Soil stack air admittance valves, permitted position, and types?

Discussion in 'Plumbers' Talk' started by always_learning, Nov 19, 2005.

  1. Hombre

    Hombre New Member

    No, I'm not a plumber, I'm an engineer. The Durgo is designed by engineers and it's designed to seal gases, not liquids.

    If you look at how the valve works you will see that a diaphragm is lifted by negative air pressure opening the vent.

    Fill it with water and the assembly could float, also opening the valve....

    Of course if an AAV manufacturer claims that their product is OK in a location where it might get wet, that's a different matter.
     
  2. Moley

    Moley New Member

  3. doitall

    doitall New Member

    And read this at the same time.

    Air admittance valves, when installed, must finish above the highest flood level of the space the valve is in...I.E. If the valve is in the same room as a wash hand basin, it must be higher than the overflow of that basin. This is so the pressure equalisation can occur without breaking the water in the traps.
     
  4. Hombre

    Hombre New Member

    Nice pic Moley, it proves my point nicely. If you imagine water coming in from the stack (not the outside) it will fill the left side of the Durgo and when the level of water reaches the height of the diagphragm a combination of bouyancy and surface tension may cause the valve to lift. The unit has not been designed to resist this (it may be OK, but the manufacturer won't guarantee it...)

    I'm afraid that the text quoted by doitall is wrong - just 'cos it's on the web doesn't make it true! As A.L. stated earlier the pressure (across the diagphragm) is the same no matter how high the AAV is located, this will not affect the operation. The flood level is about wetting, not pressure.

    Since this discussion is likely to carry on going round in circles I'm not going to post again, suffice to say follow the reg's and you'll always be OK.
     
  5. changename-12753

    changename-12753 New Member

    Many people can call themselves an engineer Hombre but failing to understand how a simple AAV operates put's you well below that level.
     
  6. MechEng

    MechEng New Member

    Given that I still didn't get a full explanation of whythere is the requirement to site the AAV above the highest flood level, I wrote to the manufacturer. They gave me the following information:

    It is a condition of the BBA Certificate that the AAV is placed above the highest flood level. The reason for this was due to worries over leaks from the soil stack system.

    So it would seem that the position of the AAV in terms of where it is on the run is important - water must always flow away from the valve for it to work - Fine, I think that was accepted already. The vertical height in relation to the flood levels does not have an impact on the function of the valve. It's just that there is a risk that you might not notice a leak if it's hidden, whilst you would always notice a leak if sewage is pouring out of the plughole in the sink! You will not risk 'pulling the trap seals' by fitting the AAV at a lower level than required.

    So there you have it. Only problem is that if BC are signing off the job then you're not going to get anywhere fast unless you satisy the requirements of the BBA certificate.

    I wonder if it is time to review the requirements. In my situation it is absolutely impossible for any fluid to rise above the level of the WC pan. So my AAV is perfectly safe & functions properly, yet still breaches building regs!!
     
  7. always_learning

    always_learning New Member

    MechEng,

    :) Well there you go..... thanks for getting to the 'bottom' of this, i admire your persistence...
    .... i had thought that it might be something like this...?

    cheers
     
  8. pmh

    pmh New Member

    Thank you for all of this!

    I too could not understand the thinking behind this requirement and for your years have operated with the air admittance valve in the bathroom, below the sink level. Primarily for reasons of cosmetics and practicality. In my case, any flooding blockage would have been immediately evident with wc pan filling! From an engineering viewpoint there was nothing wrong I could see, so I was prepared to live with my judgement call.

    Wherever it has been previously discussed, misinformation was everywhere!
     
  9. Hombre

    Hombre New Member

    Thanks MechEng,
    It's exactly as I said then.
    Hombre
     
  10. changename-12753

    changename-12753 New Member

    You will not risk 'pulling the trap seals' by fitting the AAV at a lower level than required.

    Incorrect.


    for example 2 toilets 1 upstairs 1 downstairs, you fit the aav between the 2, lower toilet will suck air in and the upstairs 1 ?????????

    This simple question posted on this thread still remains unanswered.
     
  11. bathstyle

    bathstyle Active Member

    The thing that gets me is why we are allowed to install anti vac traps for Basins. These work on the same principle of an air admittance valve but are obviously allowed to be installed below the spill over level of the Basin
     
  12. always_learning

    always_learning New Member

    Bstyle,

    At the risk of exetending this thread even further, I agree.... :)
    Weird isn't it? Perhaps because the 'regs' assume that an anti-vac trap will (or could) be more visible/accesible than an AAV, to detect leaks due to a blockage...?

    Can we draw this to an end now please, i feel guilty asking the question in the first place :(

    thanks to all
     
  13. MechEng

    MechEng New Member

    OK R2D2, I see your point! But seeing as the AAV sits on top of the stack, how can you then extend the stack above the AAV to serve the 1st floor WC?
    I suppose you could put the AAV on the branch, but that would not really make sense.

    I don't see why the requirement could not be modified slightly to state:

    "The AAV must be placed above the highest branch entry point into the stack"

    This would prevent your example from occuring, but would still allow the likes to me to fit the valve & still operate within the regs. I can't see any reason why this requirement would be detrimental. Perhaps you'd need to specify a minimum height for the AAV above the branch to ensure that flow into the stack doesn't impinge on the valve, but that wouldn't be too difficult to define?
     
  14. plumbers mate

    plumbers mate New Member

    try this for a senario!!!!!! soil pipe blocks, water rises, aav mechanism gets wet, mechanism freezes,!!!!!(you choose open or closed) you are starting to get the picture, and before anyone says why would it freeze, you be suprised where i've seen them fitted, some guy previously said follow the instuctions !!!!!!wise man
     
  15. Pat1964

    Pat1964 New Member

    Hi, can anyone tell me if this is an aav at the top of soil stack. It's in a void between 2 walls, this is taken from the floor level boxing in looking upwards.
    Many thanks for any help
     

    Attached Files:

  16. Dave does Gas

    Dave does Gas Screwfix Select

    Pat you have opened up a thread from 2005, start a fresh one please, makes it easier to find the question.

    In answer to your question hard to tell withou seeng the top of it but it certainly could be
     
  17. The Shadow

    The Shadow Member

    Why because the appliance WC, sink or WHB will over flow first if a drain blocks up before it reaches the level of the the AAV. Simples !!!
     

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