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Furnaces and thermostats are not mix-and-match equipment. There are several types of heating systems and thermostat systems, and they must be compatible to ensure safe and proper operation. There are three main types of thermostat systems used today:
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 owners manual or contact the manufacturer.Continue to 2 of 4 below.
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Low-voltage thermostats are the most common and versatile type of thermostats found in homes. They typically run on 24 volts and are 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. 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 of Low-Voltage Thermostats
Number of thermostat wires: two or three without air conditioning; four or five with air conditioning. Can have seven or more wires with heat pump systems.
Voltage: 24 volts AC
Commonly used with:
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- Conventional gas forced-air furnaces, including standing pilot and electronic ignition types
- Electric forced-air furnaces
- Single-stage heat pumps providing heating and cooling
- Multi-stage heat pumps providing heating and cooling
- Gas-fired, oil-fired, and electric hot water boilers for radiant heat or baseboard hydronic systems
- Electric central air conditioning systems (integrated with forced-air ductwork)
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Line-voltage thermostats are powered directly by a standard 120-volt or 240-volt circuit. 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 F 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 connecting 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 of Line-Voltage Thermostats
Number of thermostat wires: two to four
Voltage: 120 volts AC or 240 volts AC
Common used with:
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- 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
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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 will not 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 of Millivolt Thermostats
Number of thermostat wires: two
Commonly used with:
- Gravity wall (room) furnaces
- Direct or top-vent wall furnaces for small areas
- Mobile home direct-vent wall furnaces