Thermostat
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".
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, central heating, air conditioners, as well as kitchen equipment including ovens and refrigerators and medical and scientific incubators.Construction
Thermostats use different types of sensors to measure the temperature. In one form, the mechanical thermostat, a bimetallic strip in the form of a coil directly operates electrical contacts that control the heating or cooling source. Electronic thermostats, instead, use a thermistor or other semiconductor sensor that requires amplification and processing to control the heating or cooling equipment. A thermostat is an example of a "bang-bang controller" as the heating or cooling equipment output is not proportional 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 change the time to achieve the desired temperature. The rate at which the target system temperature can change is determined both by the capacity of the heating or cooling equipment to respectively add or remove heat to or from a target system and the capacity of the target system to store heat.To prevent excessively rapid cycling of the equipment when the temperature is near the setpoint, a thermostat can include some hysteresis. Instead of changing from "on" to "off" and vice versa instantly at the set temperature, a thermostat with hysteresis will not switch until the temperature has changed a little past the set temperature point. For example, a refrigerator set to 2°C might not start the cooling compressor until its food compartment's temperature reaches 3°C, and will keep it running until the temperature has been lowered to 1 °C. This reduces the risk of equipment wear from too frequent switching, although it introduces a target system temperature oscillation of a certain magnitude.
To improve the comfort of the occupants of heated or air-conditioned spaces, bimetal sensor thermostats can include an "anticipator" system to slightly warm the temperature sensor while the heating equipment is operating, or to slightly warm the sensor when the cooling system is not operating. When correctly adjusted this reduces any excessive hysteresis in the system and reduces the magnitude of temperature variations. Electronic thermostats have an electronic equivalent.
Sensor types
Early technologies included mercury thermometers with electrodes inserted directly through the glass, so that when a certain 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:
- Bimetallic mechanical or electrical sensors.
- Expanding wax pellets
- Electronic thermistors and semiconductor devices
- Electrical thermocouples
- Direct mechanical control
- Electrical signals
- Pneumatic signals
History
Modern thermostat control was developed in the 1830s by Andrew Ure, 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 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.
Albert Butz 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 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 or hot-water radiator bi-metallic thermostats which the individual flow. However, thermostatic radiator valves 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 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 cooling system thermostat, used to maintain the engine near its optimum operating temperature by regulating the flow of coolant to an air-cooled radiator. 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 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 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, 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 vs. temperature increase, and contraction 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 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, 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 in the control tube to activate heating or cooling when required. The control air typically is maintained on "mains" at 15-18 psi. Pneumatic thermostats typically provide output/ branch/ post-restrictor pressures of 3-15 psi which is piped to the end device.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.
A wax pellet driven valve can be analyzed by graphing the wax pellet's hysteresis which consists of two thermal expansion curves; extension vs. temperature increase, and contraction vs. temperature decrease. The spread between the up and down curves visually illustrate the valve's hysteresis; there is always hysteresis within wax driven technology due to the phase change between solids and liquids. Hysteresis can be controlled with specialized blended mixes of hydrocarbons; tight hysteresis is what most desire, however specialized engineering applications require broader ranges. Wax pellet driven valves are used in anti scald, freeze protection, over-temp purge, solar thermal, automotive, and aerospace applications among many others.
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 sensors. A principal use of the bi-metallic thermostat today is in individual electric convection heaters, where control is on/off, based on the local air temperature and the setpoint desired by the user. These are also used on air-conditioners, where local control is required.Contact configuration nomenclature
This follows the same nomenclature as described in Relay and Switch.- "NO" stands for "Normally Open". This is the same as "COR". 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". 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, to start a heater when it is becoming cold.
Simple two wire thermostats
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. 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. 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 or indirectly. Due to the variety of possible voltages and currents available at the thermostat, caution must be taken when selecting a replacement device.- Setpoint control lever. 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 wound into a coil. The center of the coil is attached to a rotating post attached to lever. As the coil gets colder the moving end — carrying — moves clockwise.
- 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. It is connected electrically by a second wire of the pair to the thermocouple and the heater's electrically operated gas valve.
- Magnet. 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 degrees before the contacts will open. As an alternative, some thermostats instead use a mercury switch 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 nature if broken. When replacing these thermostats they must be regarded as chemical waste.
Millivolt thermostats
As illustrated in the use of the thermostat above, all of the power for the control system is provided by a thermopile 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, 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 thermostats - 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 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 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. 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 rendering moot the functionality of the thermostat. In other circumstances such as piloted wall and "gravity" 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.
| Terminal Code | Color | Description |
| R | Red | 24 volt |
| Rh | Red | 24 volt HEAT load |
| Rc | Red | 24 volt COOL load |
| C | Black/Blue/Brown/Cyan | 24 volt Common |
| 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 |
| B | Varies/Blue/Black/Brown/Orange | Reversing valve Energize to Heat or Common |
| E | Varies/Blue/Pink/Gray/Tan | Emergency Heat |
| S1/S2 | Brown/Black/Blue | Temperature Sensor |
| T | Varies/Tan/Gray | Outdoor Anticipator Reset, Thermistor |
| X | Varies/Black | Emergency Heat or Common |
| X2 | Varies | 2nd stage/emergency heating or indicator lights |
| L | Varies | Service Light |
| U | Varies | User programmable |
| K | Yellow/Green | Combined Y and G |
Older, mostly depreciated designations
| Terminal Code | Description |
| V | 24 volt |
| 4 / M | 24 volt HEAT load |
| F | Fan |
| H | Heat |
| P | Heat Pump defrost relay |
| C | Cool or clock power |
| T | Transformer common |
| blank / 6 | Not heat to close valve |
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 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. Line voltage thermostats are sometimes used in other applications, such as the control of fan-coil units in large systems using centralized boilers and chillers, 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, effectively controlling the heater like a lamp dimmer, and gradually increasing and decreasing heating to ensure an extremely constant room temperature. Systems which include a fan must typically use simple on/off controls.
Digital electronic thermostats
Newer digital thermostats have no moving parts to measure temperature and instead rely on thermistors or other semiconductor devices such as a resistance thermometer. 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, makes sure that at that time the temperature will be 21 °C, 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
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.
Thermostats and HVAC operation
Ignition sequences in modern conventional systems
- Gas
- Oil
- Electric
- Coal
With zoned systems, 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 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. Four wires come to the centrally-located thermostat from the main heating/cooling unit : 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, 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.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".