Home Improvement Skills & Specialties HVAC

Choosing the Right Thermostat for Your Furnace

By
Bob Formisano
Bob Formisano
Bob Formisano is a licensed architect and builder with nearly 40 years of experience building new homes and restoring older homes. One of his specialties is repairing old systems dating back to the 1920s, including galvanized water pipes, knob-and-tube wiring, and more. His home repair articles for The Spruce span more than 10 years.
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Updated on 03/21/22
Reviewed by
Larry Campbell
Larry Campbell
Reviewed by Larry Campbell
Larry Campbell is an electrical contractor with 36 years of experience in residential and light commercial electrical wiring. He worked as an electronic technician and later as an engineer for the IBM Corp. He is also a member of The Spruce Home Improvement Review Board.
Learn more about The Spruce's Review Board
Woman adjusting digital thermostat
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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. 

  • 01 of 03

    Low-Voltage Thermostats

    A Honeywell thermostat
    Amazon

    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.

    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 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  
  • 02 of 03

    Line-Voltage Thermostats

    Line-voltage thermostat
    Amazon

    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.

    Characteristics

    • 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
  • 03 of 03

    Millivolt Thermostats

    Economy millivolt non-programmable thermostat
    Amazon

    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. 

    Characteristics

    • Number of thermostat wires: two
    • Voltage: 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