Dryer venting question

Discussion in 'HVAC Heating & Cooling' started by f_armer, Mar 24, 2010.

  1. f_armer

    f_armer New Member

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    In the process of remodeling our basement, I have moved our washer and dryer to a different location. I want to enclose the vent within the wall. I used 4" galvanized pipe and I used the foil tape on the seams with no screws.
    My question is, I added 1" styrofoam to the inside of the basement walls, before I framed the wall (2x4). The dryer vent inside the wall will be in direct contact with this foam from top to bottom. Is there a potential fire risk with this setup? I was also planning on putting R12 fiberglass insulation in the cavity also before drywall.
    Also, as it comes out the top of the wall, it then runs between open floor joists, then exits outside.
    It seems like a fairly hot pipe to be against styrofoam.....
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    I don't think it will be a problem unless the total run exceeds the manufacturer's specifications. The max run requires a calculation based on how long each leg is, plus a value for each turn. Also, the appliance manual will list any precautions about location and contact. You won't know unless you read it, but I don't think you'll have problems. You may want to blow any loose lint out of the vent on a periodic basis, though.
  3. f_armer

    f_armer New Member

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    My vent goes vertical 7.5' and horizontal 10'. There are two 90's including the one at the dryer. According to some charts, I am probably at max recommended length. If you add 5' for every 90. I would be at 27.5' Most info I can find says 25'. It is a 15 yr old dryer and I can't find the original manual.
  4. nukeman

    nukeman Nuclear Engineer

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  5. f_armer

    f_armer New Member

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    I was thinking of using one of those, but your still making a 90, it is just a longer arc. Wouldn't it still count?
  6. krow

    krow Plumber

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    For a 4" round duct. As a general rule Most dryers, unless specified, are allowed 12' run of pipe. Each elbow/90 will be considered 1'. The very first 90 at the dryer is generally not counted. If you are going a longer length, you may end up increasing the size of pipe to 6" diameter

    The thing that you need to be concerned of, is the hidden piping in the walls. Once lint starts to accumulate in the ducting, you may not have access to remove it.. The lint may cause the dryer not dry your cloths as efficient . (depending on how closed off the opening is)
    Last edited: Mar 24, 2010
  7. nukeman

    nukeman Nuclear Engineer

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    Location:
    VA
    The transition duct (flex) is not counted on the effective length.

    As a reference point, here is what the Viginia Mechanical Code says (see Section 504):

    https://www2.iccsafe.org/states/Virginia/Mechanical/Mech_Frameset.html

    They allow 35ft with 5ft counted per 90* bend. I've seen more codes that state the limit is 25ft (with 5ft counted per bend). Your best bet is to keep the effective run as short as possible. Additional infor can be found in section 15 of the Virginia Residential Code:

    https://www2.iccsafe.org/states/Virginia/Residential/Res-Frameset.html

    Also, if you are under 2009 IRC, you need to put a label near the duct that states what the effective length is (something like this: http://shop.dryerbox.com/s.nl/it.A/id.253/.f).

    I am putting in one of those dryer boxes. The one in the link I posted is really heavy duty metal. The model for a 2x6 wall takes 4" round duct (you have to oval it a bit to put through the hole). The 2x4 wall model uses 4" oval duct. Around here, Lowe's carries a plastic version of this box (for 2x4 walls, 4" oval duct).

    For the clearance issue, I don't see anything in the codes that I am under that require clearance to combusibles for a residential clothes dryer (there is for commercial, though). You may want to check you local code to confirm. If you are using round duct, you might be able to go to oval instead and gain some clearance.

    I would try to get some space if possible. The dryer vents don't get super hot normally. The moisture does a lot to control the temperature. I would estimate the normal duct temperature to be under ~200F. However, if the vents clogged with lint and the dryer ran longer than it should, the temperature could increase by a fair amount. If there was a lint fire and you had some clearance, the fire may be contained. With the metal in direct contact with the foam, I would worry about it igniting. Since it would be behind drywall, you might not know this was going on until it was too late.
  8. f_armer

    f_armer New Member

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    Good info. The dryer vent doesn't seem to get abnormally hot, it is all new pipe and working fine. I am just concerned about enclosing it all in the wall and insulating around it. Is it asking for trouble? I thought about taking some metal flashing and running it in between the vent and the styrofoam (I don't have much room to work with) would the flashing help protect the sytrofoam?
    Maybe I'm fussing over nothing.
  9. nukeman

    nukeman Nuclear Engineer

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    Location:
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    It will probably be fine. As an engineer, I always think about what *might* happen and try to prevent it. In my case, I built a 2x6 wall and spaced the duct away from combustibles, just to be safe. I only have a short run up and then across the wall. My effective length is only about 8'.

