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2 Jul 2017
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How Does a Gas Furnace Work?

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Posted By Michael B.

A furnace is a device designed to heat a home or other building. A gas furnace is one that burns gaseous fuel such as natural gas or propane. If your home has a gas furnace, you may be curious about how it actually works. Understanding its parts and how they work together can help you ensure that your unit is operating efficiently, and may even help you perform small repairs yourself rather than spending big bucks on calling in the pros.

The Heating Cycle Begins The process starts with the thermostat that is inside your home. The thermostat senses the temperature inside the room. When the thermostat senses that the temperature dips below the point at which you have it set, it kicks the system into gear. On more modern units, this means that an inducer fan then pulls air through the unit until adequate combustion airflow is established. After an appropriate level of airflow has been reached, the gas valve opens and allows gas to pass through. The gas then flows through the burners and is lit up by either a spark or the heat from an ignitor. If the unit in your home is an older one, the unit is likely to have a small flame that burns continuously, otherwise known as a standing pilot, to light the gas.

Your Home Gets Heat After the furnace senses that the thermostat has triggered the flame and that it is properly lit, the actual spark (or ignitor) is turned off. If your unit is older and contains a standing pilot, it will not be turned off. At this point, the gas will typically burn for at least two minutes before the blower starts to disperse heat throughout your home. The reason for this is to give the air an adequate period of time to warm up and so that cold air wont be pushed through the vents into the rooms in your house at the start. After either the preset time (roughly two minutes) or pre-established temperature is reached, the blowers motor is turned on and it blows air over the heat exchanger. At the heat exchanger, the air is heated and then blown through the duct system into the various rooms via vents in the floor, walls or ceiling.

The Heat Goes Off You may have noticed that just as the heat in your home turns on when a certain temperature is reached, it also turns off after the rooms are warm enough. This is also because of the way the thermostat works with your gas furnace. Basically, the thermostat again senses the temperature in the room. When the room warms up to the temperature set by you at the thermostat, the gas valve is switched off, stopping the flow of gas. After the gas is turned off, the blower motor will still run for a few minutes. The reason for this is to allow the heat exchanger to cool off a bit. How long the blower motor runs after the gas supply is shut off depends on either a timer or a temperature switch.

Basic Maintenance Whether you have a gas furnace or an oil furnace, some routine maintenance will be necessary in order keep everything running smoothly and to extend the life of your unit. Now that you understand the basics of how a gas furnace works, you can probably handle routine maintenance yourself, with a little more research and some online tutorials. If you feel more comfortable trusting the experts, you can invest in a maintenance contract, and for as little as $100 a year, a dealer will take care of it for you.v

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