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Community and Q&A

Installing ventilation conduit into larger ventilation conduit

ducks4994 | Posted in General Questions on

We used to have a gas water heater (4 inch exhaust conduit) and a gas furnace (5 inch ventilation conduit) that both fed into a 8 inch main ventilation conduit that runs from our basement up to exhaust on the roof on the second floor. The gas furnace was just replaced with a new, high-efficiency furnace that required a new exhaust line to be run. So now, only the gas water heater (4 inch exhaust conduit) runs into the main 8 inch exhaust conduit. I would like to install a new ventilation fan in the basement but avoid punching another hole in the back wall. Is it possible (not just physically but legally) to run a separate, sealed powered vent exhaust line (4 inch?) up through the existing 8 inch main exhaust conduit? (no co-mingling of natural draw water heater exhaust with powered vent exhaust) Thank you.

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  1. Expert Member
    Dana Dorsett | | #1

    Is the 8" flue a clay-lined chimney?

    If yes, it's a conditional "yes", using the chimney as a flue-chase is usually legal, provided vent pipe for the high efficiency furnace becomes stainless steel, not PVC where it will be in contact with (or close proximity to) the water heater's 4" flue liner. Stainless flue piping is on the expensive side, so it may be cheaper to swap out the water heater for a power vented type that can use PVC flues as well.

    With some forethought it might have been better/cheaper overall to replace the hot air furnace with an hydronic air handler running off a condensing water heater (serving both functions with a single burner), but it's probably too late if the new furnace is already being installed.

  2. GBA Editor
    Martin Holladay | | #2

    There are a couple of issues here. The first concerned the "orphaned water heater" venting problem.

    You note that your water heater with a 4-inch flue is now connected to an 8-inch flue. Does the water heater have a continuous stainless-steel flue inside the chimney that extends to the top of the chimney? And if so, does this continuous stainless-steel flue have a diameter of 4 inches or 8 inches?

    For more on the orphaned water heater problem, see When a Flue is Too Big.

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