How Does a Heating Thermostat Work?
Bimetallic Thermostats Older thermostats were controlled by a device called a thermometer coil. Thermometer coils are made out of bimetallic strips–two strips of metal attached together. When the temperature goes up, metals expand but different metals expand at different rates. That makes the coil expand as it heats up and contract when it cools. When it contracts enough, the coil bumps into a switch which turns on the heater. As it expands, it moves away from the switch, turning off the heating again. Digital Thermostats Bimetallic thermostats are large, bulky and not very accurate, and have been largely replaced by digital thermostats. Digital thermostats use thermistors, special resistors which are sensitive to temperature changes. Most resistors resist the flow of electricity at a constant rate, but thermistors change as it gets hotter or cooler, slowing or speeding the flow of any electric current fed through it. In a digital thermostat, a current flows through the thermistor to
