Stub stack diameter

[EvilDonut]

My architect has drawn up plans for an extension, and I’ll be tackling some plumbing bits myself.

There is a new utility room (sink+washing machine + boiler discharge) draining into an internal SVP, connected to the main drain. See pic:

He has drawn a stub stack inside the utility room to provide venting for just that room, and has specified a 100mm pipe for this.

Is that overkill for venting these items? It’s a pain for cabinets. Is an AAV needed given it already drains into an SVP?

Part H gives you allowances for flow rate for each type of thing and I worked out 65mm would do for that flow rate.

But I’ve no idea what I’m doing Does this design look sensible, or should I do things differently?

 

[Hugh Jaleak]

Looks a bit of an odd design to me, several 110mm connections into a stack like that. Fine for WC and the basin, but I'd be looking to do the Utility room wastes differently. Personally I'd run a 110mm drain in from outside, joining existing or proposed run using a chamber, to the sink/WM position, and just connect the wastes straight into that at floor level, using an appropriate connector. Wont require an AAV, and frees up the corner, no boxing in needed.

Looking at the design shown, I wouldn't want to run a length of 110mm under the floor like that, with the only access for cleaning being at the top end, and inside the house.

 

[Newboy]

I'm with HJ. I can't see a practical way of joining two/three 110mm into a single stack.

External inspection chamber with WC into main channel and two laterals for the basin and utility room All below ground pipework in 110mm with reducers at floor level.

May or may not need a vented stack depending on your existing drainage arrangement.

 

[Nige F]

I thought the same - but what's @ the top of the drawing ? It's not external is it If it was the fan duct would go out there.

 

[EvilDonut]

It's not an @, its "G3" in a circle, and it represents one of the steel beams in our kitchen. So all internal. I should have cropped a wider area of my plans The area above the utility room is kitchen. The whole area shown in my diagram is suspended floor, but further up from that (outside the diagram) will be solid floor.

I had assumed that only the AAV Stub Stack had been specified as 100mm, and the horizontal pipe joining sink/WM to stack and then stack would be less - is your interpretation that the whole pipe system would be 100mm? Why are we even talking about 100mm pipe when we're only serving a sink and W/M - is that not completely overkill? The building regs say 65mm will be sufficient for that combined flow rate. Is it because shops only seem to sell 32,40,50,110mm pipe? Is 100mm pipe something different, or does my architect mean 110?

And as far as I understand, AAVs are only necessary for long horizontal runs, but if we're running 110mm pipe horizontally between the sink on the right of my diagram and the "New SVP" on the left (which is vented from above), that's only about 4 metres.
Would a standpipe for the WM and an anti-siphon trap for the sink accomplish the same, avoiding the need for the AAV?

I'll probably just end up doing what is on the diagrams, as they have been approved, but I don't want to blindly do it all without understanding the principles and design decisions behind it so I have the chance to avoid unneccesary compromises (like losing cupboard space!)

 

[Hugh Jaleak]

It's not an @, its "G3" in a circle, and it represents one of the steel beams in our kitchen. So all internal. I should have cropped a wider area of my plans The area above the utility room is kitchen. The whole area shown in my diagram is suspended floor, but further up from that (outside the diagram) will be solid floor.

That is what Nige was asking, we wanted to know what the area was at (@) above the drawing edge, and particularly, where the drain goes from the WC shown. Plastic drainage/soil will be 110mm, there is no 100mm plastic drainage in the UK, the only time you may come across 100mm pipes, is with clay drainage.

For drainage runs under floor, then short runs of waste, (max 4m of 50mm pipework), are permissible under a suspended wooden floor, but usually these will emerge externally to discharge into a stack or gulley, and cleaning eyes are provided externally for maintenance purposes. If under a solid floor, then 110mm is used, even for a single cloakroom basin, up to floor level. These runs will join other drains externally, usually via a chamber, to allow access if required. Joining a 110mm run into another run on a blind junction is not permitted, as there is no means of access to the first run, hence why we cannot see the current proposal being allowed. When you say it has been 'Approved', who has approved it?

I would have expected you to be asked to take 2 separate runs, from the WC and the new sink position respectively, to a suitable point on the existing drain, and connect using a chamber to allow access back up both new runs. WC may benefit from being capped with an AAV although not essential, sink and WM can discharge directly to the drain.

 

[EvilDonut]

Building control has approved the architect’s plans - it is part of a house extension. I assume they at least glanced at the plumbing details but who knows.

All the area shown on my diagram is timber suspended floor so access not as much of an issue.

I had hoped I could use 50mm pipe for the horizontal run with no stub stack in the utility room. The run is just under 4m from sink to W.C. SVP.

For good measure I was thinking an AAV under the worktop just after the sink trap, the kind that are allowed beneath the flood level, e.g: http://www.floplast.co.uk/products/soil-and-waste-systems/air-admittance-valves

 

[EvilDonut]

Would rather not use 110mm under floor joists too, as then I wouldn’t be able to crawl under one key section any more

 

[Hugh Jaleak]

If Building Control are happy, then go for it, they are the one's with the casting vote. If sink is at the end of the run, then an anti vac trap on that should suffice, just allow means for cleaning out the underfloor run should need arise.