Many people know little to nothing about how their furnace works. You may know what certain parts are, like the pilot light for example, but how does your furnace use that tiny flame to generate heat for your entire home? In order to gain a better understanding of how this works, you must first understand how each of the main components of the furnace work individually. Knowing how the furnace operates can help you have a better understanding of the repairs it may need when the furnace breaks down.

Main Parts and their Function

The furnace is made up of a lot of different parts that are all necessary for the unit to function properly. However, there are a few main components that are directly involved in the furnace’s creation of heat, and those are the ones you should focus on. We should leave the more technical parts to the professional HVAC contractors.

• Standing Pilot/ Electronic Ignition- Depending on the type of furnace you have, you will either have an electronic ignition or a standing pilot that functions as the unit’s pilot light. Typically, older furnaces have standing pilots, and newer more efficient furnaces have electronic ignitions. But, what is the difference between the two?

1. Standing Pilot- What characterizes this type of pilot is its constant flame. A standing pilot consists of a pilot assembly and a thermocouple. The pilot assembly holds the flame that heats the thermocouple. Once the thermocouple is warmed to a certain temperature it will send a current of electricity to the gas valve. This current triggers the gas valve to open and release gas into the unit.  This type of pilot requires a constant stream of gas to keep the pilot lit and is therefore inefficient.
2. Electronic Ignition- An electronic ignition does not have a constant flame. This pilot will only light up when the thermostat is calling for heat. There are two types of electronic ignitions. The intermittent pilot uses an electrical spark to ignite the pilot, and the pilot will then light the burners. A hot surface ignition system uses a heating element that is similar to the filament in a light bulb, to ignite the burners.

• Flame Sensor- If you have an electronic ignition, chances are you also have a flame sensor. The flame sensor is a safety device that will detect whether or not the unit has fired up properly. If the pilot or burners do not fire up, the flame sensor will tell the control board (the electrical control box that directs furnace function) to shut down the gas valve and the burners. This will prevent any gas leaks in the event of a part failure.
• Burners- These are a plate-like surface that will ignite when the furnace calls for heat. They often glow red when they are hot. This part of the furnace is also known as the combustion chamber. Gas will mix with air here to create combustion that will heat up the burners.
• Draft Inducer Motor- If you observe the function of your furnace, you will notice a small fan-like motor that is the first component of the unit to turn on when heat is being called for. This motor has two functions. First, the inducer motor will bring in air from outside to be mixed with the gas released by the gas valve to create combustion. Its second function involves removing the harmful gases that are generated by the combustion process. The inducer motor will pull these gases through the venting system that should lead to the outdoors.
• Supply and Return Vents- These vents are parts that are associated with the duct work system in the home. Supply vents lead out of the furnace and deliver heat to each room in the home. Return vents lead into the furnace and suck air out of each room to be delivered to the furnace for reheating.
• Heat Exchanger- The heat exchanger is the part of the furnace that warms the air before it is blown into the various rooms in the home.  On one side of the heat exchanger combustion is taking place. The burners transfer the heat made by the combustion through the heat exchanger. On the other side, the air that is being sucked into the furnace by the return vents is flowing across the heat exchanger to be heated. The two sides of heat exchanger create a barrier between the toxic combustion gases and the air that is circulated through the home.
• Blower motor- The blower motor is a big fan that blows the warm air created by the furnace through the duct work and into each of the rooms in your home. This is the part that creates the circulation cycle of air. If you set the fan to “auto” at the thermostat, it will only run when the thermostat calls for heat. If you set the fan to “on,” it will run continuously and circulate air even when heat isn’t being generated. It is more energy efficient and less stressful on the motor to run it on “auto” only.

When the thermostat drops below the set temperature, it will send a signal to the furnace that calls for heat. The inducer motor will kick on and begin drawing air into the combustion chamber. The pilot will light and trigger the gas valve to open. The gas valve will release gas into the combustion chamber, and it will mix with the air to create combustion.  As the inducer motor vents the harmful combustion gases outside, the combustion will cause the burners to ignite. The hot burners will transfer heat to the heat exchanger, and the blower motor will kick on. As air is drawn through the heat exchanger from the return vents, it will be heated and then blown through the duct work and out the supply vents by the blower motor.

This is basically how your furnace creates and delivers heat in your home. So, the next time your unit breaks down, you will have a better understanding of what your serviceman is talking about. You will be able to protect yourself from dishonest contractors, and maybe even be able to do a little trouble shooting on your own.

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