Water and soil runs with AAV

[Dunc]

In the upstairs bathroom I have bath, then sink, then toilet running "horizontally" (18mm/m fall) into the vertical soil stack.

I need to install an air admittavce valve for the stack. Venting is provided separately, outside the house.

 

If I install as in (A), in the highly technical drawings below, is there risk that the bath and sink would be vacuumed by the toilet flushing?

 

Would (B) be a better arrangement?

[Russell griffiths]

I like air admittance traps as well so one under the bath as well as basin. 

[Nickfromwales]

1 hour ago, Dunc said:

In the upstairs bathroom I have bath, then sink, then toilet running "horizontally" (18mm/m fall) into the vertical soil stack.

I need to install an air admittavce valve for the stack. Venting is provided separately, outside the house.

 

If I install as in (A), in the highly technical drawings below, is there risk that the bath and sink would be vacuumed by the toilet flushing?

 

Would (B) be a better arrangement?

A: You don’t need any further air admittance here as the ‘drop’ occurs under the T outside, therefore it can access air to alleviate the vacuum of the flush from the external AAV ‘stub-stack’ you are creating.

 

B: Technically this will work perfectly well, but brings the AAV indoors meaning you need to be able to access it for maintenance and it need to be best part of 1100mm above indoor floor level.

[Nickfromwales]

23 minutes ago, Russell griffiths said:

I like air admittance traps as well so one under the bath as well as basin. 

Yours is a bungalow lol, so near zero invert, but you do like your belts and braces 🙃.

 

AAV’s are only really ever needed on a 1st floor, or where the invert (drop for flushed water from an WC) exceeds 1300mm. 

[Dunc]

To be clear @Nickfromwales whichever scenario is used, the soil stack and AAV (and everything else in the drawings) are indoors on the first floor - the soil stack does not penetrate the ceiling or wall here it just drops to below the slab and exits there. Hence the requirement for the AAV inside to let air in. Gas venting is outside.

Access not a problem as this is all in a service coomb behind a false wall.

Does this change your view of Option A?

[Nickfromwales]

1 hour ago, Dunc said:

To be clear @Nickfromwales whichever scenario is used, the soil stack and AAV (and everything else in the drawings) are indoors on the first floor - the soil stack does not penetrate the ceiling or wall here it just drops to below the slab and exits there. Hence the requirement for the AAV inside to let air in. Gas venting is outside.

Access not a problem as this is all in a service coomb behind a false wall.

Does this change your view of Option A?

Ah, sorry, read it as if the riser was outside. 
 

A is fine, and if you run the bath waste in 50mm and branch off in 40mm for the basin (reducing to 32mm at the trap) then you want even need an AAV at the bath; as per @Russell griffiths comment, if there’s room to fit an upstand and regular AAV behind the bath panel then it’s cheap insurance.