Slow draining basin confusion.

19 Sep 2010
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United Kingdom
We had a new bathroom fitted a couple of years ago. The bath and toilet are fine, however the basin drained very very slowly.

I cured this by adding an antivac trap, however the sink is draining slowly again. It takes about 5 minutes to drain 2" of water. I've replaced the anti-vac valve, but it hasn't helped.

Since then I've worked out that if I push the anti-vac valve up a little, air comes rushing out of the system, sometimes with a little water and the basin drains at a normal speed.

Any idea what this means? The bath drains normally still.
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This sounds like one of the problems which comes to the forum now and again which defies understanding! I can remember a couple or so in the last couple of years.
It seems likely that there's a blockage down the pipe somewhere, making a length of pipe between that and your trap with air in, and somehow that stops the trap emptying. You'd think the water would just run down the side of the air in the pipe, but it doesn't.
It's always when the waste pipe is a shallowish angle, but of course it's usually not practical to change it. Have a look to see if there could be any sagging or horizontal pipe where stuff could accumulate.
How long's the pipe, and what's the diameter, and the fall?

If an anti-vac trap doesn't fix it, all I can suggest is putting one of the "industrial strength" drain cleaners down which will dissolve any hair or other gunk. You'll get something from a plumbers' merchant, probably not a supermarket. You need the Sulphuric acid (nasty) one, not Spirits of Salts (Hydrochloric acid).
always worth a first try, is a large volume of boiling water from saucepans - it softens goo.
The fall is quite small, however I don't think this would be simple to change. I will investigate though. The pipe is 32mm.

I hadn't considered the possibility of sagging. I'll take a good look at that, I think it's possible to follow the pipework under the bath.

I did use a flexible drain unblocker and found no blockage, so fall and sagging are the most likely. I have the in laws around today, but will report back once I've had chance to fully investigate.

Thanks for the tips.
Do you have anyone that washes there hair in the basin.
Mine got clogged with hair.
Removed it with a length of wire, made small hook at bottom, put it down the plug hole & twisted it to wind the hair.
Just an idea.
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We had a slow draining basin in a vanity unit. After some years, I accidentally blocked it completely with bits of frame sealing foam but luckily managed to clear it with a plunger. The plug of foam etc cleared the pipe and now it drains well. :)

My girls wash their hair over the bath, but somehow it is my job to pull the hair wad out of the plug hole. :( However, this is mostly near the trap.

There are bacterial drain cleaners that supposedly digest blockages. There are chemical drain cleaners too.

I've seen reference to limits on the length of 1¼" (32mm) drain pipes. Longer lengths should be 1½".

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