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Community and Q&A

Water heater & washer emergency drains = holes in the house

user-5946022 | Posted in General Questions on

In my area the plumbing code requires a washer sit in a plastic pan, and that the pan have a drain draining direct to the outside.  The pan drain cannot drain to the sewer system under the theory that if the washer overflows it is because the drain is clogged, so you need to drain the overflow elsewhere.

They require similar for a tank water heater – sits in a pan, and the pan has a direct drain to the outside.  Note this is NOT the T&P valve drain on the water heater, it is the pan drain.

Seems like it would be simple, although unsightly, to install a small trap at the exterior and then manually keep the traps primed to prevent air leaks.  (Presumably no water will ever go through these drains unless there is an overflow, so nothing to auto prime the traps).  Short of that, is there a backflow preventor that can be used for this situation?  It would need to be weak enough to allow water with only some head pressure to flow out, and strong enough to resist air going through it.

While both of these are small 1″ drains, this creates two 1″ holes in the house.  I have never read of this issue on GBA.  What are people doing to prevent these two drains from being total airleaks in an otherwise tight house?

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  1. this_page_left_blank | | #1

    You can install a trap primer. You connect a half inch pex line to just above the trap level, and connect that to the outlet of the primer. The inlet of the primer is connected to the cold line of a regularly used faucet. Each time you run the faucet, a small amount of water goes to the trap.

  2. Expert Member
    BILL WICHERS | | #2

    Fill the trap with mineral oil and don’t worry about it — mineral oil won’t evaporate and if you ever need the drain to flow the mineral oil can flow out. Mineral oil is compatible with PVC pipe. You can also put just a thin layer of mineral oil over the water in the trap to limit or stop evaporation of the water. No need to do anything fancy.


  3. this_page_left_blank | | #3

    I forgot about the oil trick. For a rarely used drain, this makes sense. For a drain hooked up to the septic/sewer, it's usually against code, but for a direct drain to the outside it shouldn't be an issue.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      I don't think I've ever seen a code interdiction against filling the trap with something other than water. If freezing is an issue food grade anti-freeze works too.

      1. this_page_left_blank | | #5

        There's no code to prevent doing that, but there are cases where there is a code requiring some method to keep the trap filled, and filling (or covering over the water in) it won't meet the requirement.

        1. user-5946022 | | #6

          All - thanks for the mineral oil tip.
          Trevor - there is no code requirement for a trap on this drain as it is not connected to the sewer, so there are no sewer gases to trap. The pipes just go from the pans under the wash machine & water heater directly outside. I would be adding a trap for the express purpose of keeping these pipes from being holes in my otherwise tight house - the liquid in the trap will be the air barrier. It occurs to me it will also serve as a crude pressure sensor. If I can figure out a way to do this with clear pipe, it will give me an indication if my house is under positive or negative pressure. I think I'll experiment with that a bit first before adding the mineral oil. If I put a port at the top of the trap it will be easy to refill with water.

          1. Expert Member
            BILL WICHERS | | #7

            You can get clear pvc pipe and it will glue to regular pvc pipe with the usual solvents. You can get small quantities from us plastics in Lima Ohio. It’s not cheap though.

            Mark the outside of the clear pipe with a sharpie marker in whatever units you like (inches, millimeters) with a sharpie marker and how you have a manometer. Put some dye in the mineral oil, maybe just some food coloring (I’ve never tried this) so you can see it. Mineral oil isn’t the same weight as water — it’s lighter, which is why it floats on water — so you won’t be able to directly read out in “inches of water”, you’ll need to convert for the lighter weight of the mineral oil. Aside from that, you should be able to use it as a pressure gauge without any trouble.


          2. Expert Member
            MALCOLM TAYLOR | | #8

            That's a great idea. Do you have somewhere you an locate the trap that is easily accessible? Mine is buried outside the house connected to the perimeter drains.

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