Soil pipe branch pipe question

[tonyb61]

Hi,
I did ask this question a few days ago, but I wasn't quite sure from the answers if what I was doing was ok. So I'm asking again with a specific question. This is sketch of the branch pipe I am proposing to fit. This is a 40mm bath waste pipe.The total fall is much higher than the building regs H1 document 18-90mm/m. The length of the horizontal run is around 2.5 m, and will be 44mm/m gradient.
Is fitting the vertical section of 40mm pipe ok ? I was going to fit a hepvo trap/valve on the bath waste so I think it should satisfy the requirements in H1 ?
Thanks

 

[ShaunCorbs]

Is the only a bath going into it where's the basin going ??

 

[tonyb61]

Is the only a bath going into it where's the basin going ??

It's only the bath, the basin is going into the other side of the soil pipe, with an offset. The basin is in the other corner of the room to the bath and wc sadly.

 

[plumben]

what will be the total length of run from bath to soil pipe

 

[thompsonbrown]

Not sure why fitting a HepvO trap will help with your fall? Looking at your drawing you have a lot more fall that 90mm.
Is there something I'm missing?

 

[tonyb61]

what will be the total length of run from bath to soil pipe

It's 2.5m...

 

[oz-plumber]

The horizontal sections of the pipe require the fall.
Each section of horizontal pipe requires its own fall.
You don't include any vertical sections of pipes fall.

 

[tonyb61]

Not sure why fitting a HepvO trap will help with your fall?

The building regs H1 quote the fall figures for an unvented branch pipe, as I understand it with a greater fall causes a higher water flow which means there's maybe enough negative pressure to empty the waste trap. If you vent a waste pipe then you can have a greater fall. The hepvo being dry will allow the negative pressure to be released, and then seal the waste again. Am I misunderstanding ?

Looking at your drawing you have a lot more fall that 90mm.
Is there something I'm missing?

The fall on the horizontal section of the branch is 44mm/m, and I set the length of the vertical section so that the horizontal section fall is about right. The total fall is definitely higher than 90mm/m, but venting the branch should then prevent problems ?

 

[tonyb61]

The horizontal sections of the pipe require the fall.
Each section of horizontal pipe requires its own fall.
You don't include any vertical sections of pipes fall.

Ok, that's the bit the H1 regs do not mention anywhere that I can see I did think if you had a long vertical fall there is a risk the trap could have it's water seal pulled out by the pressure caused by the water falling down the vertical pipe section ?

 

[thompsonbrown]

The fall on the horizontal section of the branch is 44mm/m, and I set the length of the vertical section so that the horizontal section fall is about right. The total fall is definitely higher than 90mm/m, but venting the branch should then prevent problems ?
Its the first time you have mentioned an unvented soil pipe.
with an unvented soil pipe I would be more worried about gas pressure build up than too much water flow to drain 3" bath trap.