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PVC dryer vent?

Planeman | Posted in General Discussion on November 30, 2009 06:28am

Couple of questions regarding a dryer vent in a an existing home.  Background – My daughter and SIL bought a 6 year old house in a short sale deal with the lender, great house, excellent price, 1/2 of what the previous owners owed the bank.  This is in Riverside county, southern CA.  House was bought as is, no warranty, no negotiations.  So far no significant issues have surfaced, previous owners left the home in immaculate condition.

On doing a walk thru I noticed that the dryer vent in the laundry room was 3 inch PVC pipe with a run of about 20 feet straight up and thru the roof.  I was not aware PVC was an approved material for a gas dryer vent.  But it has been in place apparently since the house was new and seems to be functioning fine.  Is PVC code approved in other areas for a dryer vent? Just curious since most I have seen are metal.

Second question. The dryer has a 4 inch exhaust that immediatly gets reduced to 3 inches to go into the 3 inch PVC vent pipe.  In my simple mind this is a loss of 25% of the flow volume.  Will this affect the dryer performance?

Any help is appreciated.

 

Experienced, but still dangerous!
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Replies

  1. edwardh1 | Nov 30, 2009 07:08pm | #1

    loss of a LOT MORE than 25%

  2. MikeHennessy | Nov 30, 2009 07:28pm | #2

    "In my simple mind this is a loss of 25% of the flow volume."

    To be more precise, 56% less flow, since the area of the cross section of a pipe is a function of the *square* of the radius.

    That said, dunno the answer, but the fact that it heads straight up may help a bit with the draft. But all bets are off if the thing gets clogged.

    Mike Hennessy
    Pittsburgh, PA
    Everything fits, until you put glue on it.

    1. Planeman | Nov 30, 2009 11:59pm | #13

      Thanks for the math help Mike, that is a huge reduction.

       Experienced, but still dangerous!

    2. Piffin | Dec 01, 2009 12:24am | #17

      mathermatically that is about right as a measure of the cross section, BUTCompare actual airflow in smooth straight PVC 3" to the friction loss in a snarly snaking flex metal urrugated exhaust line that catches half the lint running thru it and the amt of air coming out the far end is close to same 

       

      Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

  3. User avater
    Sphere | Nov 30, 2009 07:34pm | #3

    I'd swap it out. Metal downspout in a pinch.

    Spheramid Enterprises Architectural Woodworks

    Repairs, Remodeling, Restorations

    PROUD MEMBER OF THE " I ROCKED WITH REZ" CLUB

     

  4. ChipO | Nov 30, 2009 07:58pm | #4

    When we first used the dryer that came with our house, it left us with a dryerful of warm damp clothes. Some investigation revealed that the vent pipe necked down from 4" to 3", and that's where it was completely clogged. Replaced it with 4" flex vent (it's only a ~4' run) and all has been well since.

    1. Scott | Nov 30, 2009 08:29pm | #5

      >>>Replaced it with 4" flex vent (it's only a ~4' run) and all has been well since.That's ALUMINUM flex, not the plastic stuff, right?

      1. ChipO | Nov 30, 2009 08:58pm | #6

        Correct. :-)

  5. User avater
    popawheelie | Nov 30, 2009 11:00pm | #7

    It should be 4" and metal.

    Do yourself a favor when you instal the new one and have it so you can clean it if you have to.

    Since it is a straight shot it wouldn't be that hard.

    I've cleaned them with a brush made for the job that you  pull/push through the pipe.

    But you would need access from both ends.

     

    "There are three kinds of men: The one that learns by reading, the few who learn by observation and the rest of them have to pee on the electric fence for themselves."
    Will Rogers
  6. DanH | Nov 30, 2009 11:03pm | #8

    Necking down to 3" straight up is bad and good. Very good chance of clogging at the neck-down point (hopefully cleaning out that section is easy), reduced drying efficiency (due to the loss of over half your clear area), but increased velocity, reducing somewhat the likelihood of clogging on the vertical run (which is a likely problem on a 20-foot vertical).

    Dunno about the PVC issue with regard to fire hazard -- pretty sure it wouldn't be legal for commercial use, but maybe OK for residential.

    This country will not be a permanently good place for any of us to live in unless we make it a reasonably good place for all of us to live in.  --Theodore Roosevelt
    1. Planeman | Nov 30, 2009 11:50pm | #11

      Thanks to all for the input and advice.  I think I will advise them to swap it out for metal.  The neckdown from 4" to 3" occures right at the dryer exhaust so cleaning is not a big deal.  Wish they could go 4" but it's a standard 2 x 4 interior wall so unless it was an oblong shaped 4" pipe, there is no room.  The laundry room is at the center of the house so the only way out is straight up.

