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A thermostat is a regulating device component which senses the
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
of a
physical system A physical system is a collection of physical objects. In physics, it is a portion of the physical universe chosen for analysis. Everything outside the system is known as the environment. The environment is ignored except for its effects on the ...
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 A central heating system provides warmth to a number of spaces within a building from one main source of heat. It is a component of heating, ventilation, and air conditioning (short: HVAC) systems, which can both cool and warm interior spaces. ...
,
central heating A central heating system provides warmth to a number of spaces within a building from one main source of heat. It is a component of heating, ventilation, and air conditioning (short: HVAC) systems, which can both cool and warm interior spaces. ...
, air conditioners,
HVAC Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HV ...
systems, water heaters, as well as kitchen equipment including
oven upA double oven A ceramic oven An oven is a tool which is used to expose materials to a hot environment. Ovens contain a hollow chamber and provide a means of heating the chamber in a controlled way. In use since antiquity, they have been us ...
s and refrigerators and medical and scientific
incubator An incubator is anything that performs or facilitates various forms of incubation, and may refer to: Biology and medicine * Incubator (culture), a device used to grow and maintain microbiological cultures or cell cultures * Incubator (egg), a de ...
s. 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 Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
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 are used in any device or system that heats or cools to a setpoint temperature. Examples include building
heating A central heating system provides warmth to a number of spaces within a building from one main source of heat. It is a component of heating, ventilation, and air conditioning (short: HVAC) systems, which can both cool and warm interior spaces. ...
,
central heating A central heating system provides warmth to a number of spaces within a building from one main source of heat. It is a component of heating, ventilation, and air conditioning (short: HVAC) systems, which can both cool and warm interior spaces. ...
, and air conditioners, kitchen equipment such as
oven upA double oven A ceramic oven An oven is a tool which is used to expose materials to a hot environment. Ovens contain a hollow chamber and provide a means of heating the chamber in a controlled way. In use since antiquity, they have been us ...
s and refrigerators, and medical and scientific
incubator An incubator is anything that performs or facilitates various forms of incubation, and may refer to: Biology and medicine * Incubator (culture), a device used to grow and maintain microbiological cultures or cell cultures * Incubator (egg), a de ...
s.


Construction and control

Thermostats use different types of
sensor A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
s to measure temperatures and actuate control operations. Mechanical thermostats commonly use
bimetallic strip A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated. The different expansions force the flat strip to be ...
s, converting a temperature change into mechanical displacement, to actuate control of the heating or cooling sources. Electronic thermostats, instead, use a
thermistor A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''. Thermistors are divided based on their conduction ...
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 ℃. Such control is in general inaccurate, inefficient and high-level mechanical wear, but for components like compressors, it still has a significant cost advantage compared with more advanced ones allowing 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. 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: *
Bimetal Bimetal refers to an object that is composed of two separate metals joined together. Instead of being a mixture of two or more metals, like alloys, bimetallic objects consist of layers of different metals. Trimetal and tetrametal refer to objects co ...
lic mechanical or electrical sensors. * Expanding wax pellets * Electronic
thermistor A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''. Thermistors are divided based on their conduction ...
s and
semiconductor devices A semiconductor device is an electronic component that relies on the electronic properties of a semiconductor material (primarily silicon, germanium, and gallium arsenide, as well as organic semiconductors) for its function. Its conductivity l ...
* Electrical
thermocouple A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the ...
s These may then control the heating or cooling apparatus using: * Direct mechanical control * Electrical signals * Pneumatic signals


History

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 An incubator is anything that performs or facilitates various forms of incubation, and may refer to: Biology and medicine * Incubator (culture), a device used to grow and maintain microbiological cultures or cell cultures * Incubator (egg), a de ...
. This is one of the first recorded feedback-controlled devices. Modern thermostat control was developed in the 1830s by
Andrew Ure Andrew Ure FRS (18 May 1778 – 2 January 1857) was a Scottish physician, chemist, scriptural geologist, and early business theorist who founded the Garnet Hill Observatory. He was a fellow of the Royal Astronomical Society and the Royal S ...
(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. Warren S. Johnson (1847–1911) of
Wisconsin Wisconsin () is a state in the upper Midwestern United States. Wisconsin is the 25th-largest state by total area and the 20th-most populous. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake M ...
patented a bi-metal room thermostat in 1883, and two years later filed a patent for the first multi-zone thermostatic control system.
Albert Butz Albert Butz (1849–1905) was a Swiss-born American inventor and businessman who founded the Butz Thermo-electric Regulator Company that, through a series of re-organizations, name changes, and mergers, became Honeywell, Incorporated. Early li ...
(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 An incubator is anything that performs or facilitates various forms of incubation, and may refer to: Biology and medicine * Incubator (culture), a device used to grow and maintain microbiological cultures or cell cultures * Incubator (egg), a de ...
for eggs that was taken up for use on poultry farms in 1879. The incubators incorporated an accurate thermostat to regulate the temperature so as to precisely simulate the experience of an egg being hatched naturally.


