If 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.
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 gaseous vapor. Since hot vapor naturally rises, the flue that’s attached to the heat exchanger points upwards and extends up and out of the roof. So while your furnace is running, combustion gases will rise toward the flue pipe and be safely vented outside. In some homes, the flue will be attached to the upper part of the chimney.

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 accidentally 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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