      Thanks again, great advice,Experienced, but still dangerous!

      1. DanH | Nov 30, 2009 11:53pm | #12

        There is oval duct made for that, or you can just smash 4" round "rigid" duct to fit.
        This country will not be a permanently good place for any of us to live in unless we make it a reasonably good place for all of us to live in.  --Theodore Roosevelt

      2. JTC1 | Dec 01, 2009 12:03am | #15

        What DanH said.

        Could be a challenge without having the wall open though......

        I was amazed to see that the electric dryer in my rental house allowed 90' of 4" rigid vent.  Then they supplied a table which gave a formula for calculating the vent run - a 90* elbow = 25', a louvered exhaust vent = 15', etc. still had plenty of permissible run left.

        JimNever underestimate the value of a sharp pencil or good light.

        1. Piffin | Dec 02, 2009 12:49am | #26

          most are good for 28-40 feet is all, and each elbow restricts flow 15% ROT 

           

          Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          1. JTC1 | Dec 02, 2009 03:44pm | #27

            ...28'-40'...elbow 15%...

            I understand the norm - just like the dryer in my house.

            I just read and follow the installation instructions / manuals when it comes to appliances......

            My rental house must have the Tim Taylor, turbo-charged, "more power" model!

            Or, the manufacturer may have manipulated the numbers a bit in their "vent run calculation worksheet" to make things look "better" - 90' total run when using rigid metal pipe - but then they specify the "footage allowance" for a single elbow as 25'.

            JimNever underestimate the value of a sharp pencil or good light.

      3. danski0224 | Dec 01, 2009 03:57pm | #22

        I'm pretty sure I recall seeing something in the IRC that prohibits PVC venting for dryers.

        4" metal pipe will fit in a 3.5" wall.

        Cut the top and bottom plates about 4.5" or so and stick the dryer vent piping in the wall. Securely tape pipe joints with UL listed aluminum foil tape (no screws/rivets). Do NOT use duct tape- it does not last. Top and bottom plates will need to be firestopped with approved materials.

        Install drywall on one side. When the drywall is installed on the other side, it will oval the pipe.

        Smarter thing is to have a 2 x 6 mechanical wall, but that adds so much cost and takes away so many square feet...

        1. marv | Dec 01, 2009 04:52pm | #23

          I think PVC is not allowed because of static electricity build up. 

          Dust collection in workshop has same problem.  If metal is not used, you need to run a copper ground through all the lines.You get out of life what you put into it......minus taxes.

          Marv

          1. User avater
            BillHartmann | Dec 02, 2009 05:46pm | #28

            Static is not a problem for a dryer or a average workshop. Too low a flow to generate large amounts of static. And the stuff in the pipe is well below explosive levels.And in the dryer much of the time the moisture level is way too high anyway.Now in an industrial setting with say a widebelt sander running condintously then you can get enough fine dust in the airflow for it to be explosive..
            William the Geezer, the sequel to Billy the Kid - Shoe

        2. Planeman | Dec 01, 2009 06:55pm | #24

          Good plan, I will get the SIL to do the retrofit to a 4" metal pipe.  I am amazed at the numbers and the calulations shared here.  Lots of good stuff.

           Experienced, but still dangerous!

          1. User avater
            SamT | Dec 01, 2009 09:38pm | #25

            All the numbers in the flow reduction calculations above in re transitioning from 4" to 3" assume a constant pressure source.

            Since the dryer fan attempts to be a constant volume source, pressure will increase due to the transition and the flow volume will not decrease as much as the above calcs show.

            You'll need a better flow analysist then me and a few more numbers than just pipe sizes to know what the actual reduction will be.

            I suspect it will be in the order of magnitude of 10%.SamTA Pragmatic Classical Liberal, aka Libertarian.

            I'm always right! Except when I'm not.

  7. JTC1 | Nov 30, 2009 11:33pm | #9

    >>Is PVC code approved in other areas for a dryer vent?<<

    Not in my area, can't speak for other areas.  Here I need 4" rigid metal - aluminum or galvanized inside of a wall or ceiling. No screws at joints, metal tape only for joining sections. In wall suspension becomes a bit of a challenge - generally a steel U or Z-shaped bracket screwed to a stud and the vent pipe hose-clamped to the bracket.

    Plastic flex is OK if exposed or accessible (like in an attic) - aluminum flex is much preferred and seems to work better (I have Al in my own house). Aluminum flex produces a better flow if not expanded.

    Total run of flex is limited to 8' I think - this allows a removable section of vent at the dryer end and roof jack end to allow for easy periodic cleaning.

