Most thermostats that control central heating equipment, including furnaces, boilers, and heat pumps, are low-voltage thermostats that typically use 24 volts of electricity. Electric baseboard heaters and in-wall heaters may use line-voltage thermostats that use 120 or 240 volts of electricity. Low-voltage thermostats are powered by a transformer that reduces the incoming line voltage from 120 volts down to a level between 12 and 24 volts, depending on your furnace design. Within this low-voltage thermostat category, there are four major types:
Mercury contactMechanical contactDigitalElectronic programmable
For an analog thermostat to work, something has to move as the temperature increases or decreases. That’s the job of the bimetallic strip, which is just a fancy name for two pieces of metal joined together. The reason it works is that the two metals expand at different rates at a given temperature, known as the coefficient of expansion. As one side expands more than the other side, the metal curves one way or the other. You then couple this little bit of physics knowledge with the exact rates of expansion for the two metals, and the result is a precise instrument that will move a specific amount at a given temperature. The thermostat contact is like a switch for your lights. As the bimetallic strip moves, it either opens or closes the contact. In the case of a mercury thermostat, the contact is a glass vial, called an ampoule, that contains mercury. The ampoule is attached to a coiled bimetallic strip, also called a bimetallic coil. As the coil moves, it tips the vial of mercury until the mercury moves and the switch contact is opened or closed. Finally, the heat anticipator is an electrical resistance wire mounted on a center disc connected to the bimetallic coil. It is adjustable to allow fine-tuning of when the thermostat turns the furnace blower on and off. The bimetallic thermostat has modes for heating and cooling and fan positions for on/off and automatic. It is not programmable. The latest iteration of the programmable thermostat is the smart thermostat, such as the Nest. These are fully programmable and can be controlled manually or with a smartphone or other device. They are also capable of programming themselves by learning how you typically set the temperature throughout the week.
