STACK DRAINAGE - offsets in discharge stack?

[marshr02]

Need to introduce a biggish offset in the bottom storey of a 4 storey discharge stack. The stack serves two bathrooms in one house (no flats). I've read Part H, in particular that these should be avoided etc etc, although I really need to try to get this to work.

My questions are:

1) If the horizontal offset distance is say 700mm, is the offset best formed from two large radius bends, or just two 45 degree bends. (Given the offset is near the base of the stack). I guess with the 2 large radius bends there will actually be a section of horizontal pipe which could attract blockages?

I guess that two long radius bends will encourage a smoother flow, and prevent positive pressure fluctuations? that Part H describes as possibly requiring a vent stack for greater than 3 storeys?

2) Purely out of academic interest; what's to stop the base of a stack being a large radius bend feeding into a long run of horizontal pipe (at appropriate fall to obtain self-cleansing velocity) before dropping vertically into a drain entry say 3 metres away

Cheers for any replies

 

[Bernie2]

Hmm!

Most places would not let you offset the live side of a stack, they would make you move the whole stack over, but if you must and its allowed in the local by laws then long radius bends are the way.

Most stacks today should already have long radius rest bends at their base. Many drain runs do drop inside inspection chambers but should be in a drop shaft either external or internal.

In the old days the drains often came in higher up and had wide inspection shafts, we would try and bat the turds as they hurtled down through mid air. You got a point for each one and usually covered in it if you where not quick enough or your mate sent one toward you that you missed, but it was good fun.

Still fancy being a Plumber?????

 

[marshr02]

Thanks for reply.

The reasons for this proposal is I'm trying to move the soil pipe inside; the house is being refurbished and has now a loft conversion - so has gone from 3 to 4 storeys. Next time I'll remember to run a swept underground branch off the sewer into the building BEFORE laying 10 tonnes of concrete footings 1m wide....

I've been swatting up on the various 'physics' reasons for the best practice routes; self-cleansing velocity, reducing back pressure from blowing out the traps etc.

In my case the offset is used at the base to take the base of the internal soil through the 13" brick wall and outside roughly above the existing sewer entry point. I guess long radius bends will reduce back pressure.

I'm assuming 'an adequate' drainage system is the primary purpose of the regs, with all the various ways to go being only an 'example' of how you could satisfy a functioning stack.

 

[Tong]

You don't say what kind of system it is .. Primary Vented, Secondary vented etc.?? this would have some bearing on the answer. Two 45's are the preferred method at the base of stack and would be the best solution for any offset .. 30 deg bends even better if you can (should still be long radius) if it's a vented system you might need to run a vent from the offset .. but this seems a relative light load for a 4" (assumed) stack so may get away without.

 

[marshr02]

Thanks for the reply

Was thinking it would be a 'modern single pipe' system with AAV at the top - as next door soil pipe vents the sewer run. I've found some Floplast Long Radius Bends at 45 degrees so was thinking of using these.

I'm guessing your point about venting is due to backpressure above the offset blowing about the traps? Could this be easily addressed by retro fitting a vent pipe if proved to be required after use? A heard something in the regs about a 'dry stack' which just vents to the drain and doesn't need to vent itself high up as well?

 

[justlead1]

Hi Use a bossed pipe between bends and cap it, in the event of a vent being included and provision made in the boxing it would not be a difficult task.

 

[Bernie2]

Any change in direction can cause back pressure or resistance in any pipework. A sharp short radius bend causes it more than a medium bend and a medium more than a long radius. To remove the back pressure they use a vent.

Even washing machine suds foaming at the base of a stack can cause back pressure that may blow a seal.

Fitting anti vac traps or AAV's are no use they only help where seals are being sucked out not blown out. A resealing trap may help but won't stop smells or gas coming in at first only after the back pressure has vented itself through the seal.

And no you can't just use AAV's all over the place, they do have limits as to the number that can be used in relation to the overall local drainage ventilation.

In fact over use of AAV's could cause dangerous explosive gas build ups.

Yes you can look up things like wetted perimeters and channel flows but you also perhaps need a basic working knowledge of what you are doing and one you can understand in the way you understand things not the way the writers put it.

Knowledge is a tool and like all tools, you have got to be able to understand it to be able to use it. Most Plumbing jobs are solved on the hoof and to be able to do that, you probably need to know how in principle its supposed to work the detail can come later if you need it.

If you don't know you can get back pressure from using sharp bends, you will be none the wiser if you fit them until you start to get problems. But at the early stage you don't really perhaps need to know the coefficient of resistance in a bend just perhaps that sharp ones have more resistance than slow ones.

You don't really need to know about wetted perimeters or channel flows at the first stage, just that, you may need to look them up.

 

[Tong]

As mentioned this is not a heavy load for a 4" stack with 2 bathrooms .. assuming 1 bath, 2 basins, 2 WC's and 1 shower is about 1.2 litres/sec the capacity of a 4" primary vented (Single stack) system is 5.2 l/s so well within the capacity of the pipe and should be OK provision for a vent at a later date may be good insurance but I would probably not bother.

If the stack is inside noise might be a consideration so try it well before you cover it up.

I agree the indiscriminate use of AAV's is problematic as many people do not understand their function and limitations.

 

[marshr02]

Thanks for the replies folks. Very useful, although Bernie I'm not some bookish airhead

Very interesting that even washing machine foaming can cause blow out of seals. I understand the one pipe system works because all the various compromises just keep all the various problems in check. What I really do not have is a 'feeling' for what bits can be tweaked.. I was intending to put acoustic wadding into the pipe boxwork to reduce the thudding sound while watching coronation street.

AAVs - yes I realise they are a compromise and only address negative pressure. I would prefer a more purist approach of venting the dry section to the outside (as that would also reduce the positive pressure problems); I'm struggling to see an easy clean way of routing the internal stack through the tile clad dormer or flat roof without introducing other problems. If it wasn't for the close velux I could use a tile vent in the front sloping roof?....darn..any ideas greatly received.

Tong - many thanks for your assessment of the stack load

Cheers

 

[justlead1]

Hi. You can use 75 mm pipe for vent. Its easier to get between common rafters. The vent needs to terminate 1 m above any openings to the buildings, If you want to take it through ridge i can supply you a lead slate or roof vent for purpose. Good Luck

 

[Bernie2]

Hi! marshr02,

I wasn't suggesting you where an air head, I am sorry if I came over that way, it was really intended as an open reply for guys who may know nothing about stacks or discharge velocities.

Lets be honest I don't really know what anybody else knows, some guys don't like all the technology answers.

So I try to explain as best I can without the technology, to try and give them a basic working knowledge that may enable them to be able to think it out for themselves. Sorry, and I apologise if I inadvertently offended you it was unintentional.

 

[marshr02]

No problems Bernie; it was a lighthearted reply of mine. I've read Part H forwards and backwards, but know I don't have the real world experience of a good plumber. I know from my day job that engineering systems are always a compromise; and I like to understand the issues. In this example I might leave the soil pipe positioned in the loft ensuite ready to extend through the flat roof if it proves to be required.

Any of you folk in the Brighton area? I've put a post elsewhere for a plumber. Only too happy to pay for knowledgeable folk. Cheers

 

[SimonG]

Its also 5 years old.

 

[Allgoode]

Hi,
Saniflo macerator & pumped system possibly may be the solution. slightly noisy during the night but ok during the day.
Father in law has one in his dormer bungalow which is on the loo and was needed to get the waste to the stack about 15ft away around two corners.
From what I can remember they do a system that can cater for a full bathroom.
Just an idea,
Cheers,
Andy (Allgoode)