Maximum connections to 40 mm waste

[SlightlyGrey]

I just had a visit from Building Control. The BCO said that I had too many appliances going into a 40 mm waste pipe. I have looked up Part H of the Building Regs, but I do not see any specification of how many branches you can have on a 40 mm pipe. The BCO said I had to connect them all into a 50 mm pipe.
The feeds I have are: shower, kitchen sink, dishwasher. The dishwasher and sink are connected to the 40 mm shower waste via swept-T connections. Can anybody advise me on what the rules are? Is the BCO connect - do I have to replace the final section with 50 mm and feed the 40 mm runs into that?

Second question is he said I need a 50 mm AAV - I really cant see that the physics supports that requirement. Any guidance would be much appreciated!! Thanks

 

[oz-plumber]

I would stick with what you have been told.

Even some of that sounds undersized, but if you want it passed - do what you have been told.

As for the physics of things - us plumbers follow the regulations and have a general grasp of how and why thing are done and what is required.

Our buck stops at - 'That's what the regulations state'

 

[Chalked]

Probably due to the length of the run

 

[Howsie]

A kitchen sink requires a 40mm waste run. A shower requires a 40mm waste run. If they require them on there own, irrespective of the length of the run, do you not think that a 50mm waste run sounds sensible?

What part of the 'physics' are you getting lost at?

 

[Alert]

BCO Sounds correct, AAV is to stop traps drawing water

 

[Masood]

Our buck stops at - 'That's what the regulations state'

Speak for yourself mate. I like to understand the engineering principles behind the regulations. I read, research and understand the science so I can make informed decisions.

OP - with the addition of a shower, the volume and velocity of the water draining away has the potential to create a vacuum which could blow out the water seals in the sink and dishwasher traps. A larger diameter will break the plug of water, and an AAV will prevent the vacuum forming.

The BCO is probably erring on the side of caution but it's a sound decision. Nowadays even single wastes serving just one shower are often run in 50mm, as showers become more powerful and have higher flow rates, especially with unvented cylinders.

 

[Alert]

If it was taken straight into 110mm svp there would be no need for air admittance valves

 

[oz-plumber]

Speak for yourself mate. I like to understand the engineering principles behind the regulations. I read, research and understand the science so I can make informed decisions.

Plenty of things will work and perform as required outside of the Regulatory Requirements.
We all know that the Regulatory Requirements have a correction factor - of what we don't know.

But at the end of the day, we are relieved of any retributions by Governing Authorities if we work to the Regulatory Requirements.

If something goes wrong, open the book and show the section / clause / amendment and prove that what you did is as per requirements.

If your inside the Requirements - your safe.
Outside - your cactus.

 

[Masood]

Plenty of things will work and perform as required outside of the Regulatory Requirements.
We all know that the Regulatory Requirements have a correction factor - of what we don't know.

Yes we do. It's all in the CIBSE guide. Every Building Regulation is derived from that. It is the canonical source for all architectural / construction engineering information in the UK.

But at the end of the day, we are relieved of any retributions by Governing Authorities if we work to the Regulatory Requirements.

If something goes wrong, open the book and show the section / clause / amendment and prove that what you did is as per requirements.

If your inside the Requirements - your safe.
Outside - your cactus.

Only partially correct. As qualified people, especially in the Competent Persons schemes (Gas Safe, any of the electrical industry self-certification schemes, Water Regs Approved Plumber), we are expected to apply our engineering judgement to the specific scenario on site.

In the OP's case, the BCO appears to have done just thet. The OP can't see anything in the regs requiring the pipe to be upsized, but sound engineering judgement will tell you that a larger pipe would be required.

Under UK law, you can be held responsible for such decisions, as the customer is entitled to rely on your expertise, and not just on published regs, as they can never be totally exhaustive or definitive, given the huge variation which can exist in site-specific circumstances.

 

[Howsie]

If it was taken straight into 110mm svp there would be no need for air admittance valves

Not necessarily.

 

[Alert]

Not necessarily.

What scenario would you Howsie ?

 

[Gasmk1]

therein lies the difference between plumbers and gas fitters for AAV i read automatic air vent but as a plumber you all read air admittance valves. each to his own 🙂

 

[SlightlyGrey]

Thanks for the responses, particularly Masood.

Yes it does seem to make sense to go to 50 mm if you are merging 3 separate 40 mm connections - so I will do that.
It would have been helpful if the regulations gave some guidance on this - but I can't see any.

The point about physics was that since the viscosity of air is vastly less than water, I would have guessed that a 40 mm AAV would be sufficient to break any negative pressure on a short length of 50 mm waste pipe - but hey what do I know, I'm an engineer not a plumber 🙂

 

[Howsie]

Thanks for the responses, particularly Masood.

Yes it does seem to make sense to go to 50 mm if you are merging 3 separate 40 mm connections - so I will do that.
It would have been helpful if the regulations gave some guidance on this - but I can't see any.

The point about physics was that since the viscosity of air is vastly less than water, I would have guessed that a 40 mm AAV would be sufficient to break any negative pressure on a short length of 50 mm waste pipe - but hey what do I know, I'm an engineer not a plumber 🙂

Ah, an engineer. I'm still building my bridge...

 

[Howsie]

therein lies the difference between plumbers and gas fitters for AAV i read automatic air vent but as a plumber you all read air admittance valves. each to his own 🙂

A plumber knows it all, a gas fitter only works with gas. I'm old school. Still can knock a box gutter up out of a flat piece of lead.