Kitchen sink slow to drain

23 Nov 2009
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United Kingdom

My kitchen sink is slow to drain, and when the washing machine empties it starts to fill up the sink. I have replaced the trap underneath the sink, and tried some basic drain cleaner and all of that made no difference. It also makes a fair bit of gurgling noise when it has finally finished draining.

The bathroom toilet and sink and bath all drain no problems.

Looking outside, the toilet waste seems to feed into the main drain pipe which is vented at the top of the roof. The bathroom sink and bath seem to feed into the down gutter pipe. All the houses in my row are set up like this.

The kitchen sink waste pipe, after passing through the trap, heads horizontal for a bit (30cm), then bends straight down into the floor. Just after the bend, an old waste stand pipe from the old washing machine joins (my washing machine just joins under the sink above the trap). This old waste stand pipe is currently capped off, but if you remove the cap the sink drains twice as quickly, but you still get a bit of gurgling noise.

Anyone got any ideas as to what’s wrong?

Should the kitchen sink be connected to a vent pipe (see a lot of comments about this on US sites)? So should I fit an air admittance valve or something? Can that be fitted under the kitchen sink or does it need to go outside

Why does taking the cap off the old waste pipe speed things up? Is this providing air when the cap is taken off?

Is there just a blockage in the drain? If the toilet and bathroom drain fine then is it unlikely to be something in the main drain?
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if the old washing machine stand pipe has a seal of water in it it dosent need a cap on it so take it off ;)
I would think you have a partial blockage somewhere in your sink waste pipe. The gurgling noise is suction, because the pipe is running full bore where the restriction is, then as the sink finishes draining its causing a vacuum in the pipework.

Waste pipes can be connected to a stack, or discharge into a gulley, depends on whichever is most convenient. Taking the cap off the old washing machine pipe is probably letting air OUT, this will give more space for the water to fill as the sink empties, before it slowly drains away. Id leave cap on personally otherwise the water may escape from it if the blockage gets worse.

Ideally you need a plumbers snake, Screwfix do this:
These are designed to be pushed down waste pipes to clear bloakages. It doesnt sound from your description that the waste goes into either a gully or stack, so likely joins the underground drain direct. Remove the trap, give the pipe a good clean with a snake, and hopefully this will solve the issue.

Unlikely to be the main drain if as you say, there is no problem with toilets or bathroom upstairs. Lift manhole covers outside to double check, but my guess is the sink waste has become choked with grease, washing powder or other detritus.
Thanks, a try out the snake on the drain pipe, I think there is a blockage down the somewhere as when the sink is backed up with water, you can here a gentle trickle of water somewhere near when the pipe is going below the foor.
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if the old washing machine stand pipe has a seal of water in it it dosent need a cap on it so take it off ;)
If you leave the cap off, with no water flowing into it to replenish the seal, you would soon be prompted to replace the cap!
Its not blocked, the cap you've put on your washing machine is likely the problem. You need some sort of vent/air intake for the pipework for it to drain.

If the stand pipe has a self made seal under floor made up of bends then I'd leave the cap off..
If it starts to let out sewage smell then it doesn't have a water seal & you should perhaps fit as you say an air admittance valve. I'm unsure if you could fit it where you currently have the cap on & not get a stink coming through.. only way to know is to try. If it does stink divert that pipe outside & up a wall & stick it on outside

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