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We just had our old gas water heater replaced with a tankless unit, so the vent for the old tank is now unused. I'd like to install exhaust fans in our 2 small bathrooms, and ideally use the old vent instead of cutting a new hole.

My plan is to use 4" insulated flexible duct to run from the first fan, have the second fan wye into that same duct (with a back draft damper if needed), and then run the rest of the way to the 4" exhaust from the old water heater. Like so:

Will this setup work, or will I need different/bigger ducts? An alternative could be to use the 8" vent that used to connect to the furnace (which is now in the crawlspace) that's in between Fan 1 and Fan 2, but that seems like it may require more powerful fans.

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    Why do you think that a bigger vent requires a more powerful fan? That makes no sense. The vent has to be big enough to accommodate the air that the fan is pushing out - but if it's larger than it needs to be, I don't see what problems that could cause.
    – Mike Baranczak
    Commented Feb 13, 2020 at 15:58
  • Bigger vent = more static pressure, so a lower power fan won't move the air as much as it should
    – iddings
    Commented Feb 13, 2020 at 16:09
  • fan1 will push a lot of air through fan2 ... fan2 will push some air through fan1
    – jsotola
    Commented Feb 13, 2020 at 18:11
  • @iddings -- that, if anything, is backwards. Bigger ducts have less static pressure drop than smaller ducts...the air moves slower in a bigger duct, true, but low velocity in a residential airway is about the last of your concerns
    – ThreePhaseEel
    Commented Feb 14, 2020 at 0:12
  • @ThreePhaseEel would you mind explaining that a little further? I'm certainly no expert on this, but it would seem to me that a bigger duct would mean a larger air column that needs to be moved (perhaps static pressure isn't the appropriate term here?)
    – iddings
    Commented Feb 14, 2020 at 18:57

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I would use the old gas vent with the existing exhaust fans without concern. The only issue I could see if one fan was on pressurizing the duct the second one might have trouble opening its back draft damper because of the pressure (both fans should have back draft dampers). I would check to verify that both will open when the other is running. If anything I would think a smaller fan would be needed to reduce the back pressure, but make sure it works and use it if it works.

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  • Yes, you shouldn't join vent ducts like that for the reason Ed mentioned. I doesn't meet code even if you do get it working. Any lint buildup (bathrooms have significant tissue lint) could render it inoperable later.
    – isherwood
    Commented Feb 13, 2020 at 16:21
  • You mean the old water heater vent? I was planning to use two 80 CFM fans (the bathrooms are both quite small, about 20 and 40 sq ft. Would these fans be too much? Edit: Could I run one out the old water heater vent, and one out the old furnace vent then? Or am I better off just cutting holes in the gables?
    – iddings
    Commented Feb 13, 2020 at 16:23
  • I would use both vents. with large fans that would eliminate any issues with one holding a second one closed. No need for any fan size increase on the larger vent pipe.
    – Ed Beal
    Commented Feb 13, 2020 at 16:36

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