Low Voltage System
Low voltage thermostats are the most common
and versatile type of thermostats found in the home. They are typically 24 Volt AC and powered by using a step-down transformer from the 120 Volt line voltage in the home. Some versions of this thermostat use a voltage as low as 6 volts and as high as 30 volts, but 24 volts is by far the most common. The transformer is usually mounted on or near the furnace.
These thermostats may be digital, electronic programmable, mercury bimetallic or mechanical contact type thermostats