    I don't think flashing will buy you anything unless you can get a gap in there. If the vent got hot enough to start a fire, the flashing would also get hot enough to do the same if they are in direct contact. Since there is nothing in the code about clearance for a residential dryer vent (that I can find), I think you will be safe. The codes are writen to protect dangerous situations and there would be some wording in there if it did pose a real issue. Perhaps someone with more building experience will chime in. You can also try talking to one of the local inspectors and see what they say about it.
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    I think the only issue you might have is if you get a buildup of lint, and then that catches fire. Annual cleaning, at least until you see how much has accumulated (and more would if it is on the long side slowing the flow down), is prudent for any dryer vent.
  11. f_armer

    f_armer New Member

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    Now I'm thinking just to be safe I should cut a channel in the styrofoam, say 6-8" wide, then the pipe will only be against concrete. It is onthe basement wall that the attatched garage is on, so its actually kind of a warm wall. I would still put fiberglass insulation around it.
    I think I would sleep better doing it that way.....
  12. Hube

    Hube New Member

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    Location:
    Ontario
    f_ armer; Now you are on the right track.Just make sure the foam is approx 3" away from the pipe and you should be ok. Good luck.
  13. Dana

    Dana In the trades

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    The rated max operating temp of EPS & XPS is a bit over 100C/212F. It's melting temp is a bit higher, ~240C/465F. Unless your clothes are actually on fire and drafting well it's unlikely you'll never see those temps at the exit point of a dryer vent.

    The flash-kindling temp of EPS is ~340C/645F. That means your basement is already on fire- dial 911.

    ANSI Z21.5.1 specifies high-limit safety switches on modern gas dryers to allow exit temps no higher than 250F, which will only occur in a fault-condition (blocked air flow from a never-cleaned lint screen being the most common.) If the dryer is operating normally it'll never see anything hotter than ~160F exit temps on high-heat. That's uncomfortably hot for a bare hand on bare metal, but not much of a fire hazard. There are aftermarket flex-hose dryer vents made with crummier plastic than polystyrene.

    The most you need for code-compliance on EPS to limit flame spread is 1/2" of gypsum as a thermal barrier against open flame. If you cut a 5-6"" circle with a stub of wider vent concentric around the 4" dryer vent, packing the space with just about ANYTHING with a high melting & kindling temp (like say, fireplace mortar) you'd be more than good- you'd be OBSESSIVELY good. 3" of air-gap, would be a far inferior thermal barrier to the foam, for a dryer vent that is literally already on fire. Stuffing the gap with 3" fiberglass would not be as good, since that doesn't meet code for a thermal barrier. But WTF? If your dryer VENT is on fire methinks you're already long since toast!

    It's all a tempest in a teapot, unless you have a gas-fired dryer that you're expecting to malfunction in multiple ways. But if you'd sleep better at night, a section concentric vent (or even 4" B-vent) gap-packed with mortar would keep the foam from melting even if your house burns to the ground around it.
  14. nukeman

    nukeman Nuclear Engineer

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    Location:
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    According to Owens Corning, it looks like 165F is the max service temperature for XPS. The melting point is listed at 220F on the MSDS and a flash point of >615F. In one of their spec sheets, it says that it should not come in contact with chimneys, heater vents, steam pipes or other surfaces where temperatures exceed 150F.

    You probably would still be okay, but you could use B-vent (or similar) or remove a section of the XPS so it isn't in direct contact. You could add something like Roxul in that cavity to improve thermal performance and have something that can easily handle the heat.

    You may have EPS, though, since you called it styrofoam. What color is it?

    In my case, I have no XPS on this wall as is it an interior wall. I may add Roxul for sound control, though.
    Last edited: Apr 1, 2010
  15. Dana

    Dana In the trades

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    My bad- I had mis-read the melting point of EPS as 240C, but upon re-reading (after noting nukeman's XPS spec for melting point) it's actually 240F, in which case it's remotely possible to melt it if in direct contact. The 250F safety cutout is at the dryer though, and it would only get that hot at the dryer if the vent were restricting flow, in which case the temp at the at the wall would be much lower. (But since the stuff shrinks as it melts, it would tend to create it's own clearance long before it became a fire hazard, eh? :) )

    Operational temps of polyisocyanurate are much higher, but only highly water-vapor permeable fiber-faced iso should be used on foundations (not the foil or poly faced, which would raise the moisture content of the concrete and rot out the foundation sill.)
  16. hj

    hj Moderator & Master Plumber Staff Member

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    If you are really worried, just slide a piece of sheetrock between the vent and the insulation. Even 3/8" plaster lath, if you can find it will work.
  17. f_armer

    f_armer New Member

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    21
    The styrofoam is blue DOW cladmate XL, I don't know if its XPS or EPS.
    I cut a strip out along the pipe, I'm happy with it because since I had 2x4 framed walls and a 4" pipe, by cutting it out it also gave me extra room so I didn't have to squish the pipe to drywall over it.
    I can't see it being any type of hazard now.
  18. nukeman

    nukeman Nuclear Engineer

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    711
    Location:
    VA
    The blue stuff and the pink stuff are both XPS. White panels (looks like a styrofoam coffee cup) are EPS.

    Sounds like you did the best option given what you had to work with.
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