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Distance of exhaust fan wall cap from windows and soffit intake vent?

Joseph Moore | Posted in General Questions on

How close can a wall cap for a bathroom exhaust fan be from a soffit intake vent?
I’m trying to efficiently vent a 2nd story bathroom without sending the moist air right back into the attic and I don’t have a lot of options as far as placement is concerned.

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  1. GBA Editor
    Martin Holladay | | #1

    As far as I know, there are no code requirements that govern this issue. My advice: do the best you can, and don't worry.

  2. Joseph Moore | | #2

    That's great. Thanks for the quick reply, Martin.

  3. Expert Member
    Malcolm Taylor | | #3

    You can go some small way to mitigating the problem by using a vent that directs the moist air away from the soffit.

  4. Joseph Moore | | #4

    Thanks for your response Malcolm. And thanks for the link. Those are pretty slick looking wall caps. So would those be advantageous because they point downward and therefore would send the moist air further away from the soffit?

  5. Joseph Moore | | #5

    The funny thing is- since my original post, I'm realizing my biggest challenge here is ducting the wall mount fan. I bought one that is either ceiling or wall mount meaning the duct comes out of the side
    (which would actually be the top with the wall mounted orientation). It happens to be a 3 inch diameter duct which I thought would be helpful since the cavity is only 4 1/2 inch deep. (Old house-long story)
    I thought I'd be able to source out a low profile 3 inch 90 degree elbow to go from the vertical through the wall. Evidently I was wrong. I can only find such an elbow for a 4 inch duct. Seems I'll have to use a 3 to 4 inch reducer to actually make the duct larger just so I can use the low profile elbow that is only made for 4 inch. Seems crazy.
    Has any one else run into this problem?

  6. Expert Member
    Malcolm Taylor | | #6

    Yes, the downward vent helps direct thee moisture so that what does make its way to the attic vents is very diluted.
    Unlike new constructionI realize you have to work with what you've got, but mounting a fan and it's vent pipe in an exterior wall isn't a good idea. Any chance of building a ceiling bulkhead that could hold them both?

  7. Joseph Moore | | #7

    Not really possible. The room is small and the ceiling is already low as it is. Why is the fan and ductwork in the exterior wall not a good idea? Are you thinking drafts?

  8. Expert Member
    Malcolm Taylor | | #8

    The fan housing and vent duct are large enough that they can not be separated from the exterior by sufficient insulation to prevent condensation. Moisture that condenses will drip down the vent into the fan and your bathroom.
    Have you thought of using a though-wall fan instead?

  9. Joseph Moore | | #9

    Through the wall fan was my original idea but it seemed prone to back draft

  10. Expert Member
    Malcolm Taylor | | #10

    You don't end up with two two dampers, as you would with a conventional fan, but how much does the interior one actually do?
    oBh Broan and Panasonic make good quality through-wall models.

  11. Joseph Moore | | #11

    Thanks for the suggestion. I was looking at the through the wall models before I decided on and bought the low profile model. So you're saying there's no way to make the one I have work? Running the ducting down instead of up maybe?

  12. Expert Member
    Malcolm Taylor | | #12

    I'm sure there is a way to Heath-Robinson it, but is it worth the effort? The situation is directly analogous to venting a range hood. If you aren't going directly through the wall out the back, the preferred method is to run the duct up through the cabinet above (in the conditioned space not the wall) and then exit horizontally.

  13. Joseph Moore | | #13

    Ok. I'm reviving this thread to say I returned the low profile model I had bought and am shopping for a through the wall model. There are VERY few to choose from. I only need 70 cfm so that narrows it down to pretty much 2 or 3 models. There is a panasonic that requires 8" ducting which seems a little large and it's expensive and too quiet for my taste.
    There's a Broan that looks to be a bit flimsy but the specs and price are good. I plan on retrofitting any model I get with an an exterior wall cap of my choice as well as an interior grille of my choice.
    Any suggestions? I saw an older thread where Martin mentioned installing through the wall fans without problems of back draft. I'd love to know what models those were. Thanks in advance.

  14. David Landry | | #14

    Actually current Mechanical code does have restrictions on Exhaust terminations:
    M1506.3: Exhaust openings shall terminate not less than 3' from property lines, windows, doors or 10' from any air intake. (vented Soffits)

  15. GBA Editor
    Martin Holladay | | #15

    Thanks for pointing out the requirements of M1506. In the most current version (2012) of the IRC, the relevant section is M1506.2 in Chapter 15. Here is how it is worded:

    "Exhaust openings. Air exhaust openings shall terminate not less than 3 feet (914 mm) from property lines; 3 feet (914 mm) from operable and nonoperable openings into the building and 10 feet (3048 mm) from mechanical air intakes except where the opening is located 3 feet (914 mm) above the air intake."

    The reference to "mechanical air intakes" is not a reference to soffit vents; it is a reference to the air intake of an HRV, ERV, or a central-fan-integrated supply ventilation system.

  16. Joseph Moore | | #16

    Yes. Thanks for the code reference and the clarification regarding the intakes. So how about these through the wall vents? There are so few to choose from. Does anyone have any recommendations? I'm probably going to end up with something like this-

  17. David Landry | | #17

    Thanks for the clarification Martin.
    I wonder whether " non-operable openings into the building" might include soffit vents. It probably isn't a good idea to exhaust moisture laden air from a bathroom directly under an eave with soffit vents without 3' of clearance.

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