I found this drain in my basement in the middle of a raised rectangle of concrete. Its supposed to be for a shower the previous owner installed. I have never seen this drain type. Looking for answers for what kind of drain it is and why it would have been installed. enter image description here

  • You should either plug that or fill it with water - it's venting sewer gasses when it's dry. – Ecnerwal Sep 20 '20 at 15:58
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  • @Ecnerwal It looks like the trap is full in that picture - the drain must have been used recently. – J... Sep 21 '20 at 17:38
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    There is no water line on the removed part @J... so even if it isn't completely dry, the water level is still too low to stop sewer gasses venting through the air gap above the water. – Mark Booth Sep 22 '20 at 11:24
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    @MarkBooth I wonder if we're seeing the same picture. I see a bell trap filled nearly to the level of the central drain hole and a cap with wet sides lifted out of the way. I'm not sure what the rest of you are seeing. – J... Sep 22 '20 at 11:30

That is a bone stock "bell-trap" floor drain.

They are illegal for interior use under the IPC (at least) since the trap weir depth is perhaps 1/2 an inch or so; well below the minimum 2" weir depth required, in any case - though in fact the exclusion is for ANY bell-trap, not specific to the depth of the trap, but in this case that is typically about what you'll have. 1002.3, (Prohibited traps) item number 2 (Bell traps)

They can be used for exterior or garage drains (not connected to the sewer) only, as I recall. I just removed one and "de-trapped" another (cut the inner ring seal so it just acts as a collector) and installed an actual P trap (adjusted to 4" weir depth) on its line. Fun chopping holes in concrete, yes indeed.

It probably was installed for a shower. They look like a perfectly reasonable, easy to clean option and the large box store will happily sell them without telling you that they are illegal for interior applications.

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    As to why it was installed, it's likely because of it's minimal depth, meaning that there was less need to excavate, and possibly the drain line could have been placed higher. – Hot Licks Sep 20 '20 at 23:27
  • In fact, a proper P-Trap floor drain (with the trap and a cleanout built in) gets the drain line higher than these do unless you double-trap them (also illegal under IPC and most [all?] other codes) – Ecnerwal Sep 21 '20 at 0:55
  • The reason that they're illegal is that the shallow weir dries up quickly (in as little as a few days) and the trap fails when that happens. A proper p-trap takes 4-6 months to dry out, by contrast. If OP has such a trap and it's connected to the sewer and they don't want to remove it right away, it would be important to at least throw a bit of water into it every few days to keep the trap sealed. – J... Sep 21 '20 at 17:31

Looks to me like some sort of ultra-low-profile trap. When the top section is in place, the channel will fill up and overflow into the central drain. Water will remain in the channel, to stop nasty niffs.

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