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    • CommentAuthorgoodevans
    • CommentTimeOct 16th 2017 edited
     
    I've been looking at AAV's and kept on coming across the same rules in their respective BBA certificates for the need for a open vented soil stack which differs from the regular advice normally received.

    Normal advice: "The head of each drain must have a SVP open to the air."

    Typical BBA cert statement: "for up to and including four dwellings, one, two or three storeys in height, additional drain venting is not required"

    For example this BBA cert for Wavin's AAV ( www.wavin.co.uk/web/download?uuid=de87c2f5-13c8-42f8-bf99-b854a7368a34 ) is a typical example see table 3/figure 5 - but all the AAV manufactures seem to use exactly the same text.

    Have I missed anything but can I use the BBA certs to persuade my BCO that I don't need a traditional SVP and therefore avoid the cost of creating an ugly dry stack outside the building. (There will be a total of 3 dwellings connected to the main drain in my case).

    And for the life of me I cant find the relevant text in part H which confirms the "regular advice" and I'm sure I've read it in there somewhere.
    •  
      CommentAuthordjh
    • CommentTimeOct 16th 2017
     
    Posted By: goodevansNormal advice: "The head of each drain must have a SVP open to the air."

    That advice is usually given in the context of a self-builder building themselves a house, I believe, and in most such cases is true.

    Have I missed anything but can I use the BBA certs to persuade my BCO that I don't need a traditional SVP and therefore avoid the cost of creating an ugly dry stack outside the building. (There will be a total of 3 dwellings connected to the main drain in my case).

    S/he will want a dry stack at the furthest point from the main sewer, IMHO, assuming your three houses are in a row and connected at a single point.

    What are you doing about rainwater drainage off the roof? My dry stack looks exactly like my downpipes, because it is a downpipe, just connected differently. The cost isn't worth worrying about in the big scheme of things, especially not if the cost is shared between three houses.
    • CommentAuthorgoodevans
    • CommentTimeOct 17th 2017
     
    A new sewer serving my two new dwellings will be laid down the drive and break into the manhole at the bottom of the drive which presently serves an existing house. Both the new houses will feed into this new sewer from two directions - so each will have its own branch and therefore normally two open vent pipes will be required.

    I will have gutters and down pipes but because of roof windows etc I will end up placing a dry stack on the gable end and it definitely will not look like a downpipe in that location. I think it has to be at least 75mm in diameter - if it was smaller I think I would bury it in the EWI and ventilate near the roof ridge but a 75mm+ gash in the ewi is not good.

    Paragraph 2.18 in part H covers the provision of ventilation on drain heads - but the text is a bit mixed up intermixing "Ventilation Pipes" and "Open Ventilation Pipes (without AAV's)" - but reading the guidance it looks like an open ventilation pipe is required only in the case of surcharging or a interception trap is present. Our system is open through to the main Sewer in the road. Which is why I think the BBA certs say what they say.
  1.  
    As its on the gable end, could you make it look like a chimney/flue?

    David
  2.  
    As its on the gable end, you could take it along at ground level to the nearest corner then up the corner to look like a gutter down pipe.
    •  
      CommentAuthordjh
    • CommentTimeOct 17th 2017
     
    Ultimately, it's up to your BCO, so ask them and see what they say?
    • CommentAuthorborpin
    • CommentTimeOct 17th 2017
     
    Alternatively run the soil pipe from the upstairs toilet into the roof and vent through a cowl. That was what I did.

    IIRC is depends on the distance from the branch. AAV lets air in, soil stack lets pressure out.
    • CommentAuthorgoodevans
    • CommentTimeOct 18th 2017
     
    DFB - Fake chimney - good idea - and at the top I can put on the traditional aerial.

    djh - As always down to the BCO I'll see what he'll go for.

    borpin - the difficulty with the internal stack is the heat loss - I estimate a stack in an insulated 200mm square box
    will loose about .3 watts per linear meter of pipe per degree C. With approx 5m of stack inside the house envelope and the chimney effect inside the SVP I recon the internal temp of the stack would be near external drain temps. On a cold day thats about 22 watts (with a differential of 15 deg C) between the internal and external stack temperatures.

    If the stack effect can be halted by having a reverse AAV near the top of the stack (i.e. air can be expelled to outside if the pressure difference is above say 50Pa i.e. 5mm of water head) then most of the heat loss would be avoided. But alas I see no approved product however one could be hacked together with an AAV nested inside a larger pipe.
  3.  
    Posted By: borpinAAV lets air in, soil stack lets pressure out.

    Not in my book, a soil stack lets air in as well. The aim is when the loo is flushed the water rushing down the soil pipe can create a vacuum that can suck the water out of the a couple of sink traps. This is where the AAVs work by destroying that vacuum. The soil pipe has to do the same thing. Which is why (goodevans) a reverse AAV wouldn't work.
    •  
      CommentAuthordjh
    • CommentTimeOct 18th 2017
     
    Posted By: Peter_in_Hungary
    Posted By: borpinAAV lets air in, soil stack lets pressure out.

    Not in my book, a soil stack lets air in as well. The aim is when the loo is flushed the water rushing down the soil pipe can create a vacuum that can suck the water out of the a couple of sink traps. This is where the AAVs work by destroying that vacuum. The soil pipe has to do the same thing. Which is why (goodevans) a reverse AAV wouldn't work.

    Not if it's a dry stack (SVP) as goodevans is proposing. Each actual soil stack has an AAV to destroy vacuum as you say. The SVP is only there to allow excess pressure out. In practice, it is an open pipe (with a metal basket on top) since that's easiest to build.

    It has to have an external vent, since it may be emitting smelly and noxious gases, so I don't see what point would be served by a reversed AAV in any case.
    • CommentAuthorMike1
    • CommentTimeOct 18th 2017
     
    Posted By: djhThe SVP is only there to allow excess pressure out. In practice, it is an open pipe (with a metal basket on top) since that's easiest to build.

    It has to have an external vent, since it may be emitting smelly and noxious gases, so I don't see what point would be served by a reversed AAV in any case.

    Worth noting that those gasses include methane and hydrogen sulfide, both of which are potentially explosive; allowing the safe escape of gasses is just as important for an SVP as equalising pressure.
    • CommentAuthorgoodevans
    • CommentTimeOct 18th 2017
     
    As a hack - this is what could be built if the AAV's have enough resistance to drawing in air with minor pressure difference. For a 30 degree difference in temperature you will need about 1.5 Pa per meter height of warm stack to resist the chimney effect. - 5m stack 7.5Pa resistance (0.75mm head of water)
      anti-chimney aav.jpg
    • CommentAuthorgoodevans
    • CommentTimeOct 18th 2017
     
    A better hack
      anti-chimney aav hack 2.jpg
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