A thermostat is a device for regulating the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. In the Physical sciences Quality assurance, and Engineering, Measurement is the activity of obtaining and comparing physical quantities Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature System (from Latin systēma, in turn from Greek systēma is a set of interacting or interdependent Entities, real or abstract The thermostat does this by controlling the flow of heat energy into or out of the system. In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός That is, the thermostat switches heating or cooling devices on or off as needed to maintain the correct temperature.
Thermostats can be constructed in many ways and may use a variety of sensors to measure the temperature. The output of the sensor then controls the heating or cooling apparatus.
Common sensors include:
These may then control the heating or cooling apparatus using:
In 1883, Warren S. Bi-metal refers to an object that is composed of two separate Metals joined together A thermistor is a type of Resistor with resistance varying according to its Temperature. Semiconductor devices are Electronic components that exploit the electronic properties of Semiconductor materials principally Silicon, Germanium In Electrical engineering and industry thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal Potential Johnson, a professor at the State Normal School in Whitewater, Wisconsin, received a patent for the first electric room thermostat. His invention launched the building control industry and was the impetus for a new company.
Johnson and a group of Milwaukee investors incorporated the Johnson Electric Service Company in 1885 to manufacture, install and service automatic temperature regulation systems for buildings. The company was renamed Johnson Controls in 1974. 
On a steam or hot-water radiator system, the thermostat may be an entirely mechanical device incorporating a bi-metal strip. Uses A Steam engine uses the expansion of steam in order to drive a Piston or Turbine to perform Mechanical work. Radiators and convectors are types of Heat exchangers designed to transfer Thermal energy from one medium to another for the purpose of cooling Bi-metal refers to an object that is composed of two separate Metals joined together Generally, this is an automatic valve which regulates the flow based on the temperature. For other uses see Valve (disambiguation. For the electronic component see Thermionic valve. For the most part, their use in North America is now rare, as modern under-floor radiator systems use electric valves, as do some older retrofitted systems. A floor is the walking surface of a room or vehicle Floors vary from simple dirt in a cave to many-layered surfaces using modern technology A solenoid is a three-dimensional Coil. In Physics, the term solenoid refers to a loop of wire often wrapped around a Metallic core which Retrofitting refers to the addition of new technology or features to older systems They are still widely employed on central heating radiators throughout Europe, however.
Mechanical thermostats are used to regulate dampers in rooftop turbine vents, reducing building heat loss in cool or cold periods.
An automobile passenger compartment's heating system has a thermostatically controlled valve to regulate the water flow and temperature to an adjustable level. For other uses see Valve (disambiguation. For the electronic component see Thermionic valve. 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 Automotive engineering, an intake manifold or inlet manifold is the part of an Engine that supplies the Fuel / Air mixture to An actuator is a mechanical device for moving or controlling a mechanism or system In modern vehicles, the vacuum actuators may be operated by small solenoids under the control of a central computer. A solenoid is a three-dimensional Coil. In Physics, the term solenoid refers to a loop of wire often wrapped around a Metallic core which In automotive electronics an electronic control unit (ECU also called a Control unit, or Control module, is an Embedded system that controls one or more
A thermostat is used in automobiles containing an internal combustion engine to regulate the flow of coolant.
This type of thermostat operates mechanically. It makes use of a wax pellet inside a sealed chamber. The wax is solid at low temperatures but as the engine heats up the wax melts and expands. The sealed chamber has an expansion provision that operates a rod which opens a valve when the operating temperature is exceeded. The operating temperature is fixed, but is determined by the specific composition of the wax, so thermostats of this type are available to maintain different temperatures, typically in the range of 70 to 90 °C (160 to 200 °F). The Celsius Temperature scale was previously known as the centigrade scale. Fahrenheit is a temperature scale named after Daniel Gabriel Fahrenheit (1686–1736 a German Physicist who proposed it in 1724 Modern engines run hot, that is, over 80 °C (180 °F), in order to run more efficiently and to reduce the emission of pollutants. Most thermostats have a small bypass hole to vent any gas that might get into the system (e. g. , air introduced during coolant replacement), this small bypass hole is under normal circumstances used to have a small flow when the thermostat is still closed. Without this flow it would be impossible for the thermostat to react correctly on the heating up water. Modern cooling systems contain a relief valve in the form of a spring-loaded radiator pressure cap, with a tube leading to a partially filled expansion reservoir. Owing to the high temperature, the cooling system will become pressurized to a maximum set by the relief valve. The additional pressure increases the boiling point of the coolant above that which it would be at atmospheric pressure.
