Thermostat

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Thermostats

A thermostat is a device for maintaining the temperature of a system within a range by controlling, either directly or indirectly, the flow of heat energy into or out of the system.

=Mechanical=

Bi-metal

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 is now rare, as modern under-floor radiator systems use electric valves, as do some older retrofitted systems.

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.

All thermostats have thermometers. In a classical mechanical thermostat, the thermometer is a coiled bimetallic strip. A bimetallic strip is made by uniting several layers, made up of two different types of metal, together. The metals that make up the strip get bigger or smaller when they are heated or cooled. Each type of metal expands at its own specific rate, and the two metals (usually iron and copper) that make up the strip are selected so that the rate that the strip gets bigger and smaller aren’t the same. When the strip cools off, one side of the metal on the inside of the coil gets smaller and the strip will wind more tightly. When the coil tightens, the circuit is completed by a switch attached to the coil, and the furnace turns on. The switch in the circuit is typically a mercury switch (a bead of liquid mercury metal inside a glass bulb with two leads at one end). As the temperature of the room rises, the opposite occurs, and the coil unwinds, opening the circuit and turning the furnace off. Most modern thermostats are digital, with a solid-state temperature sensor rather than a bimetallic strip, and a transistor switch instead of a mercury switch.

The thermostat senses the temperature of the room and shuts the fan on and off automatically, at the level of comfort wanted by the user. Most thermostats turn on the heating mechanism whenever the temperature gets below a certain temperature and turns it off whenever the temperature rises above another temperature. The two temperatures are separated slightly so the heating mechanism doesn't turn on and off too quickly, and are usually set a few degrees from the temperature wanted. For example, if the thermostat is set to 58 degrees (Fahrenheit), it does not stay at that exact temperature. The temperature will rise and fall between about 57 degrees, which is when the thermostat turns the heating device on, and 59 degrees, which is when it turns the heater off. More complicated thermostats can also control cooling devices like an air conditioner on to cool the room down faster, and turning it off when the air reaches the temperature that is wanted.

Wax pellet

A thermostat is used in automobiles using an internal combustion engine to regulate the flow of coolant. When the thermostat is open, coolant passes through the cylinder head where it gets hot. It is then led from the engine into the radiator where it looses the heat to the air flowing through it. A “water pump” driven from the engine propels the coolant around the system. When the thermostat is closed the flow is prevented and so the engine is allowed to heat up to its optimum operating temperature.

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=

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. The several millivolts of electricity needed to operate this valve is generated by a thermocouple heated by a pilot light (a small, constantly-burning flame) that also provides the ignition for the main burner.

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.
Bi-metal 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.
Flexible wire. The left side is connected via one wire of a pair to the heater control valve.
Moving contact attached to the bi-metal coil.
Fixed contact screw. This is adjusted by the manufacturer. It is connected electrically to the other wire to the thermocouple and thence to the heater control 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 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 their 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 indicates the actual temperature at the thermostat.

Millivolt thermostats

24 volt thermostats

The majority of heating/cooling systems are operated by low voltage (typically 20-30 volts) provided by a transformer. The advantage of this over the millivolt system is that it allows for the use of a heat anticipator. A heat anticipator is a small resistor integrated into the bi-metal coil. When the thermostat closes, electricity flows through the heat anticipator causing it to slightly heat the bi-metal coil. This is to done to compensate for the slow response time of the bi-metal coil to changes in air temperature, preventing excessively long heating cycles.

Line voltage thermostats

Line voltage thermostats are most commonly used for electric space heaters such as a baseboard heater. If a line voltage thermostat is used, system power (in the United States, 120 or 240 volts) is directly switched by the thermostat. 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.

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 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

In the case of a heat pump, which reverses the air conditioning in winter to provide heat, a second contact is usually provided for "emergency" heat – a backup of electrical heating elements which run when the temperature falls too far (usually about 1°C or 2°F) below the main setting. (These coils also run when the unit is in defrost mode, though this is controlled by the outdoor unit rather than the indoor thermostat.) There is also a slight difference in the wiring, as instead of "heat" and "cool" wires, there is one to turn on the outdoor compressor unit, and another to hold the reversing valve for cooling in summer and defrosting in winter.

Digital

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.

=See also=

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