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How Does a Furnace Vent Gas? Your Gas Furnace’s Ventilation Process

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A technician working on a furnace tune-up


4576850300_7d49f862c5_mIf you know anything about the fuel-burning process, you might be familiar with the fact that combustion gases like carbon monoxide are released as a byproduct. Given the fact that most furnaces use natural gas as their heat source, you might be wondering what happens to those combustion gases while your furnace is running. Even if you are unaware of the release of combustion gases, it’s good to know what your furnace does with them so you can make the right decisions about maintaining your system and keeping your home safe. Today we’re going to help familiarize you with this important process by answering the question, “How does a furnace vent gas?”


How combustion gases are produced in your furnace


When your furnace turns on, a set of burners are lit and the flames are directed into a metal chamber known as a heat exchanger. Inside the heat exchanger, hot combustion gases like carbon monoxide are produced and the walls of the chamber heat up. While your furnace blows air over the outside walls of the heat exchanger (protecting it from the gases inside), combustion gases build up inside of the chamber.


In order to release the combustion gases that build up inside of the heat exchanger, a venting pipe known as a “flue” is attached to the exchanger. Depending on the efficiency of your system (see below), the gases will be directed out of your home to different places.


How mid-efficiency furnaces vent gas


In a mid-efficiency furnace (below 90 AFUE), the combustion gases that are vented out are in the form of a gaseous vapor. Since hot vapor naturally rises, the flue that’s attached to the heat exchanger points upwards and extends all the way up and out of the roof. So while your furnace is running, combustion gases will rise up toward the flue pipe and be safely vented outside. In some homes, the flue will be attached to the upper part of the chimney.


What is a furnace vent and where does my furnace vent?

A furnace vent is either a flue, draft fan or an external vent that is responsible for removing combustion gases from your furnace. Your furnace vent will expel the gas generated during combustion outside of your home, typically through the roof. Having a furnace vent and knowing where it is located is key to the overall safety of your home. Without a furnace vent, unwanted gas would stay stuck inside your home posing a risk to you and your family. To learn more about furnace vents, contact us today

Does a furnace need to be vented?

Yes, a furnace needs to be vented. Just as you would not want your car’s exhaust in your home, you also do not want your furnace’s exhaust in your home. A furnace vent allows for any gases and exhaust created by combustion to be removed from the home. If your furnace is not vented, it can lead to dangerous conditions, like carbon monoxide building up within your home. To get your furnace inspected, give us a call today

Should my furnace exhaust pipe be hot?

Yes, your furnace exhaust pipe may be hot to the touch. It is normal for the exhaust pipe to be hot closest to your unit, since that air is typically hotter than the temperature in your home. If your furnace’s exhaust pipe was installed properly, then it should not pose a safety risk if the exhaust pipe is hot to the touch, since it is designed in a way to ensure that the pipe does not ignite any surrounding material with its heat. However, if the temperature exceeds 130 degrees, that can indicate there is an issue that needs to be addressed. Call us today to get your furnace inspected. 

How high-efficiency furnaces vent gas

In a high-efficiency furnace (90 AFUE or above), the combustion gases are in a liquid form. That’s because there is a second heat exchanger that extracts more heat out of the combustion gases, which cools them down into a liquid. In these systems, a fan blows the combustion byproducts out of a flue pipe that extends out of the side of your home.

How to ensure your furnace vents gas properly

No matter how your furnace vents combustion gases, it’s important to ensure that the process is working properly. The best way to do that is to schedule a furnace tune-up every year. During your tune-up, we’ll inspect your flue pipes for cracks or blockages and fix any problems we find so that combustion gases don’t accidently get directed back into your home.


If you have any questions about how a furnace vents gas, or if you’d like a heating system serviced or installed in your home, contact Jerry Kelly, your St. Louis furnace installation and repair contractor. We provide service all over the St. Louis area, including towns like Weldon Spring, Wentzville and Winghaven, MO.


photo credit: kightp via photopin (license)

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13:00 24 May 22
Aaron K did a great job. Very professional. Thank you Aaron K.
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16:56 23 May 22
Heath was very professional and did an excellent job. Highly recommend JK!
Kimberly G.
01:51 22 May 22
I had my ac tuneup for the Dpring. Aaron was very professional and thorough. He did great work.

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