A thermostat regulates the temperature of a system and keeps the system’s temperature at desired setpoint temperature. The device does this by turning heating and cooling devices on and off and regulating air flow of a heat transfer fluid as needed. This maintains the correct temperature.

Thermostats may be used to control heating or cooling a system, or may be a component part of a heater or air conditioner. There are a variety of ways that thermostats can be constructed and could use various sensors to measure the temperature. The output of the sensor then controls the heater or cooler.

In 1883, Warren S. Johnson invented the first electric room thermostat. 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.
Today sensor technologies include Bimetallic mechanical or electrical, expanding wax pellets, electronic thermistors and semiconductor devices, and electrical thermocouples. These control heating and cooling using direct mechanical control, electrical signals, and Pneumatic signals.

Traditionally domestic water and steam based systems are controlled by bi-metallic strip thermostats; however, thermostatic radiator valves are now being widely used. In rooftop turbine vents mechanical thermostats are used to regulate the dampers. This reduces building heat loss during cold periods.

In some automobiles the heating system has a valve controlled by the thermostat that regulates the water flow and the temperature. Older vehicles use the application of engine vacuums to actuators that control the water valves and flappers to direct the flow of air. Modern vehicles’ actuators are operated by small solenoids ran by the central computer. The system will then switch on or off when the temperature drops below or above the set temperature. Electronic sensors are now taking the place of many Bi-metallic sensors. Air conditioners and electric convection heaters use bi-metallic sensors, where the control is on/off, based on the local air temperature and the set point desired by the user.

Power is provided to the thermostat by a thermocouple which is heated by the pilot light. This power is very little and the system must use a low power valve to control the gas. Pilot lights waste a large amount of gas and are not really used on stoves anymore. They are still used on water heaters. The light on a water heater is usually still keeping the water warm and makes it unnecessary for an electric circuit to be run to the water heater. Pilot ignition is preferable on tankless water heaters because it is faster than hot-surface ignition and more reliable than spark ignition.

Most modern thermostats operate on a low voltage control circuit. The source of the 24 volt AC power is a control transformer installed as part of the heating/cooling equipment. Since the level of voltage is so low and safe the control system is able to operate multiple switching devices such as relays, contractors, and sequencers. The use of a heat anticipator generates a small amount of additional heat to the sensing element while the heating appliance is operating. This allows the heating contacts to open slightly early to prevent the space temperature from overshooting the thermostat setting. There is also a cooling anticipator that generates a small amount of additional heat to the sensing element while the cooling appliance is not operating. This energizes the cooling equipment early preventing the temperature from climbing excessively. These are generally non-adjustable.

Electromechanical thermostats use resistance elements as anticipators. They use either thermistor devices or integrated logic elements. Some of the enhancements of thermostats include outdoor temperature display, programmability, and system fault indication.

There are also line voltage thermostats which are commonly used for electric space heaters. In line voltage the system power is directly switched by the thermostat. Since the current often exceeds 40 amperes using a low voltage thermostat will often result in failure or possibly fire. Some of these thermostats have the ability to be programmable. Baseboard heaters benefit from programmable thermostats through continuous control thus maintaining a constant room temperature.

New digital thermostats have no moving parts to measure temperature and instead rely on thermistors or semiconducting devices such as a resistance thermometer. These thermostats usually run on batteries although some use common 24 volt AC circuits as a power source. They usually have an LCD screen showing various information. Some even have touch screens.

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. Most programmable thermostats provide a typical 30% savings if left with default programs.

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