This type of thermostat is 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 it heats up the gas expands applying pressure to the diaphragm which reduces the flow of gas to the burner.
The illustration is the interior of a common two wire heat-only household thermostat, used to regulate a gas-fired heater via an electric gas valve. Natural gas is a Gaseous Fossil fuel consisting primarily of Methane but including significant quantities of Ethane, Propane, Similar mechanisms may also be used to control oil furnaces, boilers, boiler zone valves, electric attic fans, electric furnaces, electric baseboard heaters, and household appliances such as refrigerators, coffee pots, and hair dryers. A zone valve is a specific type of Valve used to control the flow of Water or Steam in a hydronic heating or cooling system The power through the thermostat is provided by the heating device and may range from millivolts to 240 volts 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). The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. Due to the variety of possible voltages and currents available at the thermostat, caution must be taken. Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere.
Not shown in the illustration is a separate bi-metal thermometer on the outer case to show the actual temperature at the thermostat.
As illustrated in the use of the thermostat above, the power is provided by a thermocouple, heated by the pilot light. This produces little power and so the system must use a low power valve to control the gas. This type of device is generally considered obsolete as pilot lights waste a surprising amount of gas (in the same way a dripping faucet can waste a huge 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. (Their poor efficiency is acceptable in water heaters, since most of the energy "wasted" on the pilot light is still being coupled to the water and therefore helping to keep the tank warm. For tankless (on demand) water heaters, pilot ignition is preferable since it is faster than hot-surface ignition and more reliable than spark ignition. )
Existing millivolt heating systems can be made far more economical by turning off the gas supply during non-heating seasons and re-lighting the pilot when the heating season approaches. During the winter months, most of the small amount of heat generated by the pilot flame will probably radiate through the flue and into the house, meaning that the gas is wasted (during a time when the system isn't actively heating) but the pilot-warmed flue continues to add to the total thermal energy in the house. In the summer months, this is wholly undesirable.
Some programmable thermostats will control these systems. 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
The majority of heating/cooling/heat pump thermostats operate on low voltage (typically 24VAC) control circuits. The source of the 24 VAC 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 relays, 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.
Most modern gas or oil furnaces or boilers will be controlled by such systems, as will most relay-operated electric furnaces:
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 startup rituals 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. 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
Line voltage thermostats are most commonly used for electric space heaters such as a baseboard heater or a direct-wired electric furnace. In Architecture, a baseboard (also called skirting board, skirting, or mopboard) is a Wooden Board, normally two to 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 often exceeding 40 amperes, using a low voltage thermostat on a line voltage circuit will result at least in the failure of the thermostat and possibly a fire. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. The ampere, in practice often shortened to amp, (symbol A is a unit of Electric current, or amount of Electric charge per second 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 boilers and chillers, or to control circulation pumps in hydronic heating applications. A boiler is a closed vessel in which Water or other Fluid is heated A chiller is a machine that removes heat from a liquid via a vapor-compression or Absorption refrigeration cycle.
Some programmable thermostats are available to control line-voltage systems. 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 Baseboard heaters will especially benefit from a programmable thermostat which is capable of continuous control (as are at least some Honeywell 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). Honeywell ( is a major American multinational conglomerate company that produces a variety of consumer products engineering services and aerospace systems A system with hysteresis can be summarised as a system that may be in any number of states independent of the inputs to the system Systems which include a fan (electric furnaces, wall heaters, etc. ) must typically use simple on/off controls.
