very slow running shower drain

[old duffer]

Installed a mira shower base with fast flow drain (30l/min) supplied with tray

Waste (40mm) runs under floor for 2m, takes a 90 bend then 30cm into the internal soil stack (AAV which is new)
the 2m run has a self sealing valve on it

Shower tray is flooding and the flow into the stack is but a trickle

Apart from a blockage (its brand new) then what else could be causing this very poor flow rate?

 

[HERTS P&D]

Has the 2m pipework dipped?

Andy

 

[Madrab]

How much fall on the waste run, if it's just the min then some of these hi flow wastes tend to airlock. If you take out the diptube/trap does it then flow ok??

 

[old duffer]

fall is around 30mm over the 2m run and then drops into the waste (as in drops vertical)
Can't remove the self seal trap
Drain was tested before laying floor tiles so I know it was ok
Where does the airlock occur and how? That might be the problem

 

[Deleted member 267285]

What type of self sealing valve? Have you tested anything else connected to the stack with the aav? Has anyone “accidentally” rinsed grout down?

 

[Madrab]

If you've only got 15mm/m fall then that may be your issue, that's really not enough TBH. When you say a self sealing trap, is it a HEPvO or similar in the waste run or is it a Wirquinn or similar membrane shower trap?
I hate to say it but I do think the fall is your problem, there just isn't enough weight of water on the run to overcome the trap membrane. When setting out the install design and the fall is too shallow I always recommend the tray is lifted on to a plinth to allow the fall of the run to reach at least 20mm/m.
I've had trouble with Hiflow traps with dip tubes air locking due to not enough water weight to overcome the resistance with the seal but that isn't the case here as it's waterless.

 

[old duffer]

turned out to be the waste was

If you've only got 15mm/m fall then that may be your issue, that's really not enough TBH. When you say a self sealing trap, is it a HEPvO or similar in the waste run or is it a Wirquinn or similar membrane shower trap?
I hate to say it but I do think the fall is your problem, there just isn't enough weight of water on the run to overcome the trap membrane. When setting out the install design and the fall is too shallow I always recommend the tray is lifted on to a plinth to allow the fall of the run to reach at least 20mm/m.
I've had trouble with Hiflow traps with dip tubes air locking due to not enough water weight to overcome the resistance with the seal but that isn't the case here as it's waterless.

It indeed was the fall.

The pipe runs 2m from the shower then 90 degree and into the soil. The pipe after the bend into the soil goes through a double skinned wall so was much wider than I had realised (I measured the pipe from its emergence from the wall)
The angle of the core drill bit on one side of the wall was higher than the other side so the pipe was bent into an upward U while running through the wall.
The reason I was unaware of this direction shift is that it connects to the soil boss via a flexible pipe

The other effect I discovered was that the bending force also created an upward force on the pipe side into the elbow which in turn raised the main pipe run from the shower waste enough to impede the flow
I was able to prove this through applying a very small downward force on the pipe and watch the flow into the soil (removed the AA valve to see down) increase/decrease.

The differences were millimetres but were enough to create this problem

Drilling through the wall once again to create a larger aperture meant I could position the pipe to provide the drop needed and ease the forces on the pipe.

 

[bernardgreen]

Drain was tested before laying floor tiles so I know it was ok
Where does the airlock occur and how? That might be the problem

Testing the flow with soapy water with foam will give a lower flow rate than a test with plain water.

I found this the hard way with my shower drain which has very little fall along its length.

 

[Madrab]

It is one of the biggest fau pas when setting out a shower tray, everyone wants a floor level/sunken shower tray these days and that can produce a very small fall to the outflow unless it's planned properly. That then ends up with a slow drain and problems with trays not emptying quickly enough. The other problem is cleaning, the fall really needs to be as close to 40odd mm/m to ensure the run is self cleaning, over time as the run starts to fur up then it leads to all sort of drainage problems.

Glad it's running better but do advise it needs regular cleaning otherwise there could be issues/callbacks

 

[old duffer]

There is an update on this worth keeping in mind if you use Mira flight shower trays with the slotted centre waste.

Having made all of the pipe fall adjustments I ran the shower to double check for any leaks at any point so I left it running.
After 4 minutes, the shower tray had flooded once again so I knew for 100% it was related to the waste this time.

In desperation, I called the Mira support line and explained what was happening. Their immediate response was to email me a diagram on how and where to drill extra holes in the waste cover to improve drain away.

The waste has a hair guard which is removable for cleaning but when in place, the form is very restrictive to flow.
Having removed the hair guard, drain away was perfect, no flooding. Hair guard in place, flooding in 4 minutes

I measured the apertures in the guard (only 3, 2 small, 1 larger) and I calculated a drain flow to be a max of 10 l/min against their stated 30 l/min
The fact they had an immediate fix suggests they are well aware of this design fault

So It was not ALL my fault after all LOL

 

[old duffer]

middle slot is 50mm x 11mm, the two smaller slots are 11 x 20mm..

 

[Madrab]

So where did you have to drill the holes, that's the fix for when you have a standard round waste with a chrome cover, you drill holes in the top of the cover to allow the trap to expel the air when filling with water, doing that breaks the airlock.

Never seen that with a linear drain but it makes sense.

 

[old duffer]

This was their suggestion:

"You might be able to help the situation slightly by drilling a hole in the tube section, if you lift it out and turn it over you will see dimples on the underside. See above.

You can drill through this quite easily with something like an 8mm bit… try the side opposite to the waste pipe at first… see if that does it, if not try both sides."

 

[Madrab]

Yeah that'll be what it's doing, where the catcher narrows, it'll airlock and if the fall on the waste isn't the greatest then it needs that extra venting to allow it to overcome that and run easier.

 

[neesbro]

Hi,
I had the same problem with airlocking. Shower tray flooded within seconds! I fixed it by separating & putting aside the lower section of the funnel assembly then dropping a tupperware type plastic container 6cm x 6cm x 3cm deep into the waste assembly so it sits on the bottom. When the top section only of the funnel assembly, complete with 'o' ring seal, is refitted, the lower end of the funnel assembly dips into the tupperware container which fills with water when in use, thereby providing a seal from the sewer. Works perfectly for me even with shower on full flow.