Thermostat
A thermostat is a device for maintaining the temperature of a system within a range by controlling the flow of heat energy into or out of the system.
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2 Simple Two Wire Thermostats 3 Combination Heating/Cooling Regulation 4 Heat Pump Regulation 5 Digital 6 See also |
Mechanical
On a steam or hot-water radiator system, the thermostat may be entirely mechanical. 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 valvess, as do some older retrofitted systems.
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. The power for this valve is provide by a thermocouple heated by a pilot light (a small, constantly-burning flame) that also provides the ignition for the main burner. This is also the typical setup for all-electric heat, whether baseboard or centralised, but requires the use of a relay as the thermostat contacts can operate only at low voltage and current.
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-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.
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 to the other wire to the thermocouple and thence to the heater control valve.
6. Magnet. This ensures a good contact when the contact closes, and also prevents short heating cycles, as the temperature must be raised several degrees before the contacts will open.
Simple Two Wire Thermostats
As an alternative, many thermostats instead use a sealed mercury-filled capsule, with electrical contacts inside of it. The weight of the mercury on either end of the capsule tends to keep it there, also preventing short heating cycles. However, these have been banned in many countries due to their highly and permanently toxic nature if broken.
Troubleshooting: Owing to the weak current available from a thermocouple this system is sensitive to adjustment and contact corrosion problems. A rare problem is a spider's nest on the contact point, preventing closure of the circuit. A far more common problem is a weak, failed, or poorly positioned (relative to the pilot light) thermocouple in the heater itself.
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 thermostat from the main unit (usually located in a closet, basement, or occasionally attic): one 24-volt for power, one for heat, one for cool, and one for fan. The power is supplied by a transformer, and when the thermostat makes contact between power and another wire, a relay back at the main unit activates that part 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, and instead rely on thermoresistors to measure the temperature. Typically one or more regular batteries must be installed to operate it, as electrical current is not flowing when nothing is running, and there is no separate low-voltage power wire supplied from the main unit. 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 X.10 or other home automation systems.
Most of these have been designed to work on standard 24-volt systems, however some deluxe units now have millivolt systems, whereby only analog or digital signals are sent along the lines, rather than using electromechanical means.
See also
"Thermostat" is the title of a song by They Might Be Giants from their 1994 album John Henry.