    >>The dryer has a 4 inch exhaust that immediatly gets reduced to 3 inches to go into the 3 inch PVC vent pipe.  In my simple mind this is a loss of 25% of the flow volume.  Will this affect the dryer performance?<<

    It reduces flow by way more than 25%; 56% just on a strict pipe area basis, probably more in reality.

    No, it is not good for performance.

    Jim

    Never underestimate the value of a sharp pencil or good light.
    1. Bing187 | Nov 30, 2009 11:47pm | #10

      I guess that depends, as you said, on locality...

      My neck of the woods, plastic has been outlawed for at least ten years. Flex, that is. Never seen PVC used for that, good question.

      Having said that, vinyl flex is garbage, and it shouldn't be used whether exposed or not. If you've ever handled a piece that's been connected to a dryer for any length of time, it gets brittle and falls apart. It also burns readily...Been to enough dryer fires to reccomend against it, tho most of those are a result of poor maintenance and cleaning.

      Hard metal=best

      Flex alum=2nd

      Vinyl= don't

      I can't quote exact 3's as to the 3" reduction, but the poster that said it is more than 25% is correct. I would hazard to guess that the dryer recommendation is not to do that....20 ft is probly the max run with no turns with 4"; with 3" it's probly a no-no.

      Bing                                                      

       

      Edited 11/30/2009 3:48 pm ET by Bing187

      1. User avater
        Sphere | Dec 01, 2009 12:03am | #14

        area=7.5" for 3" and 12.5" for 4"Spheramid Enterprises Architectural Woodworks

        Repairs, Remodeling, Restorations

        PROUD MEMBER OF THE " I ROCKED WITH REZ" CLUB

         

        1. Bing187 | Dec 01, 2009 04:35am | #18

          You're readier with the math than I am...:)

          I was basing what I said on the #'s that I know as it relates to water flow....You can flow 2x the water (roughly) through 4" supply line as 3"....And 2x the water through 5" as 4"....Figured it would work the same with air...

          Bing

          1. User avater
            Sphere | Dec 01, 2009 04:36am | #19

            I got an Iphone, it did it, not me..LOLSpheramid Enterprises Architectural Woodworks

            Repairs, Remodeling, Restorations

            PROUD MEMBER OF THE " I ROCKED WITH REZ" CLUB

             

      2. JTC1 | Dec 01, 2009 12:07am | #16

        >>Hard metal=best

        Flex alum=2nd

        Vinyl= don't<<

        I would agree with that list, but vinyl flex is still legal here.

        RE: length of run ..... see my post .16

        JimNever underestimate the value of a sharp pencil or good light.

        1. Bing187 | Dec 01, 2009 04:45am | #20

          I'm amazed at those #'s!!

          I remember vividly the last dryer I vented on a house with a "long" run...Dryer was in a small closet off the kitchen, in the middle of a section of the house...Closest point to run the duct to outside was about 12 feet away, thru the basement, but I needed 2 90s. The instructions with the dryer were "No more than 25', deduct 5' per 90 degree turn..." I don't doubt what you're quoting, but I wonder why the huge difference? Can the dryer you put in have a way more powerful source blowing the air?

          My setup "calculated " at 22' but I remember that it just made it....I was at a structure fire about a year ago at an apartment complex where they had run flex vinyl from a laundry room in the middle of the building, up the wall, and then it was buried, one duct per joist bay, behind plastered ceiling for a 30' run to outside....Got clogged with lint, eventually started the floor joists on fire, gave us something to do...:) Again, general cause was total lack of maint, but I was amazed at the length of the run, total around 40'........Just seems to me that a dryer would have an awful hard time maintaining good flow 40' (never mind 90') away.....Interesting stuff....

          Bing

          1. JTC1 | Dec 01, 2009 05:56am | #21

            I was surprised at the 90' number also.

            Bought a retirement home (for the future), and was getting it ready to rent it in the interim.

            Previous owners did some things which were not quite proper in the house. Electrical "oddities" mostly......

            Among their odd installations was venting the dryer from a first floor laundry room directly into the unimproved basement. Well, they did put on one of those Rube Goldberg "Direct Vent Lint Traps", so I guess it wasn't direct......:^)

            One thing they did do well was retain all of the owner's manuals and installation instructions for all of the appliances.

            I was able to make a pipe run across the basement ceiling to a wall and vent the dryer out through the sidewall. Vent run info was handy!

            Turned dryer on after installing - plenty of airflow there with a clean pipe! My pipe run is about 20' actual, about 70' by the manufacturer's formulas for bends, etc.

            In service for a year now, will peek at the pipe cleanliness next time I am down there. 

            Jim  Never underestimate the value of a sharp pencil or good light.

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