Depending on what is being controlled, a forced-air air conditioning thermostat generally has an external switch for heat/off/cool, and another on/auto to turn the blower fan on constantly or only when heating and cooling are running. A Forced-air or Warm air Heating system is one which uses Air as its Heat transfer medium The term air conditioning refers to the cooling and dehumidification of indoor air for Thermal comfort. A switch is a mechanical device used to connect and disconnect an electric Circuit at will A mechanical fan is an electrically powered device used to produce an airflow for the purpose of creature comfort (particularly in the heat ventilation, exhaust Four wires come to the centrally-located thermostat from the main heating/cooling unit (usually located in a closet, basement, or occasionally attic): one wire supplies a 24 V AC power connection to the thermostat, whilst the other three supply control signals from the thermostat, one for heat, one for cooling, and one to turn on the blower fan. A wire is a single usually cylindrical, elongated string of drawn Metal. A closet (especially in North American usage is a small and enclosed space a cabinet, or a Cupboard in a House A basement is one or more floors of a building that are either completely or partially below the Ground floor. An attic is a space found directly below the pitched Roof of a house or other building (also called garret, Loft or The power is supplied by a transformer, and when the thermostat makes contact between power and another wire, a relay back at the heating/cooling unit activates the corresponding function of the unit. A transformer is a device that transfers Electrical energy from one circuit to another through inductively coupled Electrical conductors A relay is an electrical Switch that opens and closes under the control of another Electrical circuit.
The heat pump is a refrigeration based appliance that reverses refrigerant flow between the indoor and outdoor coils. This is done by energizing a "reversing", "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.
Newer digital thermostats have no moving parts to measure temperature and instead rely on thermistors. 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 A digital system uses discrete (discontinuous values usually but not always Symbolized Numerically (hence called "digital" to represent information for Measurement is the process of estimating the magnitude of some attribute of an object such as its length or weight relative to some standard ( unit of measurement) such as A thermistor is a type of Resistor with resistance varying according to its Temperature. 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). In electronics a battery is a combination of two or more Electrochemical cells which store chemical Energy which can be converted into electrical energy The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. An alternating current ( AC) is an Electric current whose direction reverses cyclically as opposed to Direct current, whose direction remains constant A thermopile is an electronic device that converts Thermal energy into Electrical energy. Each has an LCD screen showing the current temperature, and the current setting. Most also have a clock, and time-of-day (and now day-of-week) settings for the temperature, used for comfort and energy conservation. Circadian Locomotor Output Cycles Kaput, or Clock is a gene which encodes proteins regulating Circadian rhythm. In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός Energy conservation is the practice of decreasing the quantity of energy used Some now even have touch screens, or have the ability to work with X10, BACnet, LonWorks or other home automation or building automation systems. A Touch Screen is a display which can detect the presence and location of a touch within the display area X10 is an international and open industry standard for Communication among electronic devices used for Home automation, also known as Domotics BACnet is a Data Communications Protocol for Building Automation and Control Networks LonWorks is a networking platform specifically created to address the unique performance reliability installation and maintenance needs of Control applications The platform Home automation (also called smart homes or domotics) is a field within Building automation, specializing in the specific Automation requirements "Building automation" can also refer to Build automation, the practice of programmatically Compiling Software programs
Digital thermostats use either a relay or a semiconductor device such as triac to act as switch to control the HVAC unit. A relay is an electrical Switch that opens and closes under the control of another Electrical circuit. Semiconductor devices are Electronic components that exploit the electronic properties of Semiconductor materials principally Silicon, Germanium A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two Silicon-controlled rectifiers ( SCRs 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. A proportional–integral–derivative controller (PID controller is a generic Control loop Feedback mechanism widely used in industrial Control systems For instance, setting it up that temperature in the morning at 7am should be 21 degrees, makes sure that at that time the temperature will be 21 degrees (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).
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. 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 The programmable thermostat article provides basic information on the operation, selection and installation of such a 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
The thermostat 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 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.
NEMA — National Electrical Manufacturers [sic] Association in 1972 standardized the labels on thermostat terminals. The National Electrical Manufacturers Association or NEMA is a U These standards specify alphanumeric codes to be used for specific functions in thermostats: