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How a Thermostat Works

How Does a Thermostat Work?

A thermostat's basic function is to regulate the operation of heating and cooling equipment in your home. You set the thermostat to the desired temperature and the thermostat turns the equipment on and off to maintain that temperature. A thermostat is essentially an automatic switch. The thermostat senses the temperature and turns the air conditioner or heater on and off as required to maintain the desired temperature.

There are two basic types of thermostats: electromechanical and electronic. They both do the same thing but they do it in different ways. Additionally, of the two basic types, there are several varieties from which to choose.

An electromechanical thermostat typically uses a bi-metal coil or strip which moves as it expands and contracts with temperature changes. Mounted on the bi-metal strip is a glass vial partially filled with mercury. As the vial is tilted on the back on the bi-metal strip, the mercury flows to the end of the vial where two electrical contacts are exposed. When the mercury envelopes those electrical contacts, a low voltage circuit is made, thanks to the fact that mercury is conductive. This works just like a light switch and makes a circuit to turn on the heater or A/C.

An electronic thermostat operates in much the same way as an electromechanical model except that instead of the moving bi-metal strip and mercury switch, it uses a sensor to detect temperature levels. When the temperature differs from the preselected temperature the thermostat electronically makes the circuit to the heater or A/C.

Advanced features include programmable thermostats where different temperatures can be set for a certain time of day or the day of the week. A setback feature allows you to set the thermostat to a preferred temperature and then a different temperature for hours of the day when you are away and don't need to heat or cool the house to your preferred temperature. Another feature called a heat anticipator is used for certain types of heating equipment that continue to generate heat even after they are powered off. This feature shuts off the heater in advance of reaching the preset preferred temperature.

While most home thermostats use only low-voltage, the power should be shut off to the HVAC equipment at the circuit breaker when work is down on the thermostat. Accidental shorting of wires may not injure you but it may damage valuable equipment.

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