Furnance Thermostat 101

Furnance Thermostat 101

Furnaces and thermostats are not mix-and-match components. There are several types of heating systems and thermostat systems, and they must be compatible with one another to ensure safe and proper operation. There are three main types of thermostat systems used today:

  • Low-voltage
  • Line-voltage
  • Millivolt

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. 

Low-Voltage Thermostats

Low-voltage thermostats are the most common and versatile type of thermostats found in homes. A low-voltage thermostat typically runs on 24 volts of electricity and is powered by a step-down transformer wired to a standard 120-volt household circuit. Some types may use voltages as low as 6 volts or as high as 30 volts, but 24 volts is the most common type. The transformer that steps down the house voltage to low voltage is usually mounted on or near the furnace. Low-voltage thermostats include digital/programmable, mercury bimetallic, and mechanical contact types.


  • 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 systems

  • Voltage: 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  

Line-Voltage Thermostats 

Line-voltage thermostats are powered directly by a standard 120-volt or 240-volt circuit—the same circuit that powers a permanent electric heater. They are primarily used for electric resistance heating systems, such as electric baseboard heaters and in-wall heaters. These thermostats are not as sensitive as most low-voltage types, and it may take a temperature fluctuation up to 7 degrees Fahrenheit to make them respond. As a result, expect wider temperature fluctuations in any heating system controlled by these thermostats.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.


  • Number of thermostat wires: two to four

  • Voltage: 120 volts AC or 240 volts AC
They are commonly used with:

  • Electric baseboard heaters
  • Localized heating systems (with a line-voltage thermostat in each room)
  • Oil-fired hot water boiler for radiant heat or baseboard hydronic systems

Millivolt Thermostats

Millivolt systems are not very common and are primarily used in direct- or top-vent wall furnaces. These types of systems require a special thermostat and don’t work with standard low-voltage thermostats.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. 


  • Number of thermostat wires: twoVoltage: 750mV
They are commonly used with:

  • Gravity wall (room) furnaces
  • Direct-vent or top-vent wall furnaces for small areas
  • Mobile home direct-vent wall furnaces
  • Hotel room and office buildings