page_title: Thermostat
section_title: Bimetallic switching thermostats
hierarchical_section_title: Thermostat / Electrical and analog electronic thermostats / Bimetallic switching thermostats
caption_attribution_description: English: Thermostat for controlling the temperature in an office building. It is made by en:Honeywell and uses a en:bimetallic strip. Photo by Dante Alighieri
context_page_description: A thermostat is a 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. 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".
context_section_description: Water and steam based central heating systems have traditionally had overall control by wall-mounted bi-metallic strip thermostats. These sense the air temperature using the differential expansion of two metals to actuate an on/off switch. Typically the central system would be switched on when the temperature drops below the setpoint on the thermostat, and switched off when it rises above, with a few degrees of hysteresis to prevent excessive switching. Bi-metallic sensing is now being superseded by electronic sensors. A principal use of the bi-metallic thermostat today is in individual electric convection heaters, where control is on/off, based on the local air temperature and the setpoint desired by the user. These are also used on air-conditioners, where local control is required.
caption: Bimetallic thermostat for buildings.
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