Francais | English | Espanõl

Thermostat

From Wikipedia, the free encyclopedia

Jump to: navigation, search

Bi-metallic thermostat for buildings

A thermostat is a device for regulating the temperature of a system so that the system's temperature is maintained near a desired setpoint 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:

These may then control the heating or cooling apparatus using:

  • Direct mechanical control
  • Electrical signals
  • Pneumatic signals

Contents

[edit] Mechanical

[edit] 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 Fahrenheit, which is when the thermostat turns the heating device on, and 59 degrees Fahrenheit, 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.

[edit] Wax pellet

car engine thermostat

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 loses 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.

[edit] Electrical

[edit] 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 furnaces and electric baseboard heaters. 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 indicates the actual temperature at the thermostat.

[edit] 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 undesireable.

Some programmable thermostats will control these systems.

[edit] 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. By using this lower voltage the wiring may be the common low-current type used for doorbells, reducing the installation cost.

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:
  • 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.

Most digital thermostats in common residential use in North America are programmable thermostats, which will typically provide a 30% 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.

[edit] 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.

[edit] 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:

  • 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

[edit] See also

[edit] External links

es:Termostato fa:دماپا fr:Thermostat nl:Thermostaat no:Termostat pl:Termostat pt:Termostato ru:Термостат fi:Termostaatti sv:Termostat tr:Termostat

Retrieved from "http://en.wikivisual.com/index.php/Thermostat"
Personal tools