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Reviewing How Thermostats Work

Reviewing How Thermostats Work

You walk past them countless times a day in the office and at home, but have you ever stopped to wonder how thermostats work?

You probably assume there are simple circuits and mercury-filled bulbs involved in non-digital units; you’d be right, although how you think they work together might be wrong.

How non-digital thermostats work most commonly is by using coils of bimetallic strips, which expand and contract with the temperature of the room. Attached to the coil is the mercury-filled bulb, which slides to one side to complete a circuit to turn on the heating or cooling system. The needle and wheel you see on the face of the thermostat is attached to, and moves with, the coil.

Once the room temperature reaches the set temperature the bimetallic strip works the other way and breaks the circuit. For example, if a room is too warm the coil would expand and complete the circuit to turn on the AC unit. Once the room reaches the desired temperature the coil will contract, tilting the mercury-filled bulb and breaking the circuit. And that is how thermostats work.

Another type of non-digital thermostat you might find in your home replaces the coil for gas- or liquid-filled bellows, but how these thermostats work is the same. As the room air warms, the contents of the thin-metal bellows (like thin pillows) expands forcing the wall to push out and activate a switch. As the room cools the liquid or gas contracts and the bellow walls move together, releasing the switch.

Because of the much wider surface area of the bellow system compared with the bimetallic coils, the former is more sensitive to room temperatures and are quicker to react.

Unlike bellow and coil systems, digital thermostats have no moving parts as they use small semiconductor devices or thermistors, wires that conduct electricity better depending on the room temperature, as sensor elements.

If you are interested in a new thermostat, or anything for your home, contact Casteel today. Our technicians would love to help.