Electrical and analog electronic thermostats

Bimetallic switching thermostats

Traditionally, water and steam based central heating systems have been fully controlled by wall mounted bimetal thermostats.These sensors sense air temperature using the differential expansion of two metals to actuate an on/off switch.Typically, the central system will turn on when the temperature falls below the thermostat set point and turn off when the temperature rises above it, with a hysteresis of a few degrees to prevent excessive switching.Bimetal sensing is now being replaced by electronic sensors.Today, the main use of bimetallic thermostats is for individual electric convection heaters, with on/off control based on the local air temperature and the user's desired set point. These are also used in air conditioners that require local control.

Contact configuration nomenclature 

This follows the same nomenclature as described in Relay § Force-guided contacts relay and Switch § Contact terminology.

• "NO" stands for "Normally Open". This is the same as "COR" ("closing price").Can be used to start the fan when it gets hot; i.e. stop the fan when it gets cold enough.

• "NC" stands for "Normally Closed".This is the same as "OOR" ("open on rise").Can be used to start the heater when it gets cold; that is, to stop the heater when it gets hot enough.

• "CO" stands for "conversion". This is both "NO" and "NC". Can be used to start a fan when it gets hot, or (on the opposite terminal) a heater when it gets cold.Any leading number represents the number of contact groups, such as "1NO", "1NC" means that a group of contacts has two terminals. "1CO" will also have a contact set, even though it is a changeover with three terminals.

Simple two wire thermostats

1.Inset is the interior of a common two-wire heating home thermostat used to regulate a gas heater via an electric gas valve. Similar mechanisms are used to control oil furnaces, boilers, boiler zone valves, electric attic fans, electric stoves, electric baseboard heaters, and household appliances such as refrigerators, coffee makers, and hair dryers.Power through the thermostat is provided by the heating equipment, which may range from millivolts to 240 volts in common North American constructions, for direct (electric baseboard heaters and some electric furnaces) or indirect (all gas, oil, and forced hot water systems ).Because of the wide variety of  voltages and currents available with thermostats, care must be taken when selecting a replacement device.

Set point control lever. Move it to the right for higher temperatures.The circular indicator needle in the center of the second slot shows through the numbered slots in the case.The bimetal sheet is wound into a coil.The center of the coil is connected to the swivel post which is connected to the lever. As the coil cools, the moving end carrying moves clockwise.Cord the left side is connected to the heater control valve by one of a pair of wires.The moving contact is attached to the bimetallic coil.From there, go to the controller for the heater.Fix the contact screw.This is adjusted by the manufacturer. It is electrically connected by the second of a pair of wires to the thermocouple and the heater's electric gas valve.Magnets.This ensures good contact when the contacts are closed. It also provides hysteresis to prevent short heating cycles, as the temperature must rise a few degrees before the contacts open. As an alternative, some thermostats use a mercury switch at the end of the bimetallic coil.The mercury weight at 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 because it is highly toxic and permanently toxic if damaged. When replacing these thermostats, they must be treated as chemical waste.Not shown in the illustration is a separate bimetal thermometer on the outer case to show the actual temperature at the thermostat.

Millivolt thermostats

As shown in the use of the thermostat above, all power to the control system is provided by a thermopile, which is a combination of many stacked thermocouples, heated by a pilot light.The thermopile generates enough electricity to drive a low power gas valve which is under the control of one or more thermostat switches which in turn control the fuel input to the burner.This type of appliance is generally considered obsolete because the pilot light wastes a lot of gas (like a dripping faucet wastes a lot of water over a long period of time) and is no longer used on stoves, but in many gas water heaters and Gas fireplaces can still be found.Their low efficiency is acceptable in water heaters, since most of the energy "wasted" on the pilot still represents a direct heat gain to the tank.Millivolt systems also eliminate the need for special circuitry for water heaters or furnaces; these systems are usually completely self-sufficient and do not require any external power source to operate.For tankless "on-demand" water heaters, pilot ignition is preferable because it ignites faster than hot surfaces and is more reliable than spark ignition.Some programmable thermostats those that offer a simple "millivolt" or "2-wire" mode - will control these systems.