Mechanical thermostats

This covers only devices which both sense and control using purely mechanical means.


Bimetal

Domestic water and steam based central heating systems have traditionally been controlled by bi-metallic strip thermostats, and this is dealt with later in this article. Purely mechanical control has been localised
steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization ...
or hot-water
radiator Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics. A radiator is always a ...
bi-metallic thermostats which
regulated Regulation is the management of complex systems according to a set of rules and trends. In systems theory, these types of rules exist in various fields of biology and society, but the term has slightly different meanings according to context. Fo ...
the individual flow. However,
thermostatic radiator valve A thermostatic radiator valve (TRV) is a self-regulating valve fitted to hot water heating system radiator, to control the temperature of a room by changing the flow of hot water to the radiator. Functioning Conventional wax motor TRV The ...
s (TRV) are now being widely used. Purely mechanical thermostats are used to regulate dampers in some rooftop turbine vents, reducing building heat loss in cool or cold periods. Some automobile passenger heating systems have a thermostatically controlled valve to regulate the water flow and temperature to an adjustable level. In older vehicles the thermostat controls the application of engine vacuum to actuators that control water valves and flappers to direct the flow of air. In modern vehicles, the vacuum actuators may be operated by small
solenoids upright=1.20, An illustration of a solenoid upright=1.20, Magnetic field created by a seven-loop solenoid (cross-sectional view) described using field lines A solenoid () is a type of electromagnet formed by a helix, helical coil of wire whose ...
under the control of a central computer.


Wax pellet


Automotive

Perhaps the most common example of purely mechanical thermostat technology in use today is the
internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combus ...
cooling system thermostat, used to maintain the engine near its optimum
operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
by regulating the flow of
coolant A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosio ...
to an air-cooled
radiator Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics. A radiator is always a ...
. This type of thermostat operates using a sealed chamber containing a wax pellet that melts and expands at a set temperature. The expansion of the chamber operates a rod which opens a
valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings ...
when the operating temperature is exceeded. The operating temperature is determined by the composition of the wax. Once the operating temperature is reached, the thermostat progressively increases or decreases its opening in response to temperature changes, dynamically balancing the coolant recirculation flow and coolant flow to the radiator to maintain the engine temperature in the optimum range. On many automobile engines, including all Chrysler Group and General Motors products, the thermostat does not restrict flow to the heater core. The passenger side tank of the radiator is used as a bypass to the thermostat, flowing through the heater core. This prevents formation of steam pockets before the thermostat opens, and allows the heater to function before the thermostat opens. Another benefit is that there is still some flow through the radiator if the thermostat fails.


Shower and other hot water controls

A
thermostatic mixing valve A thermostatic mixing valve (TMV) is a valve that blends hot water with cold water to ensure constant, safe shower and bath outlet temperatures, preventing scalding. The storage of water at high temperature removes one possible breeding ground ...
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 ''Legionella'' is a genus of pathogenic gram-negative bacteria that includes the species '' L. pneumophila'', causing legionellosis (all illnesses caused by ''Legionella'') including a pneumonia-type illness called Legionnaires' disease and a mil ...
'' bacteria (above ), while the output of the valve produces water that is cool enough to not immediately scald ().


Analysis

A wax pellet driven valve can be analyzed through graphing the wax pellet's hysteresis which consists of two thermal expansion curves; extension (motion) vs. temperature increase, and contraction (motion) vs. temperature decrease. The spread between the up and down curves visually illustrate the valve's hysteresis; there is always hysteresis within wax driven valves due to the phase transition or phase change between solids and liquids. Hysteresis can be controlled with specialized blended mixes of hydrocarbons; tight hysteresis is what most desire, however some applications require broader ranges. Wax pellet driven valves are used in anti scald, freeze protection, over-temp purge, solar thermal energy or solar thermal, automotive, and aerospace applications among many others.


