Soil pipes: when are long radius bends and AAV advisable?

[Rickety]

Hi,

I'd appreciate some advice on a soil pipe installation.

Previous we had a two story building, with two W/Cs on upper floor, connecting to a simple direct vented stack (there are no secondary vent pipes). We've added a new floor, which will have another WC. One of existing W/Cs will be removed eventually.

Here's a simple schematic of my pipework, with existing soil in green, and new pipes I'm installing in red. Everything is in 110mm, with existing in cast iron, and new in HDPE pipe.

Untitled

• Rickety
• 7 Oct 2013
• 1

I assume we have existing long rad bend at B, though as in solid floor I can't verify. Soil flows to final manhole in cellar, with some other junctions joining, before flowing to second manhole outside at edge of property.

It's not feasible to connect the new soil directly down to the manhole junction due to physical constraints/displacement forward/backward not show in diagram.

Questions:

- Is it recommended/advisable to have a long radius, rather than normal, 90 deg bend at point B? As the waste would have dropped about 3m vertically, it might be good help it enter the horizontal section gently? Is there a rule of thumb here as normally they seem to only be fitted when going underground?

- If a long radius is a good idea, any suggestion on dimension? I'm using Geberit HDPE and they one they have is only 10cm radius, whereas a below ground one is normally 450mm? http://catalog.geberit.com/public/product.aspx?cat=GB_GB-en_1&ch=24&p=53872

- I planned to cap the old vent A, and let the stack vent above new WC> However wondering if it might be a good idea to put a air admit. valve at A also?

Any suggestions would be very helpful.

Cheers

Rick

 

[Newboy]

Is there a reason that you can't join to the existing stack at A on your diagram? - much simpler (=fewer possible blockages!). T into the existing stack and existing stack through the roof.
The coupling you have shown in the link states that it requires

a combination of buttwelding and electrofusion

do you have the necessary equipment?

 

[Rickety]

Unfortunately A falls inside a exterior wall on a bedroom, rather than the new ensuite. Also the vent section of pipe is only 80mm or so, and ripping that out would be a lot of work.

I have buttwelding plate, which has worked fine with other 110 sections. But it's a good point - seems odd it asks for both buttwelding *and* electrofusion as you'd normally use one or the other not both.

I was hoping there would be some rule of thumb people applied like you need '100mm of radius for each meter of fall'. But it doesn't seem to be the case.

 

[Hugh Jaleak]

I'd be cautious blocking off the existing vent, and then fitting a new WC upstream of the existing. Possibility when top WC is flushed it could cause a pressure differential in the pipework below, which with no ventilation could cause problems with WC's on floor below. Long radius bend should help situation but may not completely alleviate it.

Also you may need to be aware the Building regs frown upon bends in the wet part of a stack. Doesn't prohibit their use, but advice is best avoided if at all possible.

 

[Rickety]

Hugh - thanks for thoughts.

I was wondering if a good idea to put an AAV at A rather than just capping. However for the situation you are talking about I'm guess I'm likely to get positive pressure at A, so that isn't going to help....

What I could possible do is connect A to the new vertical vent pipe using a small diameter (40mm say) pipe, to relieve pressure, if that is a good idea?

 

[Nige F]

40mm vents taken from the pan connectors of all the WC`s ( MacAlpine do one ) coupled together and then taken into the new vent pipe as high as poss - forget about point A . What you`d have then is an aproximation of an Old Skool vented stack setup . Hugh is right about +`ve pressures

 

[Rickety]

Thanks for all the helpful comments! We are planning to eventually ditch the 2nd existing (right hand) wc, and renovate the other one. So I guess Ill go ahead and just cap A, and connect soil with biggest radius bend I can fit. If there is a problem, which seems quite possible, I can punch a 40mm vent pipe through from the left wc up to the new soil stack as you suggest Nige.

It's all a bit of black art this soil plumbing...!

Cheers

Rick