Thermostats regulate ambient air temperature or the air temperature of a closed system to a desired set point. They range from simple mechanical devices that only sense the temperature surrounding the device to complex
digital thermostats that include multiple zones, inputs, and outputs.
A thermostat is a device that keeps temperatures stationary; the term itself is derived from two Greek words: thermos, meaning "heat," and statos, meaning "stationary."
They are composed of one or more temperature sensing elements and one or more outputs used to regulate air temperature. The device may be comprised of dedicated sensing elements and discrete or variable outputs, or may contain a thermal switch that functions as both the sensing element and the output signal.
Thermostats control air temperature by sending a control signal to a heating or cooling device. The controller is typically either a bang-bang controller or a proportional-integral-derivative (PID) controller.
Bang-bang controllers, also known as on-off controllers or hysteresis controllers, are feedback devices that switch between two states. They use hysteresis to prevent continuous switching as the temperature passes through the set point temperature. The discrete output does not allow the heating or cooling systems to ramp-up or ramp-down, but instead switches abruptly between on and off.
Proportional-integral-derivative (PID) controllers provide an output signal that is proportional to the difference between the desired set point temperature and the measured value. They sense the rate of temperature change in response to the control signal. They enable precision regulation of air temperature to minimize temperature oscillation and temperature overshoot.
Thermostats fall into two basic categories: mechanical and digital.
Digital thermostats have no moving parts and are comprised of dedicated sensing elements such as thermistors, resistance temperature detectors (RTD), or a thermocouples and discrete outputs or variable control signals. They may differ by the control logic, the type of display, the number of outputs, and are often programmable. Interconnectivity is key feature of digital thermostats as it allows users to address temperature controls remotely.
Mechanical thermostats operate based on one of several technologies. They generally use thermal expansion of a gas or metal to operate switch contacts. Specific technology types include:
- Bimetallic disc or snap action switches operate due to thermal expansion. Two dissimilar metals expand at different rates and when the temperature threshold is reached the discs' snap action forces the switch to activate.
- Reed switches consist of a pair of contacts on ferrous metal reeds that are hermetically sealed. They are actuated by a magnetic field. The contacts may be normally open or normally closed and change state when the magnetic field surrounding the reed switch is altered.
- Mercury switches have contacts sealed in a glass envelope with a small amount of mercury. Mercury, which is in a liquid state at temperatures above -40° C, is an electrical conductor that makes or breaks the contacts depending on the angle of inclination. The thermal switch is typically actuated by mounting the switch on a metal coil and thermal expansion induces movement in the coil to tilt and actuate the switch.
- Rod and tube thermostats are composed of an outer tube and internal rod, each with dissimilar coefficients of thermal expansion that allow for a thermally-induced, plunger-style contact movement. They are characterized by rapid response times and high operating temperatures.
Gas-actuated (also referred to as vapor-tension) thermostats use the thermal expansion of a gas or vapor in a sensing bulb to create a proportional pressure on a diaphragm or piston assembly that actuates the electrical switching element.
Thermostats are powered by a line voltage, low voltage control circuit, a millivolt-induced voltage, or are otherwise battery operated. Line voltages in the United States are 120, 208, or 240 volts AC. Low voltage circuitry typically delivers 24 volts AC via a control transformer. Thermostats may be specified by any of the aforementioned voltages.
Thermostats have a set point temperature that is either fixed or field adjustable. The set point is the rated temperature at which the device undergoes a change in state associated with movement of the electrical contacts. Fixed-point devices are rated at a given temperature while field-adjustable device have a temperature range to which the set point can be adjusted.
Hysteresis, also referred to as the temperature differential, is a finite temperature range over which switching takes place. It is designed to prevent circuitry from constantly changing between states. A hysteresis of 0.5 C will typically actuate 0.5 degrees past the set-point temperature and not change state again until the temperature drops to 0.5C below the set-point temperature.
Thermostats act as the control unit for HVAC systems such as boilers, furnaces, electric heaters, central air, space heaters, or other devices that regulate ambient air temperature. They are also commonly used in household appliances including ovens, stoves, refrigerators, air conditioners, and freezers.
ANSI Z21.23 — Standard for electric and certain gas appliance thermostats
NEMA DC 13 — Features, performance, and applications of line-voltage integrally-mounted thermostats for electric heaters
NEMA DC 3 — Standards for electric, wall-mounted room thermostats