Gas expansion

Thermostats are sometimes used to regulate gas ovens. It consists of a gas-filled bulb connected to the control unit by a slender copper tube. The bulb is normally located at the top of the oven. The tube ends in a chamber sealed by a diaphragm. As the thermostat heats up, the gas expands applying pressure to the diaphragm which reduces the flow of gas to the burner.


Pneumatic thermostats

A pneumatic thermostat is a thermostat that controls a heating or cooling system via a series of air-filled control tubes. This "control air" system responds to the pressure changes (due to temperature) in the control tube to activate heating or cooling when required. The control air typically is maintained on "mains" at 15-18 
psi Psi, PSI or Ψ may refer to: Alphabetic letters * Psi (Greek) (Ψ, ψ), the 23rd letter of the Greek alphabet * Psi (Cyrillic) (Ѱ, ѱ), letter of the early Cyrillic alphabet, adopted from Greek Arts and entertainment * "Psi" as an abbreviatio ...
(although usually operable up to 20 psi). Pneumatic thermostats typically provide output/ branch/ post-restrictor (for single-pipe operation) pressures of 3-15 psi which is piped to the end device (valve/ damper actuator/ pneumatic-electric switch, etc.). The pneumatic thermostat was invented by Warren Johnson in 1895 soon after he invented the electric thermostat. In 2009, Harry Sim was awarded a patent for a pneumatic-to-digital interface that allows pneumatically controlled buildings to be integrated with building automation systems to provide similar benefits as
direct digital control Direct digital control is the automated control of a condition or process by a digital device (computer). Direct digital control takes a centralized network-oriented approach. All instrumentation is gathered by various analog and digital converter ...
(DDC).


Electrical and analog electronic thermostats


Bimetallic switching thermostats

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
sensor A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
s. 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.


Contact configuration nomenclature

This follows the same nomenclature as described in and . * "NO" stands for "normally open". This is the same as "COR" ("close on rise"). May be used to start a fan when it is becoming hot; i.e., stop the fan when it has become cold enough. * "NC" stands for "normally closed". This is the same as "OOR" ("open on rise"). May be used to start a heater when it is becoming cold; i.e., stop the heater when it has become warm enough. * "CO" stands for "change over". This serves both as "NO" and "NC". May be used to start a fan when it is becoming hot, but also (on the opposite terminal), to start a heater when it is becoming cold. Any leading number stands for number of contact sets, like "1NO", "1NC" for one contact set with two terminals. "1CO" will also have one contact set, even if it is a switch-over with three terminals.


Simple two wire thermostats

The illustration is the interior of a common two wire heat-only household thermostat, used to regulate a
gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
-fired heater via an electric gas valve. Similar mechanisms may also be used to control oil furnaces, boilers, boiler
zone valve Zone or The Zone may refer to: Places Climate and altitude zones * Death zone (originally the lethal zone), altitudes above a certain point where the amount of oxygen is insufficient to sustain human life for an extended time span * Frigid zone, ...
s, electric attic fans, electric furnaces, electric baseboard heaters, and household appliances such as refrigerators, coffee pots and hair dryers. The power through the thermostat is provided by the heating device and may range from millivolts to 240
volt The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Defi ...
s in common North American construction, and is used to control the heating system either directly (electric baseboard heaters and some electric furnaces) or indirectly (all gas, oil and forced hot water systems). ''Due to the variety of possible
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
s and
current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
s available at the thermostat, caution must be taken when selecting a replacement device.'' # Setpoint control
lever A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or ''fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is div ...
. This is moved to the right for a higher temperature. The round indicator pin in the center of the second slot shows through a numbered slot in the outer case. #
Bimetallic strip A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated. The different expansions force the flat strip to be ...
wound into a coil. The center of the coil is attached to a rotating post attached to lever (1). As the coil gets colder the moving end — carrying (4) — moves
clockwise Two-dimensional rotation can occur in two possible directions. Clockwise motion (abbreviated CW) proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back up to the top. The opposite ...
. # Flexible wire. The left side is connected via one wire of a pair to the heater control valve. # Moving contact attached to the bimetal coil. Thence, to the heater's controller. # Fixed contact screw. This is adjusted by the
manufacturer Manufacturing is the creation or production of goods with the help of equipment, labor, machines, tools, and chemical or biological processing or formulation. It is the essence of secondary sector of the economy. The term may refer to a ran ...
. It is connected electrically by a second wire of the pair to the thermocouple and the heater's electrically operated gas valve. #
Magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...
. This ensures a good contact when the contact closes. It also provides hysteresis to prevent short heating cycles, as the temperature must be raised several
degree Degree may refer to: As a unit of measurement * Degree (angle), a unit of angle measurement ** Degree of geographical latitude ** Degree of geographical longitude * Degree symbol (°), a notation used in science, engineering, and mathematics ...
s before the contacts will open. As an alternative, some thermostats instead use a
mercury switch A mercury switch is an electrical switch that opens and closes a circuit when a small amount of the liquid metal mercury connects metal electrodes to close the circuit. There are several different basic designs (tilt, displacement, radial, etc ...
on the end of the bimetal coil. The weight of the mercury on the end of the coil tends to keep it there, also preventing short heating cycles. However, this type of thermostat is banned in many countries due to its highly and permanently
toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...
nature if broken. When replacing these thermostats they must be regarded as
chemical waste Chemical waste is any excess, unusable, or unwanted chemical, especially those that cause damage to human health or the environment. Chemical waste may be classified as hazardous waste, non-hazardous waste, universal waste, and household haz ...
. Not shown in the illustration is a separate bimetal thermometer on the outer case to show the actual temperature at the thermostat.


