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How does a gas-furnace works...

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How a gas-fired unit works...

The typical residential gas heating system burns either natural or liquified petroleum gas.
The heating system switches on when a thermostat senses the room is a lower temperature, than what it was set at.
In the new units, upon sensing the room temperature is below what the thermostat is set at, the nducer fan pulls air through your unit, in order to establish combustion airflow. When the airflow is adequate, the gas valve opens to allow gas to flow through the burners where a spark or the heat from the ignitor sparks the gas. (An older unit ignites the gas with a standing pilot, which is a flame that burns continuously, in order to light the gas.)
Upon sensing the unit sense the flame is lite, the spark or ignitor shuts off. The gas burns for about 2 minutes, before the blower starts. By consequence of the blower starting after a couple of minutes, cold air does not blow through the vents, while it is heating up.
After a set time or a set temperature, the blower motor gets energized and it blows air over the heat exchanger. The air flowing through is heated and flows into the duct system.
When the thermostat senses the pre set temperature, the gas valves de-energizes and the gas shuts off. The blower motor will usually run for a few more minutes, in order to cool the heat exchanger. This process is controlled by a timer or a temperature switch.