A water heater thermostat senses temperature primarily through a temperature-sensitive element that expands or contracts in response to temperature changes, which then triggers the heating mechanism.
Let's break down the specifics:
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Expansion and Contraction: Most water heater thermostats rely on the principle of thermal expansion. This means that a material inside the thermostat changes its volume (expands when heated, contracts when cooled) in direct relation to the water temperature surrounding it.
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Rod and Tube Thermostats: Many older and some current water heaters use a rod and tube thermostat. This type of thermostat consists of a metal rod inside a metal tube. The two metals have different rates of thermal expansion. As the water temperature changes, the difference in expansion or contraction between the rod and tube causes a mechanical movement.
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Mechanical Linkage: This mechanical movement is connected to a switch. When the water temperature drops below a setpoint, the contraction activates the switch, turning on the heating element (in electric heaters) or the gas burner (in gas heaters). Once the water reaches the desired temperature, the expansion reverses the switch, shutting off the heating.
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Bimetallic Strip Thermostats: Some thermostats utilize a bimetallic strip. This strip is made of two different metals bonded together. Because each metal expands at a different rate when heated, the bimetallic strip bends or curves in response to temperature changes. This bending motion is then used to open or close an electrical circuit, controlling the heating element or gas valve.
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Gas Water Heaters (Thermocouple and Mercury Sensor): As indicated in the reference, gas water heaters often have a more complex system. While a thermocouple monitors the pilot light (to ensure it's lit and safe), a separate temperature sensor, historically containing mercury, monitors the water temperature in the tank. This sensor triggers the main gas valve to open when the water needs heating.
In summary, the thermostat senses temperature through physical changes (expansion/contraction or bending) in a material that directly corresponds to the water's temperature. This physical change then mechanically or electrically controls the heating process.