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Common Questions About Heating, Ventilation and Cooling
How do thermostats work?
In general, thermostats work by turning on a heating and/or cooling system in response to the temperature rising or failing below a certain target temperature and turning the system off once the target setting has been restored. The target temperature setting is arbitrary, as you program the thermostat to your liking.
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A thermostat works with the help of a pretty simple triggering device. In the average home thermostat, a coil is used as a reactor to temperature changes. This coil contains two different kinds of metal strips. As the temperature changes, the two metals expand (in different ways) causing their coiled shape to either wind up tighter or unwind. Setting the temperature on this type of thermostat changes the amount of tension of the coil.
As the coil changes shape, it affects another part of the thermostat: the mercury switch. This switch consists of a tiny glass bottle that contains mercury and two electrical contacts at one end. As the coil expands and contracts it changes the tilt of the glass bottle causing the mercury to flow from one end to the other. When the bottle is tilted enough the mercury connects the two leads together and allows an electrical current to flow between the two contacts indicating that the temperature in the room temperature is lower than the thermostat's setting. This current is used by the thermostat to turn the heating/cooling system on and off.
Newer digital thermostats have moved away from mercury switches and now use a thermistor. A thermistor is a resistor whose resistance changes with temperature and therefore the amount of electrical current that flows through it. The thermostat monitors that current passing through the thermistor to determine the temperature. The thermostat compares the reading to its current setting to control whether the heating and/or cooling system should be running.
This off-and-on process doesn’t happen instantly. It takes time for the warm or cold air from the heating/cooling system to travel to the thermostat. It also takes time for the thermostat itself to react to temperature changes in the air that it receives. This process is best recognized when a house gets too warm or too cold, even though the temperature reading on the thermostat box hasn’t changed. This is what’s known as an oscillation in temperature. The heat anticipator on a thermostat controls the amount of oscillations, and its setting can be changed if necessary.