Air admittance valve / soil vent pipe

[Joss]

I am planning a bungalow loft conversion, and want a bathroom on the new first floor.  At the same time, I am splitting an existing ground floor bedroom into two to make a new bathroom and cloakroom/wetroom.  Existing bathroom to be removed (to make way for the stairs)

 

I have had some building control drawings done which show a new soil vent pipe up through the roof.  It was suggested in a previous post that I could perhaps have an air admittance valve inside the building instead, but that I would need a soil vent pipe outside the building too. 

 

I have read the part H building regs that relate to drainage and waste disposal, but am still not certain whether the existing soil vent pipe would be sufficient.  If it is, would it need to be extended above the height of the new upstairs bathroom, and does it matter if it is upstream or downstream of the new drain connection. (mains drainage).

 

Thanks for any help.

[Jeremy Harris]

Essentially there are two different requirements in the Building Regulations themselves, but these aren't 100% clear in the guidance in the Approved Documents. 

 

The first requirement is that the foul drain has to be vented to atmosphere, such that there can be no build up of pressure inside it - the idea is to allow any gases to escape via a vent.

 

The second requirement is that there has to be a means to prevent a partial vacuum forming in a soil pipe when a toilet is flushed or a basin or bath emptied.

 

Common usage was to combine both of these requirements into a single vent pipe that does both functions, and either run it through the roof or run it up the outside of the house if the soil pipe is external.

 

However, there's no need to do this at all, you can split the two functions and solve each individually.  You can fit a vent pipe, to allow the foul drain to be at atmospheric pressure, pretty much anywhere along the pipe run.  You can even fit one beyond the entry point of the soil pipe inside a hedge or flower bed, at fairly low level if you wish, or run an external vent pipe outside the house and up above eaves level.

 

To resolve the partial vacuum problem you can fit an AAV inside the house, well above the highest outlet, in a position where it can be accessed for maintenance.  This will open if there is a partial vacuum in the soil pipe and allow air to be drawn in, then close to prevent smells from escaping.  An AAV cannot stop any build up of pressure in the foul drain, so if you fit one then you need an external vent as well.

 

The advantage of fitting an AAV and an external vent is that there is less heat loss.  Internal vent pipes act a bit like chimneys and constantly draw cool air up through them, so unless well-insulated they will draw heat from the house and can act as a focus for condensation.

 

If you have an existing vent to the foul drain, then you should be fine with an AAV for the new upstairs bathroom.  It needs to be well above the highest drain and accessible for occasional maintenance/replacement, so you can't build it into a wall without some form of access cover, plus it needs a reasonable air supply.

Edited December 30, 2017 by JSHarris

[Joss]

Thanks JSH for your very clear explanation. Splitting these as you have described is just what I was hoping to be able to do. 

[curlewhouse]

This has given me food for thought too. I've gone for an external stack for the reasons JSH gives, (though the architect had originally drawn an internal one), but one issue I have is that unless I want to run pipes from the bathroom diagonally some distance across the outside wall to the SVP, (if fitted directly behind the upstairs main bathroom loo, the top of the vented stack would be nearer a bathroom roof window than I'd like for obvious odour reasons, & presumably there is a specified distance anyway). But the idea that I could vent the line out in the garden somewhere instead would solve that nicely, and just add an AAV in above the bathroom somewhere.

Edited December 30, 2017 by curlewhouse
Spelling!

[Jeremy Harris]

If venting in the garden, then it ideally needs to be at the end of the foul drain run, and to avoid odours, hide the vent in a hedge or flower bed, as soil and vegetation are pretty good at eliminating odours.  The external vent doesn't need to be very high, just high enough to be above flood level.

 

An AAV at  the top of the stack inside the house will resolve the possible partial vacuum problem, without contributing to the heat loss or affecting airtightness.

Edited December 30, 2017 by JSHarris

[willbish]

Does each soil pipe run need to be vented to atmosphere? Or would a single vent for all pipes entering an inspection chamber suffice?

 

I think I'm going to need 3 different runs meeting at a single inspection chamber and then flowing on to our shared septic tank as pictured. The constraints of the site and the proximity to the road mean that no soil pipe can run around the side of the property.

 

Really hoping I can do without an external stack or a roof penetration.

Drainage plan2.pdf

[Temp]

Normally you have several stacks on each run to the main sewer. Most BCO will want the furthest stack to be open vented but the rest can have AAV.

Edited December 31, 2017 by Temp

[PeterW]

Worth fitting these on every basin and then you have no issues at all with syphoning when the toilets flush. 

 

McAlpine Waste

 

 

 

 

[willbish]

15 minutes ago, Temp said:

Normally you have several stacks on each run to the main sewer. Most BCO will want the furthest stack to be open vented but the rest can have AAV.

 

Great, so I think my plan conforms.

 

8 minutes ago, PeterW said:

Worth fitting these on every basin and then you have no issues at all with syphoning when the toilets flush. 

