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Dryer vent routing – up or down?

wwilme | Posted in General Discussion on June 1, 2004 10:53am

We are finishing the former attic of a Cape Cod and have placed the laundry closet between two new dormers.  Since the back of the closet is a knee wall,  I can’t vent directly to the outside in the conventional manor.  The question is where to send the dryer exhaust?  The void behind the knee wall falls over a “set in” porch on the first floor.  This would easily allow the exhaust to be sent down and through the porch ceiling if that is “legal.”  I have not seen a “hood” that is meant to exit vertically through a ceiling or overhang instead of horizontally through a wall.  Would the condensation from the exhaust leave a big puddle on the concrete porch floor below?   Above is the 9:12 pitch roof between the dormers.  I could route the duct vertically (about 5′) and exit the exhaust through the roof.  I would have to find a special boot for the 4″ round duct similar to what is use for plumbing vent pipes.  How far above the roof would I have to go?   I am assuming that I would have to find some sort of goose neck “hood” to keep the rain out.  What to do??

Any experts out there dealt with this dilemma before?

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  1. butch | Jun 01, 2004 11:30pm | #1

    If it were me I'd go through the roof using galv. 4" pipe all the way. When you

    terminate above the roof use two vent elbows to create a shepards hook.  You

    need to keep the end high enough to clear  any snow accumultions you might have

    for your area. I don't think I'd want  a dryer venting into a porch, and you didn't

    specify gas or electric, because you legally couldn't vent  under a soffit(ceiling) w/gas

    dryer.  One other thing, I would use the least amount of elbows as they add 

    something like 5' per elbow to your vent run and the shorter the vent run the better

    1. SRDC | Jun 02, 2004 03:12am | #2

      I agree completely with running through the roof. Less piping, less hassle of running down then out through a porch ceiling, and easy to put a boot over and seal nicely. I am also always leary of creating condensation on the outside of the piping if it goes anywhere but up. I suppose it would not be so much of an issue as it would with heat ducting. I am more familiar with condensation problems that relate to air conditioning rather than I am with heating in general, but since heat rises it makes more sense to me to go up with exhaust.

      I would recommend screening the end of the pipe if it comes out to a non enclosed outdoor space. I had a problem with wasps getting in a basement window this year that a dryer exhaust goes through. The house is a rental, and the problem was not the vent itself but satellite cable that was drilled in through the frame.

      The glass panel provided a nice green house for the wasps to live in during cold part of early spring. We had a couple of days that were warm enough to thaw a bunch of them out and they got into the house. The little buggers got into the basement and went into the outside wall of the bathroom. I can't decide if they were coming in through the exhaust fan opening from the outside through the basement or coming in through the bathroom exhaust duct, but I found a couple of cavities in the walls in the basement that don't have fire blocking. The problem is that there is a large piece of ductwork that would have to be moved to install the fire blocking. Anyway they were getting into my bathroom I assume in search of a water supply.

      It was an interesting two week period of once a day spraying the exhaust fan panel in the bathroom with Raid, waiting 5 minutes, removing the panel, collecting the dead wasps out of the tub, spraying up and down the wall cavity through holes in the fan housing, and then spraying outside into the bathroom vent opening. After spraying one day I actually watched one crawl through the frame from the bottom side inside the box area of the wall. Kinda funny now, but was really annoying at the time. Ayway I have gone on a huge tangent, the whole exhaust thing made me think of the wasps.

  2. User avater
    JDRHI | Jun 02, 2004 04:57am | #3

    How long a run would it be to a gable end wall, if you were to run it behind the knee wall?

    If it falls within the manufacturers recomended maximum, it might be worth it. I`m big on as few roof penetrations as possible. The porch ceiling is a definite no-no.

    J. D. Reynolds

    Home Improvements

    "DO IT RIGHT, DO IT ONCE"

    1. wwilme | Jun 02, 2004 05:21am | #4

      Thanks for the advice folks.  The dryer is an electric.  Can't go to the nearest gable b/c there are rooms in each of the adjacent dormers blocking the run.  Looks as though it will have to go up.  What about converting to PVC as I exit the roof - that way I could use a typical rubber boot like for a plumbing vent??

