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Should a Gas Furnace Flue Pipe Go Through the Roof or the Side of the House?

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Jerry Kelly Furnace Interior

Have you ever walked around your neighborhood and noticed that some people have flue pipes extending out of the roof while others have them coming out of the side of the home? The type of furnace flue pipe that you have is not something that’s up to personal preference, but rather a result of the efficiency of your system. Today we’re going to help you understand the difference between the two styles of furnace venting by answering the question, “Should a gas furnace flue pipe go through the roof or the side of the house?”

Why does your furnace need a venting system?

When your furnace burns fuel, it produces combustion gases as a byproduct. These gases would be very dangerous if they got into your home’s air circulation and your family breathed them in, which is why furnaces have venting systems that are used to carry combustion gases out of your home.

High efficiency furnaces produce different combustion gases than standard efficiency furnaces

With standard-efficiency furnaces, the byproducts of the combustion process contain a certain amount of heat and are released from the furnace in a gaseous form.

High efficiency gas furnaces take advantage of the fact that combustion gases contain heat, and they send those gases to a second heat exchanger. This heat exchanger extracts extra heat from the combustion gases that would otherwise be wasted. This allows your furnace to get more heat out of the gas that it consumes. One side effect of removing that extra heat from combustion gases is that the combustion byproducts are cooled down and released in a liquid form in high efficiency furnaces.

Should my furnace flue pipe go through the roof or the side of the house?

So which type of venting system should your furnace have? That depends on whether you have a standard efficiency or a high efficiency furnace.

If your furnace has an AFUE rating below 90 percent, it will most likely have a flue pipe that goes up through your roof. That’s because the combustion byproducts are in a gaseous form, so the gases can float up through the flue pipe and out of your roof.

If your furnace has an AFUE rating of 90 percent or above, it will have a flue pipe that goes out of the side of your house. That’s because the combustion byproducts are in a liquid form, so they need to be drained out of your home in a similar way that the condensation from your air conditioner is drained outside.

If you have any questions about whether your furnace flue pipe should go through the roof or the side of the house, or if you’d like a furnace installed in your St. Louis area home, contact Jerry Kelly, your St. Louis furnace repair and installation contractor. We service the St. Louis area, including towns like Ballwin, Chesterfield, Clarkson Valley, MO.

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