Slow Shower Drain

[scotcruz]

Hi folks,

I have a McAlpine Fast-flowing shower drain which is not fast flowing as the product claims.

The plumbing is correct and no issues or blockages.

When I lift the Part A (which breaks the water seal) the shower flows as it should so I'm suspecting it's to do with airlock.

I have seen some posts online where folk suggested drilling holes but could not access the photos showing the holes.

Please can someone advise where should I drills holes? Part A or B in the picture?

 

[Madrab]

I find that when there's a draining issue with high flow waste then it's invariably down to the design of the waste run, primarily the length, fall or direction changes. Basically there's not enough gravity head of water to overcome the resistance of the trap seal.

Holes would normally be drilled into the chrome top of the dip tube (A) but there are already holes in the top of that one. McAlpine changed their design a few years ago to add holes for that reason. Holes dilled/cut down the side of the catch cup (B) but still above the level of the end of A would be the next idea and usually works when all else fails but then you are reducing the seal depth and if anything else is on the shower run then the seal can be pulled and then drain smells can enter.

I find McAlpine wastes are very well designed and when the run is set out properly then there are rarely any issues.

 

[scotcruz]

Thanks Madrab... so I drilled 4 holes on Part A and also two 20mm holes on part B.

Ran the shower and hey presto.. all sorted.

I was confident it was the airlock issue, just wasn't sure where to drill the holes

 

[Madrab]

I was confident it was the airlock issue

It isn't an airlock per sé, basically there isn't enough head of water in the tray to overcome the resistance in the trap, this is usually down to the fall in the waste pipe being quite shallow

As suggested drilling holes in the catch cup (B) effectively lowers or removes the height of the water seal, which isn't ideal but if it works and you don't get any drain smell issues then sorted.

 

[scotcruz]

No drain smell so far.. as long as you don't go overboard (which I've seen some threads where folk have cut deep slots on the cup (part B) then you are OK..

 

[Madrab]

It all depends whether there is anything else on the run that can pull on the trap.

A shower trap will rarely self syphon as the volume of water isn't enough to fill the waste pipe and create a vacuum but if there are other outlets on the same run that can drain higher flows then they can pull on any other traps on the same waste run, hence the need for a min depth of water seal, as per the regs.

As suggested though, see how it goes and if it's working without issues then that's a result