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Thread: Drain line in exterior wall?

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  1. #1
    In the Trades mtcummins's Avatar
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    Default Drain line in exterior wall?

    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. #2
    DIY Senior Member dlarrivee's Avatar
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    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. #3
    In the Trades mtcummins's Avatar
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    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. #4
    Plumbing Contractor for 49 years johnjh2o1's Avatar
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    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. #5
    DIY Senior Member dlarrivee's Avatar
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    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. #6
    In the Trades Gary Swart's Avatar
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    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.

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