I was wondering if just replacing those sharp angled elbows (see picture below) will solve the slow drain on my toilet sink.

I've already take them apart and they are clean and clear, nothing blocking the water flow.

Those are a inch pipes I believe - ~115mm of circumference.

enter image description here enter image description here

  • That is a code violation for a plumbing drain configuration. What you have is called an "S" trap. That by itself will not cause a slow draining flow. What type of drain opening do you have in the sink? Provide a picture of the drain in the sink. Is the drain flow always slow or slows down after flowing good for a while? Dec 2, 2021 at 4:22
  • I'm in Australia, this S trap seems to be pretty standard here. I've uploaded a picture of the drain in the sink. The flow is always slow, except if I pump it down using the palm of my hand.
    – Zedzdead
    Dec 2, 2021 at 6:11
  • It could be gunk further down the pipe. Have you tried leaving a bottle of drain cleaner down it overnight?
    – Tetsujin
    Dec 2, 2021 at 8:01
  • I did before. A couple of times for 30m and another time overnight because I forgot. No difference at all. I think it has to be a combination of that sharp angle plus perhaps the trap being to close to the sink drain. Looking at the other two sinks I got here, the S trap seems to be placed further down.
    – Zedzdead
    Dec 2, 2021 at 9:33
  • Ok i think this guy has the answer youtube.com/watch?v=Mka2w1SwpDk
    – Zedzdead
    Dec 2, 2021 at 10:33

2 Answers 2


As mentioned in comments, the issue is the air getting trapped inside the pipes. I made 6 small holes in the top part of the S trap to prove this theory and worked like a charm. I'll now replace the upper elbows with a T join or whatever people call that and add a air valve on the top end.

enter image description here CREDIT: ILLUSTRATION BY DAVE BRANDON


  • While this may work for you, it seems rather unconventional. Do you have proper venting for this particular sink? Fixing that would likely be a better way of going about it.
    – FreeMan
    Dec 3, 2021 at 12:44
  • Not sure how to proper vent it. Can you share some resources?
    – Zedzdead
    Dec 5, 2021 at 22:37
  • Doesn't this defeat the whole purpose of having a trap? Or does your air valve only allow air in and not out?
    – Glen Yates
    May 6, 2022 at 15:22
  • 1
    Using the AAV (Air Admittance Valve) is quite standard practice in New Zealand (and therefore probably in Australia too). There is a description here. And according to the above article, they are permitted in many parts of USA as well. Jan 1, 2023 at 11:23

There are a few problems here.

  1. Even though the drain pipes below are clear you can often get hair and sludge clogs up just under the stopper. Did you check there?

  2. The design is all wrong. You have an S trap rather than a P trap. Not sure where you are but S traps are against code in the US. The problem with an S trap is that when the water is draining down the waste pipe it will suction out the water in the trap allowing sewer gasses to enter the home.

So why is my drain slow?

  1. The other problem with the design is that the seal depth should not be more than four inches. Your's looks to be more. With that much depth the water entering the trap may be slow to drain especially if you aren't emptying an entire sinkful of water.

The diagram below compares an S trap to a P trap and how to construct a proper P trap.

enter image description here

  • 1. I've just check and I noticed no blockage at all when pouring water from a bottle down the pipe after disconnecting the last elbow piece. 2. I'm Australia, this kind of trap seems to be pretty standard here. 3. That makes sense, I'll check that. Thanks!
    – Zedzdead
    Dec 2, 2021 at 6:23
  • Also, S-traps aren't code violations in existing installations. It's odd... I've got 2 (maybe 3?) in my house and have lived here for 30 years and have never had either siphon dry. They do get regular use, so maybe that keeps them wet, but the implication is that they' siphon dry during use. Guess I'm just good that way... :D
    – FreeMan
    Dec 2, 2021 at 13:35
  • Properly configured S traps may be code legal. My LAHJ strikes that section from the UPC, but essentially the down-pipe is bigger than the trap and that maintains venting. Without going and looking, I think it's referred to as "One-pipe" and there are various rules, but that size differential is the big one. Does not appear to apply to the questioner's S-trap, though.
    – Ecnerwal
    Dec 2, 2021 at 14:25

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