McAlpine ST90 Shower Trap Issue

[Bobthebuilder95]

I have a newly installed shower tray with a McAlpine high flow shower trap (ST90) connected via a flexible hose to a 40mm solid run to the main stack.

Ever since installing the shower trap cant keep up with the water flow from our pumped aqualisa shower. However if I take the grill (and therefore the water seal) off it drains fine. The water seal/trap is clear of all hair etc but appears to be the limiting factor despite being a 'high flow' waste

Any ideas much appreciated!

 

[Hugh Jaleak]

I would think the outgoing pipework may be struggling to keep up with the flow, taking the seal out allows it to flow slightly faster and the pipework effectively then begins to siphon the water away. Do you notice the level build up slightly before dropping? This would indicate siphonage is taking place.

Long term solution would be to uprate the pipework to 50mm for as much as possible, to allow a better gravity flow away from the shower.

 

[dilalio]

If your shower can deliver 20lpm and the waste can only discharge at 15lpm there'll be a conflict

 

[Bobthebuilder95]

Thanks for the reply guys. Just checked the shower puts out 12 litres a minute and the trap is rated to 40 litres a minute so it should be able to cope.

One idea I did have was the run to the main soil stack is approx 4/5 meters and there's no air emitance valve on the shower run or the sink run connected into it also. Could there be a slight syphonic pressure causing this? Hence when I take the water trap bit out it allowed air in and it flows fine?

Many thanks

 

[Hugh Jaleak]

Fitting an AAV isnt likely to do much IMO. Rating of the trap is irrelevant, it's the capacity of the outgoing pipework that's the issue I think. 90° bends and a poor fall are 2 factors that could cause poor flow. I still think removing the seal section is allowing the pipework to siphon, hence is then sucking the water out and away, rather than relying on gravity flow at atmospheric pressure.

 

[just pumps]

My money is on the flexible sagging.

 

[Madrab]

How long is the 40mm run to the stack and what is the fall on the pipe/height difference between the tray and the stack outflow?

As suggested, if the fall/drop from the shower to the stack isn't sufficient, then the 'weight' of water in the tray isn't enough to overcome the resistance of the seal in the trap properly and then out into the pipe run. Just because the trap can flow 40L/Min, that is under optimum conditions where the fall and size of the pipework is maximised.

Really the only thing that can be done is install a low resistance waste or either raise the tray or lower the connection into the stack, so the overall height difference at the tray and the main part of the pipe run and outflow to the stack is increased.

The flexible pipe can also an issue if, as suggested, the pipe is sagging.