires, like those connected to a wall outlet or light switch. By contrast, wires for low-voltage thermostats are very thin, similar to wiring for doorbells or telephone jacks. You can also check for a voltage listing inside the cover of the thermostat, or on the heat pump, furnace (inside the access door), boiler, or the electrical baseboard unit itself.
Characteristics
Number of thermostat wires: two to fourVoltage: 120 volts AC or 240 volts AC
They are commonly used with:
Electric baseboard heatersLocalized heating systems (with a line-voltage thermostat in each room)Oil-fired hot water boiler for radiant heat or baseboard hydronic systems
Millivolt thermostats use very low voltage, usually 750 millivolts (mV) or 0.75 volts. Unlike low-voltage systems, they do not require a step-down transformer and are not connected to the household wiring system. Instead, they are self-powered by a special thermocouple device called a powerpile or thermopile generator that produces direct current (DC) electricity.
Characteristics
Number of thermostat wires: twoVoltage: 750mV
They are commonly used with:
Gravity wall (room) furnacesDirect-vent or top-vent wall furnaces for small areasMobile home direct-vent wall furnacesHotel room and office buildings