 

McAlpine Waste

 

Yep as I thought, thanks

[jamiehamy]

39 minutes ago, Temp said:

Normally you have several stacks on each run to the main sewer. Most BCO will want the furthest stack to be open vented but the rest can have AAV.

That's what my BCO 'suggested' when I asked if we needed the open vent in the middle - said with a tank, at least one must vent to air, but he wasn't bothered what one. 

[joe90]

I too have a stack externally on the side of my detached garage ( which has a toilet) at the high end of the pipe run and an aav within the house to avoid vacuum. The internal aav is a 50mm one on a 50mm pipe as this is all that’s required so negating a 110mm vertical pipe for the aav.

[Temp]

Just to add that AAV need to be accessible in case maintenance is required.

[Lin]

Interesting that the vent can be in the garden rather than penetrating the roof etc and have an internal durgo.

Do these regs apply the same way to septic tanks (rather than town sewers) also ??

[Gone West]

13 hours ago, Lin said:

Interesting that the vent can be in the garden rather than penetrating the roof etc and have an internal durgo.

Do these regs apply the same way to septic tanks (rather than town sewers) also ??

I have a sewage treatment plant and have an external vent in the flower bed a short distance away. The vent is around 500mm above the ground and close to the tank. I have AAVs in the house and the BCO was quite happy with the arrangement.

[Simon Ennis]

Guys I have a similar issue.

 

I'm wondering if I can use something like this externally?

 

I'll contact the seller but I always worry people just say yes to anything to get a sale.

Cheers.

[Jeremy Harris]

22 minutes ago, Simon Ennis said:

Guys I have a similar issue.

 

I'm wondering if I can use something like this externally?

 

I'll contact the seller but I always worry people just say yes to anything to get a sale.

Cheers.

 

 

Not sure why you'd want to fit an AAV externally.  Why not just have a standard soil vent pipe grill?

[Simon Ennis]

2 minutes ago, JSHarris said:

 

 

Not sure why you'd want to fit an AAV externally.  Why not just have a standard soil vent pipe grill?

 

Thanks for the reply!

It's because there are near by properties with loft conversions / extra stories and I'm worried about any gasses or smells escaping.

Cheers.

[Jeremy Harris]

2 minutes ago, Simon Ennis said:

 

Thanks for the reply!

It's because there are near by properties with loft conversions / extra stories and I'm worried about any gasses or smells escaping.

Cheers.

 

The snag is that an external vent has to work both ways, so that it can release any pressure build up in the foul drain. 

 

An AAV only works one way and is only used to prevent a partial vacuum from forming in a vertical run of soil pipe.

[Simon Ennis]

20 minutes ago, JSHarris said:

 

The snag is that an external vent has to work both ways, so that it can release any pressure build up in the foul drain. 

 

An AAV only works one way and is only used to prevent a partial vacuum from forming in a vertical run of soil pipe.

 

Hmm, I see. Ok perhaps installing a trap at the bottom of the run, and using a regular mushroom vent at the top would be okay?

Would a gully like this work ok do you think?

 

https://www.easymerchant.co.uk/110mm-low-back-p-trap/

Thanks again.

[Jeremy Harris]

You don't want a trap in a soil vent pipe either, as that also prevents the pipe from venting to atmosphere.  Part of the function of a foul drain vent is to allow sewer gas to vent away to the atmosphere, and not build up in the unventilated section of drain.

[Simon Ennis]

18 minutes ago, JSHarris said:

You don't want a trap in a soil vent pipe either, as that also prevents the pipe from venting to atmosphere.  Part of the function of a foul drain vent is to allow sewer gas to vent away to the atmosphere, and not build up in the unventilated section of drain.

 

Ok thanks so much for the help.

May have to hire a professional I guess.

[Jeremy Harris]

14 minutes ago, Simon Ennis said:

 

Ok thanks so much for the help.

May have to hire a professional I guess.

 

 

As long as you follow the guidance in building regs you should be fine.  The relevant details are in Part H, sections H1.29 to H1.33: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/442889/BR_PDF_AD_H_2015.pdf

[Simon Ennis]

8 hours ago, JSHarris said:

 

 

As long as you follow the guidance in building regs you should be fine.  The relevant details are in Part H, sections H1.29 to H1.33: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/442889/BR_PDF_AD_H_2015.pdf

Thank you once again!

[Lin]

Any thoughts about durgos that are specified to be  fitted below flood level. Eg Marley 82 SVD3

 

My sloping ceilings are making it hard to site the durgo above flood height so these could be the answer. They could fit in the eaves.

[Tin Soldier]

sorry to hijack thread, but I guess its relevant question

 

If you are converting a room downstairs to a bathroom of an existing 2 storey property, can you use an aav, given there is another bathroom on the 1st floor above (with vent through the roof? or with it have to have its own soil vent pipe through the roof?

 

it would be joining the existing  foul drains outside the property