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My furnace exhaust pipe vents out through the roof. How do I prevent ice accumulation from blocking the exhaust pipe? I have a high efficency furnace.

We replaced our gas furnace with a high efficency one this fall. Our furnace is located in the center of the house. Both the exhaust and intake vents are run up through the roof. They are PVC pipes. Previously, we have had issues wiht the intake pipe freezing at the roof line, but never had issues with the exhaust until this winter.

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  • @UnhandledExcepSean Our gas furnace was replaced with a high efficiency one about 15 years ago, with exhaust going straight up through roof, as furnace is in a "landlocked" closet 15 feet from the nearest exterior wall. Fresh air comes from vent to crawlspace underneath, and our California location almost never freezes.
    – Armand
    Dec 27, 2022 at 18:30
  • Could you post more details about the path of the vent to outside? Tubing material (insulated or not), does it go through heated or unheated areas or both? Where is ice forming the blockage? At roof vent? Inside pipe? How did you discover the problem?
    – Armand
    Dec 27, 2022 at 18:34
  • Does this answer your question? How do I prevent ice stalagmites from building up on the furnace exhaust? It includes a list of possible issues to check.
    – Armand
    Dec 27, 2022 at 18:36
  • @Armand, solutions would be different on the roof.
    – isherwood
    Dec 27, 2022 at 19:27
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    Lori, please provide a better description of your situation. Revise to tell us vent type, size, shape, etc. We're taking wild guesses here.
    – isherwood
    Dec 27, 2022 at 19:28

2 Answers 2

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One strategy is to extend the exhaust pipe with a reducer to a larger pipe. This way, if ice accumulates, there's enough diameter to prevent problems.

I'm guessing that you have a 3" PVC vent through the roof deck. Install a 4" to 3" reducer and a 12" length of 4" pipe. You wouldn't even need to glue it on, and I probably wouldn't anyway until you verify that it solves your issue.

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If the furnace exhaust vent is a very low to the roof surface type it may very well have been incorrectly installed in the first place. Having this type on your roof is directly implied by your question regarding ice buildup on the roof.

For a more conventional furnace a through the roof furnace exhaust vent system is typical should have several key features as follows:

  • A metal boot assembly that isolates any combustible building materials from the hot exhaust pipe.
  • An extended stand pipe to raise the exit point of the hot exhaust exit well away from the nearby roof surface.
  • A cap that is designed to prevent wind downdraft and rain an snow intrusion into the exhaust stack.

A high efficiency furnace uses a feature of a concentric piping system where the hot exhaust passes through the center pipe and prewarms the intake air in the outer pipe. Some other type of heat exchanger system may also be used.

The exhaust is also cooled so that at the exit the exhaust is composed of primarily water vapor and carbon dioxide. These components become acidic and must be properly managed. Because of the acidic nature of the exhaust the exit piping used is typically PVC and run through the sidewall of the house. Venting through the roof can be problematic and cause damage to roofing, especially metal parts.

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