A thermostat is a regulating device component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint. Thermostats are used in any device or system that heats or cools to a setpoint temperature. Examples include building heating, central heating, air conditioners, HVAC systems, water heaters, as well as kitchen equipment including ovens and refrigerators and medical and scientific incubators. In scientific literature, these devices are often broadly classified as thermostatically controlled loads (TCLs). Thermostatically controlled loads comprise roughly 50% of the overall electricity demand in the United States. A thermostat operates as a "closed loop" control device, as it seeks to reduce the error between the desired and measured temperatures. Sometimes a thermostat combines both the sensing and control action elements of a controlled system, such as in an automotive thermostat. The word thermostat is derived from the Greek words θερμός thermos, "hot" and στατός statos, "standing, stationary".
Overview A thermostat exerts control by switching heating or cooling devices on or off, or by regulating the flow of a heat transfer fluid as needed, to maintain the correct temperature. A thermostat can often be the main control unit for a heating or cooling system, in applications ranging from ambient air control to automotive coolant control. Thermostats regulate temperature across a wide range of residential, commercial, and industrial applications, from household HVAC systems and kitchen appliances to medical incubators and automotive cooling systems.
Construction and control Thermostats use different types of sensors to measure temperatures and actuate control operations. Mechanical thermostats commonly use bimetallic strips, converting a temperature change into mechanical displacement, to actuate control of the heating or cooling sources. Electronic thermostats, instead, use a thermistor or other semiconductor sensor, processing temperature change as electronic signals, to control the heating or cooling equipment. Conventional thermostats are example of "bang-bang controllers" as the controlled system either operates at full capacity once the setpoint is reached, or keeps completely off. Although it is the simplest program to implement, such control method requires to include some hysteresis in order to prevent excessively rapid cycling of the equipment around the setpoint. As a consequence, conventional thermostats cannot control temperatures very precisely. Instead, there are oscillations of a certain magnitude, usually 1-2 °C. Such control is in general inaccurate, inefficient and may induce more mechanical wear; it however, allows for more cost-effective compressors compared to ones with continuously variable capacity. Another consideration is the time delay of the controlled system. To improve the control performance of the system, thermostats can include an "anticipator", which stops heating/cooling slightly earlier than reaching the setpoint, as the system will continue to produce heat for a short while. Turning off exactly at the setpoint will cause actual temperature to exceed the desired range, known as "overshoot". Bimetallic sensors can include a physical "anticipator", which has a thin wire touched on the thermostat. When current passes the wire, a small amount of heat is generated and transferred to the bimetallic coil. This small amount of heat artificially warms the bimetallic coil, causing it to open the heating contacts slightly before the room air reaches the setpoint, thereby compensating for residual thermal inertia. Electronic thermostats have an electronic equivalent. When higher control precision is required, a PID or MPC controller is preferred. However, they are nowadays mainly adopted for industrial purposes, for example, for semiconductor manufacturing factories or museums.
Sensor types
Early technologies included mercury thermometers with electrodes inserted directly through the glass, so that when a certain (fixed) temperature was reached the contacts would be closed by the mercury. These were accurate to within a degree of temperature. Common sensor technologies in use today include:
Bimetallic mechanical or electrical sensors. Expanding wax pellets Electronic thermistors and semiconductor devices Electrical thermocouples These may then control the heating or cooling apparatus using:
Direct mechanical control Electrical signals Pneumatic signals
History Possibly the earliest recorded examples of thermostatic control were built by a Dutch innovator, Cornelis Drebbel (1572–1633), about 1620 in England. He invented a mercury thermostat to regulate the temperature of a chicken incubator. This is one of the first recorded feedback-controlled devices. Modern thermostatic control was developed in the 1830s by Andrew Ure (1778–1857), a Scottish chemist. The textile mills of the time needed a constant and steady temperature to operate optimally, so Ure designed the bimetallic thermostat, which would bend as one of the metals expanded in response to the increased temperature and cut off the energy supply. Warren S. Johnson (1847–1911), of Wisconsin, patented a bi-metal room thermostat in 1883, and two years later sought a patent for the first multi-zone thermostatic control system. Albert Butz (1849–1905) invented the electric thermostat and patented it in 1886. One of the first industrial uses of the thermostat was in the regulation of the temperature in poultry incubators. Charles Hearson, a British engineer, designed the first modern incubator for eggs, which was taken up for use on poultry farms in 1879.
Mechanical thermostats This covers only devices which both sense and control using purely mechanical means.
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