Drain line in exterior wall?

Discussion in 'Plumbing Code Questions' started by mtcummins, Nov 18, 2011.

  1. mtcummins

    mtcummins In the Trades

    Messages:
    380
    Location:
    Pittsburgh PA
    Good evening/weekend all,

    I'm doing some remodeling that requires moving the drain line for a shower. There is almost no good way to do this, so what I'd like to do is kill 2 birds with one drain line and put it into a new furring wall on an exterior double brick wall, and then spray the furring space with spray foam. This way I can route my new pipe to the stack, and get some insulation value in the cold wall of the bathroom below it at the same time.

    The question is, can I run a 2" drain line with the fittings literally touching the brick of a double withe exterior brick wall without concern of freezing? I'm in Pittsburgh, PA, we get fairly cold winters here. I'll spray foam all the way around the pipe, but the side of the drain line will be essentially touching the brick and therefore not really protected by the foam. I have 3" absolute max to fur out, or I'll encroach within 15" off this wall to toilet flange, so there's no room to hold the drain off the wall. I plan to attach 2x3 (2 1/2" actual depth) studs to the brick and leave a channel for the drain line. With the variations in the brick surface, I should be able to just squeeze the fittings into that much space. I have enough vertical room that I can put a decent amount of fall in (an inch/foot shouldn't be a problem), to make sure that water moves through this section of pipe quickly and has no chance of water resting in the pipe. The run against the wall would be about 6-7 feet long, then drop into the 3" stack.

    The vent is fine in its current location, so not worried about it.

    Related to this, I'm assuming that I should use a 2" drain, as the shower has 2 shower heads... is this correct, or would 1.5" be sufficient? Last thing I want is any chance of a slow draining stand up shower, but the extra 1/2" would be helpful if the 2" pipe is not necessary...

    Thanks for your help guys!

    Mike
  2. dlarrivee

    dlarrivee New Member

    Messages:
    1,172
    Location:
    Canada
    Terrible idea.

    Insulation does not create heat.

    Having the pipe touching the brick and then spraying foam around the rest of the pipe will just help it to freeze.

    You need to route this drain elsewhere, there is no work around.
  3. mtcummins

    mtcummins In the Trades

    Messages:
    380
    Location:
    Pittsburgh PA
    Do you think it will get that cold in there? I would never put a water line in a wall like that, but a drain with decent slope, that will always have warm water running down it, seems like it might not be so bad. This surface is currently the wall of one of my bathrooms, and its not like I have the humid air condensing on the wall and freezing, so I'm not sure if the pipe would ever get cold enough to freeze even if it was standing water.

    Your point about putting insulation around the rest of the pipe possibly hurting more than helping is noted and a good point. I could leave the insulation just up to the pipe on both sides and behind it as far as it will go, but not covering the inside wall of it, so some of the room's heat would get to the inside of the pipe through the drywall... might help a little.

    As far as the local climate, most winters we get down into the low 20s with a few dips into the teens. Occasionally we'll have a day that drops below zero, but this is pretty rare and almost never lasts more than a night. Most assuredly, on those days the water doing down the drain will be piping hot :)

    Any other thoughts?
  4. johnjh2o1

    johnjh2o1 Plumbing Contractor for 49 years

    Messages:
    1,142
    Location:
    South*East
    The line won't freeze. Unless the trap is exposed to cold temperatures. If the shower is vented the warm sewer gas moving through the line will prevent it from freezing. If this wasn't true what would prevent the line from the house to a septic tank from freezing? Most of these lines are not buried below the frost line in colder climates.

    John
  5. dlarrivee

    dlarrivee New Member

    Messages:
    1,172
    Location:
    Canada
    A perfectly healthy drain will not freeze. One with a little crud in it might.


    It is just bad practice, you can justify it all you want, but I wouldn't do it in my own home.
  6. Gary Swart

    Gary Swart In the Trades

    Messages:
    7,329
    Location:
    Yakima WA
    Freezing drains is not a problem except perhaps in the Far North with temperature below zero for weeks on end. As already mentioned, don't insulate between the interior wall and pipe as insulation does not heat anything, it only slows heat transfer. An insulated vacuum bottle does not heat your coffee, it will only keep it hot longer than a non insulated container because the insulation slows the heat of the hot coffee from being lost. Insulation will not warm the house or cool it, it just slows heat loss in the winter and slow heat entering the house in the hot summer.
  7. mtcummins

    mtcummins In the Trades

    Messages:
    380
    Location:
    Pittsburgh PA
    Thanks everyone for your input. I tend to think it would be fine in this situation. I imagine that up in Canada it would be more concerning, as the winters are generally much more brutal than here. The extra fall on the line should keep water moving through it fast enough to prevent any crud build-up. If there's a lot of crud in my shower drain line, I have bigger problems to worry about (my personal hygiene) than a potentially freezing drain anyway... gross.

    Even if it did manage to freeze up a tiny bit (not sure how it would, but if it did), the hot water from a shower would melt through it quick. The trap will be 3 feet away from the cold wall, in a heated/insulated floor (my whole house is radiant floor heated), so no chance of the trap ever getting cold. And it is vented properly.

    Any thoughts about 1.5" vs 2" line for this 2 shower head setup? I guess you have a potential load of 5 GPM. I don't remember the fixture units for this setup off hand... would it be a bad idea to run this through a 1.5" drain? I def don't want to skimp there, but if its a perfectly acceptable load on a 1.5", it would make it a lot easier to get it through there, and I could keep the pipe from touching the outside wall.

    Thanks,
    -mc-
  8. dlarrivee

    dlarrivee New Member

    Messages:
    1,172
    Location:
    Canada
    2", if you must.
  9. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,244
    Location:
    Land of Cheese
    2" is the minimum (and common) drain size for a shower in the U.S.A.
  10. mtcummins

    mtcummins In the Trades

    Messages:
    380
    Location:
    Pittsburgh PA
    Hmm, 2" is minimum? My plumber originally plumbed this with 1.5" and it passed inspection (it was only a single shower at that point). Pittsburgh tends to be really over-zealous on code stuff (we have to have 2 traps and 2 vents on a double bowl kitchen sink, for one stupid example), so I'd be surprised if they'd let something like that fly.

    That said, I generally plumb all of my tubs and showers 2" when I can fit the pipe anyway, just to be safe. I'll only put one lav into a 1.5, if there's two I'll bump it up to 2" where they join. Might as well do it a tad overkill if its feasible.

    Ok, so I'll def go with 2" for this. I looked at it again today, and I have a little more than 18" to my toilet flange, so I'll prolly try to get another half inch to an inch of furring in there, and get a good 1/2" - 3/4" gap between the cold wall and the pipe which i can spray foam behind. That will give me between R3.5 and R5 to help keep the cold away from the pipe, and the bathroom heat through the drywall to the uninsulated inside wall of the pipe should def keep this line above freezing no matter how cold it is outside. It will be up near the ceiling where the room is warmest, so I really don't think there will be any problems.

    Thanks all, for your input. Have a great weekend!
  11. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,244
    Location:
    Land of Cheese
    Once past the vent take-off, the pitch can be increased to ensure that the pipe drains quickly as the years go by.
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,896
    Location:
    New England
    Long ago, in the USA, they allowed 1.5" drains for showers, but that hasn't been true for a long time (it's still allowed in Canada from what I hear). It might be grandfathered under certain circumstances, but never for new or tearout remodels. Now, the vent might work at the smaller size, but that depends on the fixture units.
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