  3. DanH | Jun 02, 2004 05:47pm | #5

    Generally a vertical (upward) run is not a good idea because lint collects in the pipe and the falls back down, clogging it. But if you can keep the upward run short then it might be OK. (Also, insulate the duct where it runs through the attic, so condensation won't form so easily inside during winter months.) My preference would be down, though, if it doesn't require too many turns. (Though why can't you vent horizontally through the roof?)

    In any event, be sure that the vent piping is accessible for cleaning (especially at elbows).

    Be aware that if you vent through the roof lint will collect on the roof near the vent opening.

    1. wwilme | Jun 02, 2004 06:48pm | #6

      Ok, been doing a little surfing and found lots of products and ideas out there.  Looks as though "Defloct-o" makes both roof and soffit vent caps with back draft flappers, both of which would fit the bill. 

      The "Undereve Vent" kit would require (from the laundry closet) a 6' run of 4" hard pipe (slightly pitched down to the eves) to the included semi-rigid flex fitting to the "Undereve-Vent" mounted to the underside of the soffit.   The vent itself is made of white plastic and would be fairly unoffensive if mounted on the back of the house.  Since my vent location is near the front door, I think I'll go with the roof vent.

      The "Deflect-o" roof vent will require (from the laundry closet) a 90 degree elbow to a 4" hard pipe extending about 3' up to a piece of semi rigid flex to the roof vent itself.  The one piece roof vent has a wide flange so it is easy to flash,  is black and fairly low profile, so it won't really be noticeable. 

      Another cool product I found was the the "Dryerbox."  It's recessed box that fits between two studs behind the dryer, creating a space that allows the flex hose from the back of the dryer to be hooked up without getting kinked.  It also allows the dryer to be pushed all the back against the wall.  The box is hooked to the hard exhaust duct leading to outside.  Where the flex duct from the dryer hooks to the hard exhaust duct in the "Dryerbox" makes a perfect cleanout point.

      1. JimBaird | Jun 02, 2004 10:58pm | #7

        Looks like you could find a way to get it out the dormer end or side wall.

        I agree that up thru roof doesn't sound too good.  Int. Res. Code restricts length of dryer smooth wall metal to 25' max, with 5' subtracted for each 90 deg. and 2.5' subtracted for each 45 deg.

        If you stick it out the porch ceiling anybody on porch will get a head full of lint.

        Those box terminals are a sure way to fill your attic with moisture and lint, plus give somebody a great chore (lint removal) to remember periodically.

        Some dryer manufacturers actually claim their equipment will blow exhaust up to 40'.  Otherwise all I've seen the codebooks say about it is if you can't meet the length restriction to size up your duct.

        JB

        Building Inspector

        Madison County, GA

        1. wwilme | Jun 03, 2004 03:53pm | #8

          It's definatley going need to be out through the roof.  A very short vertical run.  The box I was talking about is not a terminal point.  Check it out at http://www.dryerbox.com

          1. Kyle | Jun 08, 2004 04:16am | #11

            That nice long radius curve will sure make a dryer run nice. However, I have to put my dryer closer to the wall then that. When I do I am still creating a 90 degree bend to the vent. JFMA

  4. akatheharv | Jun 07, 2004 07:20am | #9

    Down is better; shorter is better, lest amount of turns is better, not blowing in your face priceless. Send it down the wall on the porch and let it out below the knee.  Make sure you all ways clean you lint after every drying.  Cleaning will stop there from having heavy lint on your floor.  Will keep it fine and let it blow away.

  5. kostello | Jun 08, 2004 12:42am | #10

    i read all the posts and it then struck me that you could consider getting a condensing dryer so you won't need a vent and no hole in the roof seems much simpler to me!!????

    eg: http://www.ebuild.com/guide/products/productDetail.asp?catCode=1560&prodID=303494&pageNum=0&recNum=1&sortCode=5&E_-1=3107

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