Do you include the vertical run when deciding waste pipe diameter size?

[SJRSJR]

Hello

I’m wondering if I’ve made a mistake here. I’ve got a 40mm shower waste outlet which runs less than a metre horizontally before it drops vertically for 2.5m, then goes outside for a further 2m horizontal run to the soil stack.

I see regs say anything over 3m should be 50mm but does that include the vertical run too?

It’s not too late to change but I’d prefer not to! Thanks.

 

[denso13]

Yes, unfortunately. You might get issues with the large vertical drop on 40mm.

 

[SJRSJR]

Thanks - would you mind describing what the issue might be. I'm reading about laminar and turbulent flow but don't really understand it. Is it about sucking the water out of the trap?

Would it help if the last horizontal run (on the outside) was 50 but the vertical remained at 40? Thanks

 

[denso13]

Is it about sucking the water out of the trap

Yes, exactly that.

If it is all connected just use it for a bit and see if you need to revisit it.

 

[SJRSJR]

Thanks. It’s not connected at the shower end.

So I’m going down a rabbit hole to try to understand this! I’m reading that on vertical drops water will flow in a vortex (so long as it’s vented), which lets air in and stops the suction effect. Does the open end of the soil stack mean that it’s vented?

Anything to avoid re-doing it!

 

[denso13]

Does the open end of the soil stack mean that it’s vented?

Yes, but not until it reaches the stack. I wouldn't worry about the theory, try it and see if it is ok.

Or, if you want to follow the regs, it'll need changed anyway.

 

[Madrab]

Does the open end of the soil stack mean that it’s vented?

'Fraid not - The waste pipe run itself would need to be vented, ideally upstream or very close to the trap. If the run was vented then that would probably mitigate self syphoning - i.e. the water from the shower filling the waste pipe and then creating a vacuum. You don't tend to get anti vac shower traps though, which are designed to break that vacuum. Venting the shower run may be awkward.

That being said, given it is a shower then the volume of water being drained p/min isn't huge, as say a full bath or basin being emptied in a oner, so the waste may never fill completely creating a vacuum that would pull on the trap seal. In a vertical drop though the risk of a vacuum may be greater, due to increase in water velocity.

Best idea would be to test it before it's all sealed and inaccessible - fit a trap to the end of the run and dump a load of water down it and see if it then pulls the seal.

 

[SJRSJR]

Okay thanks for the replies, much appreciated

 

[SJRSJR]

If I put a small handbasin nearby could I put an anti syphon trap on that?

 

[denso13]

Try it and see. There is no way of knowing if it will help or not.

It really is severely undersized for what you are doing.

 

[SJRSJR]

Okay thanks. I'll give it a go.

So now I'm wondering about the bath waste (different room). It runs horizontally for a metre to a swept tee where the basin joins. Then another 1.5m through the outside wall with a 30cm turn to the soil stack. Seeing as it's a bath should that be 50mm too?

 

[denso13]

Not a fan of linking wastes but it should be ok at 40mm.

 

[Hugh Jaleak]

Bath should be fine, you're within the 3m max permitted run allowed for 40mm.

 

[Madrab]

So now I'm wondering about the bath waste (different room). It runs horizontally for a metre to a swept tee where the basin joins

I'd just add an anti vac basin trap onto that, just to stop any gurgling @ the basin trap if either the basin empties or if a full bath is let go.

 

[robinbanks]

Anything to avoid re-doing it!

Can't suck the water out of a waterless trap