Millivolt thermostats

As illustrated in the use of the thermostat above, all of the power for the control system is provided by a
thermopile A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel. Such a device works on the principle of the thermoele ...
which is a combination of many stacked thermocouples, heated by the pilot light. The thermopile produces sufficient electrical power to drive a low-power gas valve, which under control of one or more thermostat switches, in turn controls the input of fuel to the burner. This type of device is generally considered obsolete as pilot lights can waste a surprising amount of gas (in the same way a dripping faucet can waste a large amount of water over an extended period), and are also no longer used on stoves, but are still to be found in many gas water heaters and gas fireplaces. Their poor efficiency is acceptable in water heaters, since most of the energy "wasted" on the pilot still represents a direct heat gain for the water tank. The Millivolt system also makes it unnecessary for a special electrical circuit to be run to the water heater or furnace; these systems are often completely self-sufficient and can run without any external electrical power supply. For tankless "on demand" water heaters, pilot ignition is preferable because it is faster than hot-surface ignition and more reliable than spark ignition. Some
programmable thermostat A programmable thermostat is a thermostat which is designed to adjust the temperature according to a series of programmed settings that take effect at different times of the day. Programmable thermostats are also known as setback thermostats o ...
s - those that offer simple "millivolt" or "two-wire" modes - will control these systems.


24-volt thermostats

The majority of modern heating/cooling/heat pump thermostats operate on low voltage (typically 24 volts AC) control circuits. The source of the 24 volt AC power is a control transformer installed as part of the heating/cooling equipment. The advantage of the low voltage control system is the ability to operate multiple electromechanical switching devices such as
relay A relay Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts An automotive-style miniature relay with the dust cover taken off A relay is an electrically operated switch ...
s, contactors, and sequencers using inherently safe voltage and current levels. Built into the thermostat is a provision for enhanced temperature control using anticipation. A heat anticipator generates a small amount of additional heat to the sensing element while the heating appliance is operating. This opens the heating contacts slightly early to prevent the space temperature from greatly overshooting the thermostat setting. A mechanical heat anticipator is generally adjustable and should be set to the current flowing in the heating control circuit when the system is operating. A cooling anticipator generates a small amount of additional heat to the sensing element while the cooling appliance is not operating. This causes the contacts to energize the cooling equipment slightly early, preventing the space temperature from climbing excessively. Cooling anticipators are generally non-adjustable. Electromechanical thermostats use resistance elements as anticipators. Most electronic thermostats use either thermistor devices or integrated logic elements for the anticipation function. In some electronic thermostats, the thermistor anticipator may be located outdoors, providing a variable anticipation depending on the outdoor temperature. Thermostat enhancements include outdoor temperature display, programmability, and system fault indication. While such 24 volt thermostats are incapable of operating a furnace when the mains power fails, most such furnaces require mains power for heated air fans (and often also hot-surface or electronic spark ignition) rendering moot the functionality of the thermostat. In other circumstances such as piloted wall and "gravity" (fanless) floor and central heaters the low voltage system described previously may be capable of remaining functional when electrical power is unavailable. There are no standards for wiring color codes, but convention has settled on the following terminal codes and colors. In all cases, the manufacturer's instructions should be considered definitive. Older, mostly deprecated designations:


