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. 
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You should either plug that or fill it with water - it's venting sewer gasses when it's dry.– EcnerwalCommented Sep 20, 2020 at 15:58
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Hello, and welcome to Home Improvement. If an answer is helpful, please click the large check mark next to it to accept. And, thanks for taking our tour before posting; few newbies do.– Daniel GriscomCommented Sep 20, 2020 at 16:53
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@Ecnerwal It looks like the trap is full in that picture - the drain must have been used recently.– J...Commented Sep 21, 2020 at 17:38
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1There 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 BoothCommented Sep 22, 2020 at 11:24
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1@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...Commented Sep 22, 2020 at 11:30
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2 Answers
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.
answered Sep 20, 2020 at 15:45
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1As 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. Commented Sep 20, 2020 at 23:27
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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)– EcnerwalCommented Sep 21, 2020 at 0:55
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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...Commented Sep 21, 2020 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.
