page_title: Thermostat
section_title: Digital electronic thermostats
hierarchical_section_title: Thermostat / Digital electronic thermostats
caption_attribution_description: English: A residential electronic thermostat
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: Newer digital thermostats have no moving parts to measure temperature and instead rely on thermistors or other semiconductor devices such as a resistance thermometer (resistance temperature detector). Typically one or more regular batteries must be installed to operate it, although some so-called "power stealing" digital thermostats use the common 24 volt AC circuits as a power source, but will not operate on thermopile powered "millivolt" circuits used in some furnaces.  Each has an LCD screen showing the current temperature, and the current setting.  Most also have a clock, and time-of-day and even day-of-week settings for the temperature, used for comfort and energy conservation.  Some advanced models have touch screens, or the ability to work with home automation or building automation systems.
  Digital thermostats use either a relay or a semiconductor device such as triac to act as a switch to control the HVAC unit.  Units with relays will operate millivolt systems, but often make an audible "click" noise when switching on or off.
  HVAC systems with the ability to modulate their output can be combined with thermostats that have a built-in PID controller to achieve smoother operation.
  There are also modern thermostats featuring adaptive algorithms to further improve the inertia prone system behaviour. For instance, setting those up so that the temperature in the morning at 7 a.m. should be 21 °C (69.8 °F), makes sure that at that time the temperature will be 21 °C (69.8 °F), where a conventional thermostat would just start working at that time.  The algorithms decide at what time the system should be activated in order to reach the desired temperature at the desired time. Other thermostat used for process/industrial control where ON/OFF control is not suitable the PID control can also makes sure that the temperature is very stable (for instance, by reducing overshoots by fine tuning PID constants for set value(SV) or maintaining temperature in a band by deploying hysteresis control.)
  Most digital thermostats in common residential use in North America and Europe are programmable thermostats, which will typically provide a 30% energy savings if left with their default programs; adjustments to these defaults may increase or reduce energy savings. The programmable thermostat article provides basic information on the operation, selection and installation of such a thermostat.
caption: Residential digital thermostat
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