Line-voltage thermostats

Line voltage thermostats are most commonly used for electric space heaters such as a baseboard heater or a direct-wired electric furnace. If a line voltage thermostat is used, system power (in the United States, 120 or 240 volts) is directly switched by the thermostat. With switching
current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
often exceeding 40
ampere The ampere (, ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to elect ...
s, using a low voltage thermostat on a line voltage circuit will result at least in the failure of the thermostat and possibly a fire. Line voltage thermostats are sometimes used in other applications, such as the control of fan-coil (fan powered from line voltage blowing through a coil of tubing which is either heated or cooled by a larger system) units in large systems using centralized
boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centr ...
s and
chiller A chiller is a machine that removes heat from a liquid coolant via a vapor-compression refrigeration, vapor-compression, adsorption refrigeration, or absorption refrigerator, absorption refrigeration cycles. This liquid can then be circulated th ...
s, or to control circulation pumps in hydronic heating applications. Some programmable thermostats are available to control line-voltage systems. Baseboard heaters will especially benefit from a programmable thermostat which is capable of continuous control (as are at least some
Honeywell Honeywell International Inc. is an American publicly traded, multinational conglomerate corporation headquartered in Charlotte, North Carolina. It primarily operates in four areas of business: aerospace, building technologies, performance ma ...
models), effectively controlling the heater like a lamp dimmer, and gradually increasing and decreasing heating to ensure an extremely constant room temperature (continuous control rather than relying on the averaging effects of hysteresis). Systems which include a fan (electric furnaces, wall heaters, etc.) must typically use simple on/off controls.


Digital electronic thermostats

Newer digital thermostats have no
moving parts Machines include both fixed and moving parts. The moving parts have controlled and constrained motions. Moving parts are machine components excluding any moving fluids, such as fuel, coolant or hydraulic fluid. Moving parts also do not include ...
to
measure Measure may refer to: * Measurement, the assignment of a number to a characteristic of an object or event Law * Ballot measure, proposed legislation in the United States * Church of England Measure, legislation of the Church of England * Mea ...
temperature and instead rely on
thermistor A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''. Thermistors are divided based on their conduction ...
s or other
semiconductor devices A semiconductor device is an electronic component that relies on the electronic properties of a semiconductor material (primarily silicon, germanium, and gallium arsenide, as well as organic semiconductors) for its function. Its conductivity l ...
such as a
resistance thermometer Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature. Many RTD elements consist of a length of fine wire wrapped around a heat-resistant ceramic or glass core but other constructio ...
(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 A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel. Such a device works on the principle of the thermoele ...
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 A clock or a timepiece is a device used to measure and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month and the ...
, and time-of-day and even day-of-week settings for the temperature, used for
comfort Comfort (or being comfortable'')'' is a sense of physical or psychological ease, often characterized as a lack of hardship. Persons who are lacking in comfort are uncomfortable, or experiencing discomfort. A degree of psychological comfort c ...
and energy conservation. Some advanced models have
touch screen A touchscreen or touch screen is the assembly of both an input ('touch panel') and output ('display') device. The touch panel is normally layered on the top of an electronic visual display of an information processing system. The display is ofte ...
s, or the ability to work with home automation or
building automation Building automation (BAS), also known as building management system (BMS) or building energy management system (BEMS), is the automatic centralized control of a building's HVAC (heating, ventilation and air conditioning), electrical, lighting, ...
systems. Digital thermostats use either a relay or a semiconductor device such as
triac A TRIAC (triode for alternating current; also bidirectional triode thyristor or bilateral triode thyristor) is a three terminal electronic component that conducts current in either direction when triggered. The term TRIAC is a genericised tradem ...
to act as a switch to control the
HVAC Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HV ...
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 A proportional–integral–derivative controller (PID controller or three-term controller) is a control loop mechanism employing feedback that is widely used in industrial control systems and a variety of other applications requiring continuou ...
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 thermostat A programmable thermostat is a thermostat which is designed to adjust the temperature according to a series of programmed settings that take effect at different times of the day. Programmable thermostats are also known as setback thermostats o ...
s, 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 A programmable thermostat is a thermostat which is designed to adjust the temperature according to a series of programmed settings that take effect at different times of the day. Programmable thermostats are also known as setback thermostats o ...
article provides basic information on the operation, selection and installation of such a thermostat.


