Soil pipe venting - contradictory plumbers.

[dhc]

Hi,
I'm refitting a bathroom as part of a renovation and have been given contradictory advice by plumbers. The 3 floor property has its original bathroom and toilet in a back room on the first floor with the vent stack immediately outside and rising above the eaves.
A second bathroom was added at some time in a second floor front room. This bathroom has the basin and bath waste pipes feeding into the toilet soil pipe and the soil pipe has a long run of about 8 metres with a fall of about a metre along a side wall and the back wall to join the original stack at the back of the house. Within this bathroom there's also a vent from the soil pipe going up through the ceiling . . . and opening within the attic.
Completely wrong, illegal and potentially smelly.

However, one plumber has said that this vent is necessary to prevent vacuums and syphonage as it's at the highest point in the system and all that's needed is an AAV in the attic to put it right.
But another plumber has said the original stack gives all the venting that's needed and removing the second one will tidy up the bathroom.

Who's right?

dhc.

 

[Slugbabydotcom]

Plumber #2 is probably right but I'd play safe and go for a compromise. Reduce the vent to just over toilet rim height and fit an AAV there rather than in the loft.
Theres nothing wrong with trying out #2's idea blanking off closer to the toilet and adding the AAV later if you do have any problems.

 

[Softus]

Who's right?

...Within this bathroom there's also a vent from the soil pipe going up through the ceiling . . . and opening within the attic.
Completely wrong, illegal and potentially smelly.

You're right.

However, one plumber has said that this vent is necessary to prevent vacuums and syphonage as it's at the highest point in the system and all that's needed is an AAV in the attic to put it right.

This person is also right.

But another plumber has said the original stack gives all the venting that's needed and removing the second one will tidy up the bathroom.

This person is wrong.

An open soil-n-vent pipe (with suitable cage) will relieve negative pressure in the stack, and also positive pressure in the entire foul system, including from neighbouring properties. It's for this reason that your LABC exercises control over which properties must have an open soil-n-vent pipe.

An AAV will not relieve positive pressure, so if you seal the sub stack (e.g. by removing the dry section entirely) then there's a risk of pulling out the water seal from basin and/or bath when flushing the adjacent WC. You can't leave the open pipe in the loft space, so the quickest correct fix is to pop an AAV onto that pipe.

You have the alternative of increasing the height of this section and taking it into a tile vent, but this is more work, and risks causing the roof to leak if not done correctly.

If you use an AAV, you have the additional option of reducing the height of the vent section of the sub stack, but this has only a cosmetic advantage, and is more work than you need to do.

 

[Richard C]

The maximum length for an unvented w/c branch is 6m so you must keep & vent the second stack. As you have another, external vented stack on the system you can fit an AAV as suggested; LABC will only allow use of an AAV if there is another open vent on the foul system - to relieve positive pressure as Softus says. You could reduce the height of this stub to just above the highest overflow on the system as suggested & box it in.

Although the AAV will vent the w/c branch, connecting other waste outlets into the same branch is not recommended as flushing the loo may well lead to siphoning of the waste traps if they too are not vented; separate waste branches are always best; if your renovation is subject to LABC inspection, the BI may well reject it unless you have one or the other.

 

[ChrisR]

You only need one vented trap on a common part of the branch. That'll prevent siphonage on other traps connected to the same bit of plumbing while the "piston" passes, if you see what I mean.