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 :)

We can see why you might call this an air lock but most of us would say you have a blocked vent stack. Air is the issue so we can stay with air lock for now. Your vent stack goes up to the roofline, usually. It needs to be rodded from there. Do you own or rent this home? Need to know this bit of info because landlords do not like us giving advice to tenants who may start taking apart the plumbing. We can help either way.

We can see why you might call this an air lock but most of us would say you have a blocked vent stack. Air is the issue so we can stay with air lock for now. Your vent stack goes up to the roofline, usually. It needs to be rodded from there. Do you own or rent this home? need to know this bit of info because landlords do not like us giving advice to tenants who may start taking apart the plumbing. We can help either way.
Hiya, too true about tenants and very diligent of you! ;)

Yes, I do own the place and this actually did occur to me as well. The vertical stack terminates in the attic so I went up there and checked. I expected it to have some kind of vent or diaphragm type system to allow air out but it transpires that's not the case. It's capped tightly and looks as though it's supposed to be like that.

I didn't really investigate that avenue further as both the bath and toilet seem to drain without any real issue. (Actually, having said that the bath has always drained ever so slightly slower than expected but that issue has been around since we bought the place and is probably unrelated).

:confused:

Somebody added that cap, it should not be there. It must be open to allow air to enter the drainage system to replace what is displaced when the water rushes down the pipe. You own the unit but is there a management firm or HOA that applies to the building. You own the unit but probably not the roof, the vent should extend through the roof line. I would definitely get this repaired for your use and future sales potential.

Somebody added that cap, it should not be there. It must be open to allow air to enter the drainage system to replace what is displaced when the water rushes down the pipe. You own the unit but is there a management firm or HOA that applies to the building. You own the unit but probably not the roof, the vent should extend through the roof line. I would definitely get this repaired for your use and future sales potential.
Hi again. We run the management ourselves. It's only a small group of flats - 12 in total.

Thanks for the advice and I might try removing the cap tonight just to see what happens. However, it doesn't make sense really.
a) this was a new build when we moved in so no one would have installed a cap unless the builders themselves got it wrong.
b) why would this occur all of a sudden. It's been absolutely fine for the last 4 years?

If that cap was there when under construction it should have been caught during a final inspection. Half of all inspectors graduated in the bottom half of their class, it may have been over looked. Opening the cap without routing it to the roof may allow sewer gas into your attic and even the rest of the home. If you try it be prepared to cap it with plastic wrap and duct tape until a correct roof termination can be done. A blocked vent can cause an immediate drainage problem or it can slowly build up as the vent connection to the drain accumulates debris.

K-Lye, Follow Bobs and Johns advice. What they didn't mention is that by capping off the vent in the attic it not only prevented your lavatory from draining properly but it turned the "P" trap into a "S" trap which is outlawed by every plumbing code
Uncap that vent and tie it back into a existing roof vent. Do not simply uncap that line, if you do that you will be allowing sewer gas to enter your home. Good luck, Tom

Heya, I'm going to check this more thoroughly this weekend. One question through. I'm based in the UK so would in terms of configuration and legality perhaps things are different?

One question through. I'm based in the UK so would in terms of configuration and legality perhaps things are different?
Good plumbing practice's the same whether it's in the US, UK, or China.
Vents work the same all over the world. Good luck. Tom