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I want to install a backup drain for a bathroom where there might be overflow onto the floor. The problem is how to seal that drain against sewer gas. The usual P-trap cannot be used because the drain will ordinarily never be used. It is just there in case of an unusual overflow condition, like someone flooding a sink for example.

How can I seal the drain from sewer gas?

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The ordinary P trap is required to be used, as I'm quite sure that you live in an area covered by ordinary plumbing codes.

A Trap Filler AKA Trap Primer and/or a floor drain seal may also be required to protect the trap seal.

https://codes.iccsafe.org/content/IPC2018/chapter-10-traps,-interceptors-and-separators#IPC2018_Ch10_Sec1002.4

IPC 1002.4 Other codes will be similar.

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  • I was asking for solutions not legal opinions. If I wanted a legal opinion I would post to law.stackexchange.com. – Tyler Durden Sep 27 '20 at 16:53
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    ...and the solutions are there, in the code and in this answer. Fell free to gas yourself with some idiotic home-brew illegal approach rather than using the standard viable approaches that already exist for exactly this problem. – Ecnerwal Sep 27 '20 at 16:58
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    @TylerDurden -- a trap primer is the typical solution to the problem. "Oiling the trap" can work to prevent evaporation, but I'm sure has some drawbacks as well – ThreePhaseEel Sep 27 '20 at 19:51
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Normal p trap filled with mineral oil

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    Is it OK (and legal) to pour mineral oil down the sanitary drain? – Daniel Griscom Sep 27 '20 at 20:38
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    It is both legal and ok to pour mineral oil or vegetable oil down a drain, yes. Is it to code to build a floor drain without a trap primer such that mineral oil is the only lasting solution? No, but there there are tons of these around and it is a common solution – redlude97 Sep 27 '20 at 20:53
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Plumb one of the washing fixtures into the floor drain. (like a shower or a hand basin)

Then it will see periodic use, and not dry out.

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