Thermostat Overview

A thermostat is a regulating device component that senses the temperature of a physical system and acts so that the temperature of the system remains near a desired set point.A thermostat is used to heat or cool any device or system to a set point temperature. Examples include building heating, central heating, air conditioning,HVAC systems, water heaters, and kitchen equipment including ovens and refrigerators, as well as medical and scientific incubators.In the scientific literature, these devices are often broadly classified as thermostatically controlled loads (TCLs). Thermostatically controlled loads account for approximately 50% of total U.S. electricity demand.A thermostat operates as a "closed loop" control device because it attempts to reduce the error between the desired temperature and the measured temperature.Sometimes a thermostat combines the sensing and control action elements of the system being controlled, such as a car thermostat.The word thermostat is derived from the Greek θερμός thermos, "heat" and στατός statos, "standing, standing still"".

Overview 

Thermostats control by turning heating or cooling equipment on or off, or by adjusting the flow of heat transfer fluid as needed, to maintain the correct temperature.A thermostat can often serve as the primary control unit for a heating or cooling system, with applications ranging from ambient air control to automotive coolant control.A thermostat is used to heat or cool any device or system to a set point temperature.Examples include building heating, central heating and air conditioning, kitchen equipment such as ovens and refrigerators, and medical and scientific incubators.

Construction and control

Thermostats use different types of sensors to measure temperature and initiate control operations. Mechanical thermostats typically use bimetallic strips that convert temperature changes into mechanical displacements to initiate control of a heating or cooling source. In contrast, electronic thermostats use a thermistor or other semiconductor sensor that processes temperature changes into an electronic signal to control heating or cooling the device.A traditional thermostat is an example of an "on-off controller" because the system being controlled either runs at full capacity once the set point is reached, or shuts off completely.While this is the simplest procedure, this method of control needs to include some hysteresis to prevent the device from cycling too quickly around the set point. Therefore, conventional thermostats cannot control the temperature very precisely. Instead, there are oscillations of a certain magnitude, usually 1-2 °C. [citation needed] This control is usually inaccurate, inefficient and highly mechanically abrasive, but for components such as.

Another consideration is the time delay of the controlled system.To improve the control performance of the system, the thermostat can include a "predictor" which will stop the heating/cooling slightly earlier than the set point is reached, since the system will continue to generate heat for a short while.Turning off exactly at the set point will cause the actual temperature to exceed the desired range, known as "overshoot".Bimetallic sensors can include a physical "predictor" that has a thin wire touching the thermostat.When current passes through the wire, a small amount of heat is generated and transferred to the bimetallic coil. Electronic thermostats have an electronic equivalent.When higher control accuracy is required, a PID or MPC controller is preferred. However, they are now mainly used for industrial purposes, such as semiconductor manufacturing plants or museums.