Thermostats and HVAC operation


Ignition sequences in modern conventional systems

; Gas :# Start draft inducer fan/blower (if the furnace is relatively recent) to create a column of air flowing up the chimney :# Heat ignitor or start spark-ignition system :# Open gas valve to ignite main burners :# Wait (if furnace is relatively recent) until the
heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...
is at proper operating temperature before starting main blower fan or circulator pump ; Oil : Similar to gas, except rather than opening a valve, the furnace will start an oil pump to inject oil into the burner ; Electric : The blower fan or circulator pump will be started, and a large electromechanical relay or
TRIAC A TRIAC (triode for alternating current; also bidirectional triode thyristor or bilateral triode thyristor) is a three terminal electronic component that conducts current in either direction when triggered. The term TRIAC is a genericised tradem ...
will turn on the
heating element A heating element converts electrical energy into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element. Unlike the Peltier effect, this process is indepen ...
s ; Coal, grain or pellet : Generally rare today (though grains such as corn, wheat, and barley, or pellets made of wood, bark, or cardboard are increasing in popularity); similar to gas, except rather than opening a valve, the furnace will start a
screw A screw and a bolt (see '' Differentiation between bolt and screw'' below) are similar types of fastener typically made of metal and characterized by a helical ridge, called a ''male thread'' (external thread). Screws and bolts are used to fa ...
to drive coal/grain/pellets into the firebox 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. With zoned systems (some residential, many commercial systems — several thermostats controlling different "zones" in the building), the thermostat will cause small electric motors to open valves or dampers and start the furnace or boiler if it's not already running. Most programmable thermostats will control these systems.


Combination heating/cooling regulation

Depending on what is being controlled, a
forced-air A forced-air central heating system is one which uses air as its heat transfer medium. These systems rely on ductwork, vents, and plenums as means of air distribution, separate from the actual heating and air conditioning systems. The return ...
air conditioning Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...
thermostat generally has an external
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
for heat/off/cool, and another on/auto to turn the blower fan on constantly or only when heating and cooling are running. Four
wire Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm A wire is a flexible strand of metal. Wire is c ...
s come to the centrally-located thermostat from the main heating/cooling unit (usually located in a
closet A closet (especially in North American usage) is an enclosed space, with a door, used for storage, particularly that of clothes. ''Fitted closets'' are built into the walls of the house so that they take up no apparent space in the room. Closet ...
,
basement A basement or cellar is one or more floors of a building that are completely or partly below the ground floor. It generally is used as a utility space for a building, where such items as the furnace, water heater, breaker panel or fuse box, ...
, or occasionally in the attic): One wire, usually red, supplies 24 volts AC power to the thermostat, while the other three supply control signals from the thermostat, usually white for heat, yellow for cooling, and green to turn on the blower fan. The power is supplied by a
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
, and when the thermostat makes contact between the 24 volt power and one or two of the other wires, a relay back at the heating/cooling unit activates the corresponding heat/fan/cool function of the unit(s). A thermostat, when set to "cool", will only turn on when the ambient temperature of the surrounding room is above the set temperature. Thus, if the controlled space has a temperature normally above the desired setting when the heating/cooling system is off, it would be wise to keep the thermostat set to "cool", despite what the temperature is outside. On the other hand, if the temperature of the controlled area falls below the desired degree, then it is advisable to turn the thermostat to "heat".


