Slow drain in bathroom sink - possible airlock?

[KLye]

Hi, hoping someone can help as I've reached the end of my knowledge with this particular issue. Essentially I've got a slow draining bathroom sink. It hasn't always been like this and occurred suddenly. The installation is relatively new - only 4 years ago and the property is on the top floor of a 3-story purpose built apartment block.

I've had a fair amount of chemical down the drain to try and unclog it and I've had the trap off and cleaned enough I could drink out of it! I've even had a 15 foot plumbers scope down the drain to try and clear the blockage - which takes it into the main vertical drain.

Interestingly, with the trap off, I'm able to pour water directly down the pipe without issue. However, with the trap back on and when draining from the sink, the water backs up almost immediately suggesting it's only really getting as far as the trap. This suggests an airlock of sorts but I'm really at a loss as I can't think what else the problem could be.

Any suggestions gratefully received

 

[chr15]

If the waste outlet on your basin is clear then the obvious answer would be a faulty trap, it may be the type with an auto air vent fitted, you may be able to repair it, if not just replace it like for like.

 

[KLye]

If the waste outlet on your basin is clear then the obvious answer would be a faulty trap, it may be the type with an auto air vent fitted, you may be able to repair it, if not just replace it like for like.

Hi Chris - thanks for the tip.

I don't think this is some sort of auto-vent trap. It looks fairly normal to me without any gubbins or such. Is there something in particular I should be looking for?

 

[Bahco]

Experiment!

Change the trap for an anti syphon one and see if that cures it. If not you have partial blockage.

 

[tryitandsee]

Is air getting in through the overflow to the sink, thereby allowing the water to run down the trap/waste.

Sometimes in a ceramic basin the overflow is integral to the whole thing, try pushing some thin bendy wire and 'wiggle'

 

[KLye]

Is air getting in through the overflow to the sink, thereby allowing the water to run down the trap/waste.

Sometimes in a ceramic basin the overflow is integral to the whole thing, try pushing some thin bendy wire and 'wiggle'

Yea, you see. My current thinking was perhaps this is worth persuing. Although, when I plunged the sink initially, water was coming back out the overflow thereby requiring the rag/tissue trick.

 

[tryitandsee]

Sometimes something gets in there which floats, when water goes out, it drops to the bottom and blocks the hole/outlet. When filled, it floats up again, unblocking outlet.

It's getting late mind wandering, but just another thought.

 

[TicklyT]

On some adjustable bottle traps it is possible to push the central inlet pipe down far enough to touch the bottom of the bottle once it is assembled.
In this state, the trap will only drain very slowly.

There should be about a 10 - 12mm gap between the bottom of the standpipe and the bottom of the bottle. More may compromise the depth of the water seal, less may impede the flow.

Sometimes a little surgery with a hacksaw is called for.