Bathroom sink u-bend always open to drain

[goandkickit]

Hi guys, I have a strange problem that I can't figure out the cause of - my bathroom sink plughole often smells of the drains, and after investigating it, I've found that the water somehow goes through the u bend until it is below the level of the top of the bottom part of the u bend (i.e. there is no longer an airlock).
There should be about three inches of water in the u bend, and I can't work out how it's being sucked out. When I turn the tap on to try to fill the u bend back up, the water just somehow goes out of the u bend and down the drain, with a 'glug glug glug' sound.
I can only imagine that there is some kind of suction somewhere else in the drainage system - is that possible, or likely? I've dismantled the u bend and refitted it, to check it was all okay inside, with no luck.

 

[oldplumber]

replace it with an anti suction trap or a hepvotrap.

 

[goandkickit]

Thanks for your help, oldplumber, I didn't know such things existed! I will have a look tomorrow and see what I can find.

 

[leeking]

a.k.a anti vac trap.

 

[Puddle]

As said above however would put some drain cleaner down first as if waste pipe from basin/sink partly blocked ,causing decreasing waste pipe diameter, siphonage will happen easier ,ie reduced available bore waste outlet, filling with waste water


imho

 

[goandkickit]

Hi Puddle, that's a very good idea, because I was just about to post that it has only been happening for the past few months, and I've lived here for over ten years, and that I thought that maybe there must be some partical blockage which is stopping air from coming back up the pipe while the water is going down (probably further down the drain, i.e. a few metres away from the sink).

I've got plenty of caustic soda (I make soap as a hobby), should I put some of that down to see if it shifts the blockage?

 

[oldplumber]

all that homemade soap/fat probably caused the problem then