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This article is about the temperature regulating device. For the French cooking oven temperature scale, see Gas Mark § Other cooking temperature scales.
Next Generation Lux Products TX9600TS Universal 7-Day Programmable Touch Screen Thermostat.
A Honeywell electronic thermostat in a retail store


A thermostat can often be the main control unit for a heating or cooling system, in applications ranging from ambient air control, to such as automotive coolant control, but is also used in many other applications, such as an electric clothes iron.

It is 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 control system, such as in an automotive thermostat.


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. Thermostats are used in any device or system that heats or cools to a set-point temperature, examples include building heating, central heating, air conditioners, HVAC systems, as well as kitchen equipment including ovens and refrigerators and medical and scientific incubators.


Thermostats can be constructed in many ways and may use a variety of sensors to measure the temperature, commonly a thermistor or bimetallic strip. The output of the sensor then controls the heating or cooling apparatus. A thermostat is most often an instance of a "bang-bang controller" as the heating or cooling equipment interface is not typically controlled in a proportional manner to the difference between actual temperature and the temperature setpoint. Instead, the heating or cooling equipment runs at full capacity until the set temperature is reached, then shuts off. Increasing the difference between the thermostat setting and the desired temperature therefore does not shorten the time to achieve the desired temperature. A thermostat may have a maximum switching frequency, or switch heating and cooling equipment on and off at temperatures either side of the setpoint. This reduces the risk of equipment damage from frequent switching.

The term thermostat is derived from the Greek words θερμός thermos, "hot" and στατός statos, "standing, stationary".

Sensor types[edit]


Common sensor technologies in use today include:

These may then control the heating or cooling apparatus using:

  • Direct mechanical control
  • Electrical signals
  • Pneumatic signals


Possibly the earliest recorded examples of thermostat control were built by the Dutch innovator Cornelis Drebbel (1572–1633) around 1620 in England. He invented a mercury thermostat to regulate the temperature of a chicken incubator.[1] This is one of the first recorded feedback-controlled devices.

Modern thermostat control was developed in the 1830s by Andrew Ure (1778–1857), a Scottish chemist, who invented the bi-metallic thermostat. The textile mills of the time needed a constant and steady temperature to operate optimally, so to achieve this 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.[2]

Warren S. Johnson (1847–1911) of Wisconsin patented a bi-metal room thermostat in 1883, and two years later filed a patent for the first multi-zone thermostatic control system.[3][4] Albert Butz (1849–1905) invented the electric thermostat and patented it in 1886.


Mechanical thermostats[edit]






Wax pellet[edit]


Car engine thermostat



Shower and other hot water controls[edit]

A thermostatic mixing valve uses a wax pellet to control the mixing of hot and cold water. A common application is to permit operation of an electric water heater at a temperature hot enough to kill Legionella bacteria (above 60 °C (140 °F)), while the output of the valve produces water that is cool enough to not immediately scald (49 °C (120 °F)).

Gas expansion[edit]


Pneumatic thermostats[edit]


The pneumatic thermostat was invented by Warren Johnson in 1895[7] soon after he invented the electric thermostat. In 2009, Harry Sim was awarded a patent for a pneumatic-to-digital interface[8] that allows pneumatically controlled buildings to be integrated with building automation systems to provide similar benefits as DDC.

Electrical and analog electronic thermostats[edit]

Bimetallic switching thermostats[edit]

Bimetallic thermostat for buildings.


Simple two wire thermostats[edit]

Millivolt thermostat mechanism




Millivolt thermostats[edit]




24 volt thermostats[edit]



There are no standards for wiring color codes, but convention has settled on the following terminal codes and colors.[10][11] In all cases, the manufacturer's instructions should be considered definitive.

