Thermostats

A thermostat is a device for regulating the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by controlling the flow of heat energy into or out of the system. more...

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That is, the thermostat switches heating or cooling devices on or off as needed to maintain the correct temperature.

Thermostats can be constructed in many ways and may use a variety of sensors to measure the temperature. The output of the sensor then controls the heating or cooling apparatus.

Common sensors include:

Bi-metal mechanical sensors; Expanding wax pellets; Electronic thermistors; Electrical thermocouples;

These may then control the heating or cooling apparatus using:

Direct mechanical control; Electrical signals; Pneumatic signals;

Mechanical

Bi-metal

On a steam or hot-water radiator system, the thermostat may be an entirely mechanical device incorporating a bi-metal strip. Generally, this is an automatic valve which regulates the flow based on the temperature. For the most part, their use is now rare, as modern under-floor radiator systems use electric valves, as do some older retrofitted systems.

Mechanical thermostats are used to regulate dampers in rooftop turbine vents, reducing building heat loss in cool or cold periods.

An automobile passenger compartment's heating system has a thermostatically controlled valve to regulate the water flow and temperature to an adjustable level. In older vehicles the thermostat controls the application of engine vacuum to actuators that control water valves and flappers to direct the flow of air. In modern vehicles, the vacuum actuators may be operated by small solenoids under the control of a central computer.

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Wax pellet

A thermostat is used in automobiles using an internal combustion engine to regulate the flow of coolant.

This type of thermostat operates mechanically. It makes use of a wax pellet inside a sealed chamber. The wax is solid at low temperatures but as the engine heats up the wax melts and expands. The sealed chamber has an expansion provision that operates a rod which opens a valve when the operating temperature is exceeded. The operating temperature is fixed, but is determined by the specific composition of the wax, so thermostats of this type are available to maintain different temperatures, typically in the range of 70 to 90 °C (160 to 200 °F). Modern engines are run hot, that is, over 80 °C (180 °F), in order to run more efficiently and to reduce the emission of pollutants. Most thermostats have a small bypass hole to vent any gas that might get into the system (e.g., air introduced during coolant replacement). Modern cooling systems contain a relief valve in the form of a spring-loaded radiator pressure cap, with a tube leading to a partially filled expansion reservoir. Owing to the high temperature, the cooling system will become pressurized to a maximum set by the relief valve. The additional pressure increases the boiling point of the coolant above that which it would be at atmospheric pressure.

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