A "heat only" thermostat is for systems that control a source of heat, but not air conditioning (cooling). Heat-only systems are common in urban areas, older buildings, and parts of the country where winters are relatively mild. Heat-only applications typically use the simplest wiring, which are usually just two wires. These thermostats operate on low voltage 24 volt control circuits.
CAVEAT: If your heating source is electric heat or your thermostat connects to thick wires (about the size of pencil lead), then check out the line voltage section.
1 HEAT / 1 COOL:
Also known as "single stage," a 1 Heat / 1 Cool thermostat is the most common thermostat used to control both heating and cooling in your home. Around 60 percent of heating and cooling systems in the U.S. are this type. These systems are essentially "on-or-off" systems with only one level of heating or cooling. If you have a forced air gas furnace, plus an air conditioning unit, then this thermostat is for you. These types of heating and cooling systems usually use four control wires - red, white, yellow, and green. These thermostats operate on low voltage 24 volt control circuits.
Heat Pump system thermostats are relatively common. They're typically located in areas with milder winters or in very warm parts of the country where there is little call for heating. Heat pumps use a refrigerant cycle to provide both heating and cooling. The only difference in whether they are heating or cooling an area is based on the way the refrigerant flows through the system. A special thermostat is needed to control a heat pump - especially if the system has an auxiliary or emergency heat function for abnormally cold days. Heat pumps thermostats typically use five or six control wires. These thermostats operate on low voltage 24 volt control circuits.
Multistage thermostats can control up to four stages of heat and three stages of cooling. Multistage systems are usually "higher end" systems designed to use energy more efficiently than single stage systems. The easiest way to figure out if your system is multistage is to look at the wiring. Multistage systems will have multiple wires for heating or cooling connected to the thermostat (example - W1, W2, W3). See our article "Thermostat Wiring Information" if you'd like to learn more about the purpose of these wires. Multistage thermostats typically use six to eight control wires. These thermostats operate on low voltage 24 volt control circuits.
Line voltage thermostats connect to much thicker wires than other thermostats. They typically control resistive, heat-only systems such as electric baseboard radiators and electric ceiling cable heat. These thermostats operate on high voltage 110 or 240 volt control circuits.
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