Sink Drain Gurgling

[Traveller 47]

I have installed a new wash basin in a bathroom using (mostly) the original drain plumbing. The drain plug is a slotted click/clack type and the drain drops vertically into a standard U bend before feeding horizontally for about 5 or 6 inches to another bend where it drops vertically into the original drain pipework (see attached pic).

What I don't understand is why the drain now gurges so loudly when the water is drained away. OK, air being released somehow but why did the original setup, which was very similar, not do the same thing? A further irritation is that the drain continues to emit isolated "gulp" type gurges about every couple of minutes well after all the water has emptied from the sink - maybe up to about 20 minutes later.

Any comments appreciated.

 

[Keefy]

Try this.

 

[snowhead]

Where does the original, still in use pipe go, stack or gulley?
How long and is it angled down towards it's destination?

I'd suspect the original is partially blocked or angled back towards the sink.

 

[Traveller 47]

Thanks for the reply.

The original pipework goes vertically down to just below floor level (upstairs bathroom) and then runs for about 2 metres between floor and ceiling to a soil stack. I can't access the "near-horizontal" run without major problems because the floor is large sheets of chipboard etc. However I assume it descends towards the soil stack fairly gradually because the joists are only 8 " high - so not too much room for anything but a shallow decline.

If blockage in the underfloor run is the problem, why would it not have behaved the same before installing the new sink/drain, and how could I go about clearing it?

 

[Ben-gee]

If you disconnect the last nut of the trap, where it connects to the pipework.

Remove the pipe from the trap; can you lift this pipework vertically up?
If so you may hear water run away, if you do - cut off the elbow and shorten this vertical pipe.

( it may be too long for the new basin position and this would make the section under the floor go uphill to the stack)...

 

[Traveller 47]

Thanks for the comments. The basin position is exactly the same as before - the drain hasn't moved at all. I'll try lifting the pipework as you suggest but I expect there to be only very slight movement possible.

 

[Ben-gee]

Slight movement may be enough if it changes from uphill pipe to downhill pipe, 1/2” could do it.

 

[Traveller 47]

Thanks again. I tried undoing the pipework as you suggested and found that there is no vertical movement available at all - not even a few millimetres. Shame, because I thought it was an excellent suggestion and expected to get to the bottom of the problem.