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I have an existing bathroom with sunken shower area 30" x 72". I will install a drop-in bath into this area (procedure: four brick walls, tile on the open wall, plaster base to support).

There is a floor drain in the existing area, which is essentially a 3" hole which drains into a 1.5" pipe. I think the installer misunderstood '3" floor drain" as meaning the exit hole, not the input hole, as they have connected it directly to a 3" pipe, not a 1.5" pipe

enter image description here

That 3" pipe is punched through the slab and goes immediately into the actual, unvented, 2" waste pipe

enter image description here

The floor drain looks like this one:

enter image description here

There is a related question here, where it's argued that the drain cover should not have holes in the middle

What type of floor drain is this and can I connect pipes to it? and here: How to connect bathtub drain pipe to existing floor drain?

This is not the case for most floor drains in use here in Indonesia.

There is another kind of trap here: Is this shower trap better than a P-Trap?

It is not that either. It's similar to this except 1.5", not 2" pipe:

enter image description here

So questions basically:

  1. Is it ok for me to dump my bath water over it, while continuing to direct water towards it from the rest of the bathroom
  2. it seems that it's a slightly questionable design in terms of sewer gases etc. Should I fit a 2" PVC P-trap in place of the current 2" elbow below the slab?
  3. my bath comes with plastic (p/s-trap): enter image description here

If I am to dump my water into the floor drain, then I guess the bath's trap isn't really doing anything and I can leave it out if convenient?

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Answering your bullet points...

  1. I don't see any reason you can't drain a bath into the floor drain. You'll want to make sure the drain hose stays in place so the water doesn't spill all over the floor, but this should work.

  2. Not having a trap is a problem, and I'm not sure why anyone would have installed it this way unless there is a trap just beyond the pictured area, or if there was a trap mechanism fitted to the top. You ask if you should add a trap where the elbow is as if this is a trivial task, but that seems like a very major job. You do need a trap, but I would make sure there isn't one already that you can't see.

  3. Yes, if you drain into a floor drain you are adding an implicit air gap, and you don't have to worry about gasses so the trap on the hose is not needed.

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  • there is no competent plumbing here, so it's to be expected that anything is done badly. You cannot even buy a p-trap in the local hardware stores (you have to order online). I'm not sure if the floor drain itself is considered to be a trap of some kind, but perhaps not. There definitely would not be a separate trap.... I don't think it's too major a job to refit the p-trap - essentially just saw open the elbow and melt the glue, then re-fit the trap
    – thelawnet
    Commented Nov 2, 2021 at 20:28
  • @thelawnet PVC cement (assuming that's what was used, which may be an incorrect assumption) does not respond well to attempts to "melt the glue". PVC cement works by slightly dissolving the PVC on both the pipe and fitting. Once the solvent has evaporated, the PVC reforms into a solid. Essentially, you now have one piece of PVC, not two pieces glued together.
    – FreeMan
    Commented Nov 3, 2021 at 15:38
  • @FreeMan, that's true, but fortunately when people are doing sloppy work they rarely use enough glue on these larger diameter connections and you can chip pieces out of the fitting hubs and reuse them. There is some luck involved tho, and its not as easy as just pointing a heat gun at it.
    – JPhi1618
    Commented Nov 3, 2021 at 15:48

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