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How does a thermostat control a furnace or boiler?

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heating In a thermoelectric generating system a heat source—usually fueled by coal, oil, or gas—is used within a boiler to convert water to high-pressure steam. The steam expands and turns the blades of a turbine, which turns the armature of a generator, producing electric power. A condenser converts any remaining steam to water, and a pump returns the water to the boiler.

A thermostat functions as a closed-loop feedback control system designed to maintain a constant temperature in an enclosed area. It operates by comparing the actual room temperature (the output) to a desired temperature setting (the input or set point).12

The process relies on a sensing element, typically a bimetallic strip, which flexes in response to temperature changes due to the different thermal expansion coefficients of the metals used. This movement acts as a controller and actuating device by triggering an electrical switch.21

When the room temperature falls below the set point, the thermostat closes the switch to activate the heating system, such as a furnace or boiler, by controlling the flow of fuel or electric current. Once the temperature exceeds the set point, the thermostat signals the system to turn off, thereby reducing the difference between the actual and desired temperature.12