Terminal Code Color Description
R / V Red 24 volt
Rh / 4 Red 24 volt HEAT load
Rc Red 24 volt COOL load
C Black/Blue/Brown 24 volt Common (Ground)
W / W1 White Heat
W2 Varies/White/Black 2nd Stage / Backup Heat
Y / Y1 Yellow Cool
Y2 Blue/Orange/Purple/Yellow/White 2nd Stage Cool
G Green Fan
O Varies/Orange/Black Reversing valve Energize to Cool (Heat Pump)
B Varies/Blue/Black/Brown/Orange Reversing valve Energize to Heat (Heat Pump) or Common
E Varies/Blue/Pink/Gray/Tan Emergency Heat (Heat Pump)
S1/S2 Brown/Black/Blue Temperature Sensor (Usually outdoors on a Heat Pump System)
T Varies/Tan/Gray Outdoor Anticipator Reset
X Varies Emergency Heat (Heat Pump) or Common
X2 Varies 2nd stage/emergency heating or indicator lights
L Varies Service Light

Line voltage thermostats[edit]



Digital electronic thermostats[edit]

Residential digital thermostat
Lux Products' Model TX9000TS Touch Screen Thermostat.
Lux Products WIN100 Heating & Cooling Programmable Outlet Thermostat shown with control door closed and open.


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.

More expensive models have a built-in PID controller, so that the thermostat knows ahead how the system will react to its commands. For instance, setting it 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 PID controller decides at what time the system should be activated in order to reach the desired temperature at the desired time. It also makes sure that the temperature is very stable (for instance, by reducing overshoots[citation needed]).


Thermostats and HVAC operation[edit]

Ignition sequences in modern conventional systems[edit]

  • Gas
  2. Heat ignitor or start spark-ignition system
  3. Open gas valve to ignite main burners
  • Oil
  • Electric

With non-zoned (typical residential, one thermostat for the whole house) systems, when the thermostat's R (or Rh) and W terminals are connected, the furnace will go through its start-up procedure and produce heat.


Most programmable thermostats will control these systems.

Combination heating/cooling regulation[edit]



Heat pump regulation[edit]

Thermostat design


Thermostat location[edit]


Dummy thermostats[edit]

It has been reported that many thermostats in office buildings are non-functional dummy devices, installed to give tenants' employees an illusion of control.[13][14] These dummy thermostats are in effect a type of placebo button. However, these thermostats are often used to detect the temperature in the zone, even though their controls are disabled. This function is often referred to as "lockout".[15]

See also[edit]

Notes and references[edit]

  1. ^ "Tierie, Gerrit. Cornelis Drebbel. Amsterdam: HJ Paris, 1932." (PDF). Retrieved May 3, 2013. 
  2. ^ "An Early History Of Comfort Heating". The NEWS Magazine. Troy, Michigan: BNP Media. November 6, 2001. Retrieved November 2, 2014. 
  3. ^ "Thermostat Maker Deploys Climate Control Against Climate Change". Retrieved October 3, 2009. 
  4. ^ "Johnson Controls Inc. | History". November 7, 2007. Retrieved October 3, 2009. 
  5. ^ Falk, Cynthia G. (2012). Barns of New York: Rural Architecture of the Empire State (paperback) (First ed.). Ithaca, New York: Cornell University Press (published May 1, 2012). ISBN 978-0-8014-7780-5. Retrieved November 2, 2014. 
  6. ^ "Dr-Fix-It Explains a Common Pneumatic Comfort Control Circuit". RTWEB. 2005. Retrieved November 2, 2014. 
  7. ^ Fehring, T.H., ed., Mechanical Engineering: A Century of Progress, NorCENergy Consultants, LLC, October 10, 1980 - Technology & Engineering, p. 22
  8. ^
  10. ^ Sawyer, "Doc". "Thermostat Wire Color Codes". Retrieved 7 March 2015. 
  11. ^ Transtronics, Inc. "Thermostat signals and wiring". Retrieved 7 March 2015. 
  12. ^ KMC Controls. "Thinking about Thermostats". Retrieved April 22, 2013. 
  13. ^ Sandberg, Jared (January 15, 2003). "Employees Only Think They Control Thermostat". The Wall Street Journal. Retrieved September 2, 2009. 
  14. ^ Katrina C. Arabe (April 11, 2003). ""Dummy" Thermostats Cool Down Tempers, Not Temperatures". Retrieved February 13, 2010. 
  15. ^ Example datasheet of current art thermostat, exhibiting lockout functionality :

External links[edit]