Thermostat

[1] A thermostat operates as a "closed loop" control device, as it seeks to reduce the error between the desired and measured temperatures.

Although it is the simplest program to implement, such control method requires to include some hysteresis in order to prevent excessively rapid cycling of the equipment around the setpoint.

[2] Such control is in general inaccurate, inefficient and may induce more mechanical wear; it however, allows for more cost-effective compressors compared to ones with continuously variable capacity.

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 regulated the individual flow.

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.

This type of thermostat operates using a sealed chamber containing a wax pellet that melts and expands at a set temperature.

On many automobile engines, including all Chrysler Group and General Motors products, the thermostat does not restrict flow to the heater core.

Pneumatic thermostats typically provide output/ branch/ post-restrictor (for single-pipe operation) pressures of 3-15 psi which is piped to the end device (valve/ damper actuator/ pneumatic-electric switch, etc.).

Water and steam based central heating systems have traditionally had overall control by wall-mounted bi-metallic strip thermostats.

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.

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.

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).

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

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 majority of modern heating/cooling/heat pump thermostats operate on low voltage (typically 24 volts AC) control circuits.

This opens the heating contacts slightly early to prevent the space temperature from greatly overshooting the thermostat setting.

This causes the contacts to energize the cooling equipment slightly early, preventing the space temperature from climbing excessively.

In other circumstances such as piloted wall and "gravity" (fanless) floor and central heaters the low voltage system described previously may be capable of remaining functional when electrical power is unavailable.

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.

Digital thermostats use either a relay or a semiconductor device such as triac to act as a switch to control the HVAC unit.

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.

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.

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".

If the thermostat is too close to the source controlled then the system will tend to "short a 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.

HVAC systems take a long time, usually one to several hours, to cool down or warm up the space from near outdoor conditions in summer or winter.

[23] On the other hand, compared with turning off the system completely, it avoids room temperature drifting too much from the comfort zone, thus reducing the time of possible discomfort when the space is again occupied.

It has been reported that many thermostats in office buildings are non-functional dummy devices, installed to give tenants' employees an illusion of control.