Heat pump regulation

The
heat pump A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing ...
is a refrigeration based appliance which reverses refrigerant flow between the indoor and outdoor coils. This is done by energizing a
reversing valve A reversing valve is a type of valve and is a component in a heat pump, that changes the direction of refrigerant flow. By reversing the flow of refrigerant, the heat pump refrigeration cycle is changed from cooling to heating or vice versa. This ...
(also known as a "4-way" or "change-over" valve). During cooling, the indoor coil is an evaporator removing heat from the indoor air and transferring it to the outdoor coil where it is rejected to the outdoor air. During heating, the outdoor coil becomes the evaporator and heat is removed from the outdoor air and transferred to the indoor air through the indoor coil. The reversing valve, controlled by the thermostat, causes the change-over from heat to cool. Residential heat pump thermostats generally have an "O" terminal to energize the reversing valve in cooling. Some residential and many commercial heat pump thermostats use a "B" terminal to energize the reversing valve in heating. The heating capacity of a heat pump decreases as outdoor temperatures fall. At some outdoor temperature (called the balance point) the ability of the refrigeration system to transfer heat into the building falls below the heating needs of the building. A typical heat pump is fitted with electric heating elements to supplement the refrigeration heat when the outdoor temperature is below this balance point. Operation of the supplemental heat is controlled by a second stage heating contact in the heat pump thermostat. During heating, the outdoor coil is operating at a temperature below the outdoor temperature and condensation on the coil may take place. This condensation may then freeze onto the coil, reducing its heat transfer capacity. Heat pumps therefore have a provision for occasional defrost of the outdoor coil. This is done by reversing the cycle to the cooling mode, shutting off the outdoor fan, and energizing the electric heating elements. The electric heat in defrost mode is needed to keep the system from blowing cold air inside the building. The elements are then used in the "reheat" function. Although the thermostat may indicate the system is in defrost and electric heat is activated, the defrost function is not controlled by the thermostat. Since the heat pump has electric heat elements for supplemental and reheats, the heat pump thermostat provides for use of the electric heat elements should the refrigeration system fail. This function is normally activated by an "E" terminal on the thermostat. When in emergency heat, the thermostat makes no attempt to operate the compressor or outdoor fan.


Thermostat location

The thermostat should not be located on an outside wall or where it could be exposed to direct sunlight at any time during the day. It should be located away from the room's cooling or heating vents or device, yet exposed to general airflow from the room(s) to be regulated. An open hallway may be most appropriate for a single zone system, where living rooms and bedrooms are operated as a single zone. If the hallway may be closed by doors from the regulated spaces then these should be left open when the system is in use. If the thermostat is too close to the source controlled then the system will tend to "short a cycle", and numerous starts and stops can be annoying and in some cases shorten equipment life. A multiple zoned system can save considerable energy by regulating individual spaces, allowing unused rooms to vary in temperature by turning off the heating and cooling.


Setback temperature

HVAC systems take a long time, usually one to several hours, to cool down or  warm up the space from near outdoor conditions in summer or winter. Thus, it is a common practice to set setback temperatures when the space is not occupied (night and/or holidays). On the one hand, compared with maintaining at the original setpoint, substantial energy consumption can be saved. On the other hand, compared with turning off the system completely, it avoids room temperature drifting too much from the comfort zone, thus reducing the time of possible discomfort when the space is again occupied.  New thermostats are mostly programmable and include an internal clock that allows this setback feature to be easily incorporated.


Dummy thermostats

It has been reported that many thermostats in office buildings are non-functional dummy devices, installed to give tenants' employees an
illusion of control The illusion of control is the tendency for people to overestimate their ability to control events. It was named by U.S. psychologist Ellen Langer and is thought to influence gambling behavior and belief in the paranormal. Along with illusory supe ...
. These dummy thermostats are in effect a type of
placebo button A placebo button is a push-button or other control that appears to have functionality but has no physical effect when pressed. Such buttons can appear to work, by lighting up or otherwise reacting, which rewards the user by giving them an illusion ...
. 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".Example datasheet of current art thermostat, exhibiting lockout functionality : http://cgproducts.johnsoncontrols.com/MET_PDF/12011079.pdf


See also

*
Smart thermostat Smart thermostats are Wi-Fi thermostats that can be used with home automation and are responsible for controlling a home's heating, ventilation, and air conditioning. They perform similar functions as a Programmable thermostat as they allow the ...
(and Wi-Fi thermostat) *
Automatic control Automation describes a wide range of technologies that reduce human intervention in processes, namely by predetermining decision criteria, subprocess relationships, and related actions, as well as embodying those predeterminations in machines ...
* On-off control *
OpenTherm OpenTherm (OT) is a standard communications protocol used in central heating systems for the communication between a central heating boiler and a thermostatic controller. As a standard, OpenTherm is independent of any single manufacturer. A cont ...


Notes and references


External links


Professional Reference Guide

'' "How A Thermostat Tends Your Furnance" ''
1951 article on the basics of the automatic furnace thermostats—this reference has good drawings and illustrations.

* ttp://www.vernatherm.com/thermal_valves.html "How Wax Pellet Thermostatic Valves Function" An Illustration
"Diagrams of Wax Motors i.e. "Wax Pellets""


{{HVAC Switches Temperature control Dutch inventions American inventions 17th-century inventions Articles containing video clips