From New World Encyclopedia
A bi-metallic thermostat of the type used in many buildings.

A thermostat is a device for regulating the temperature of a system so that the system's temperature is maintained near a desired temperature. The thermostat does this by controlling the flow of heat energy into or out of the system. 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:

  • Bi-metal mechanical sensors
  • Expanding wax pellets
  • Electronic thermistors
  • Electrical thermocouples

These may then control the heating or cooling apparatus using:

  • Direct mechanical control
  • Electrical signals
  • Pneumatic signals

History and Development

The thermostat was invented in 1885 by Albert Butz and is the first known example of process control methodology. This invention was the genesis for what is now Honeywell corporation.

Mechanical thermostats


On a steam or hot-water radiator system, the thermostat may be an entirely mechanical device incorporating a bi-metal strip. Generally, this is an automatic valve which regulates the flow based on the temperature. 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. 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. 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

A car engine thermostat.

An automobile operating on an internal combustion engine requires a thermostat 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).

Modern engines are 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). 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.

Electrical thermostats

Simple two-wire thermostats

Thermostat Mechanism

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 (electric baseboard heaters and some electric furnaces) or indirectly (all gas, oil and forced hot water systems). Due to the variety of possible voltages and currents available at the thermostat, caution must be taken.

1. Set point 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.

2. Bi-metallic strip 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.

3. Flexible wire. The left side is connected via one wire of a pair to the heater control valve.

4. Moving contact attached to the bi-metal coil.

5. Fixed contact screw. This is adjusted by the manufacturer. It is connected electrically by a second wire of the pair to the thermocouple and thence to the heater's controller.

6. 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 bi-metal 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.

Not shown in the illustration is a separate bi-metal thermometer on the outer case to show the actual temperature at the thermostat.

Millivolt thermostats

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.

24 volt thermostats

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 voltge 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:

  • Gas:
    • start drafting fan (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 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 furnace or boiler:
    • the blower fan or circulator pump will be started, and a large relay or triac will turn on the heating elements.
  • Coal:
    • though rare today, worth a mention; similar to gas, except rather than opening a valve, the furnace will start a coal screw to drive coal 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 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.

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 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 (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.

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 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 hysterisis). Systems which include a fan (electric furnaces, wall heaters, etc.) must typically use simple on/off controls.

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 (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. 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.

Heat Pump Regulation

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 tempeature 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.


See also Programmable thermostat.

Newer digital thermostats have no moving parts to measure temperature and instead rely on thermistors. 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 now day-of-week) settings for the temperature, used for comfort and energy conservation. Some now even have touch screens, or have the ability to work with X10, BACnet, LonWorks or other home automation or building automation systems.

Digital thermostats use either a relay or a semiconductor device such as triac to act as switch to control the HVAC unit. Units with relays will operate millivolt systems, but often make an audible "click" noise when switching on or off. More expensive models have a built-in PID controller, so that the thermostat knows ahead how the system will react to its commands. For instance, setting it up that temperature in the morning at 7:00 A.M. should be 21 degrees, makes sure that at that time the temperature will be 21 degrees (a conventional thermostate 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 are programmable thermostats, which will typically provide a 30 percent 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.

Household thermostat location

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 multiply zoned system can save considerable energy by regulating individual spaces, allowing unused rooms to vary in temperature by turning off the heating and cooling.

Thermostat Terminal Codes

NEMA — National Electrical Manufacturers [sic] Association in 1972 standardized the labels on thermostat terminals. These standards specify alphanumeric codes to be used for specific functions in thermostats:

Thermostat Terminal Codes
Letter Color Definition
R, or RH for heat or RC for cool red "hot" side of transformer
W white heat control
W2 pink or other color heat, second stage
Y2 blue or pink cool, second compressor stage
C or X black common side of transformer (24 V)
G green fan
O orange Energize to cool (heat pumps)
L tan, brown, grey or blue service indicator lamp
X2 blue, brown, grey or tan heat, second stage (electric)
B blue or orange energize to heat
B or X blue, brown or black common side of transformer
E blue, pink, gray or tan emergency heat relay on a heat pump
T tan or gray outdoor anticipator reset

See also

  • Automatic control
  • Delta Dore
  • Honeywell Chronotherm thermostat
  • Sensor

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External links

All links retrieved April 30, 2023.

  • HVAC News & Directory – New product features, news, events, training calendar and directory for industry professionals.


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