Running a soil stack vent in a cavity?

[Zikki Malambo]

We have a problem with the drains..........

For some reason there seems to be positive pressure in the local sewer which due to the extremely stupid bodge the previous owner carried out (Fitting an air admitance valve to the top of the soil stack at 1st floor level rather than taking it up above the roof line). Is causing foul smells to enter the house via basin & bath wastes.

I want to take the a vent pipe up to the top of the house, however, there isn't an obvious route. The favorite, would be to run it for a short distance in the cavity of the cavity wall and then out and up. However, the cavity isn't wide enough to accomodate 110mm pipe :=((

I was thinking about running some or all of the vent in some thing smaller. However, do I take this right up to the eaves, or switch to

110mm outside (if so, how to join the two?

Alternatively, I may just have to open up the roof on the ground floor ext (interlocking tiles) and take the 110mm out through that.

What does the team suggest?

[Andy Dingley]

So fix it. Anything else is just a bodge.

Even if you vent this positive pressure by a roof mounted stack, you can still get smells from it, depending on your local wind. My house has a deep "canyon" between houses at the back and you can get chimney smoke in that if the wind is the right way !

This is unlikely to cause a problem (although it may not be good practice). The AAV is a non-exit valve so it doesn't release smells (unless it leaks, or it's installed upside down). What it will do compared to an open stack is to prevent the release of positive pressure, which then vents through your waste traps instead. Simply moving the AAV higher won't fix this.

BTW - I have seen inverted AAVs, and they caused just the problem you describe. Worth checking.

[Zikki Malambo]

HI Andy,

Thanks for the reply. The AAV is the right way up(and in fact has been replaced in an effort to stop the problem). Only thing I can think to do is to do what should have been done in the first place. Throw away the AAV and extend the Soil Stack up above the gutter. Plus shout at the council/water Co to sort out the problem with the sewer (Apparently, it's to do with a pump "upstream" which when it runs fills the sewer full bore in a big hurry pushing the gas that's in it out of the way).

[Andy Dingley]

OK, that sucks (or rather doesn't). I think you've got a problem alright, and a high level pressure outlet might be your best option.

Keeping the AAV shouldn't hurt, and you seem reasonably confident that it's the traps leaking backwards, not the valve. So how about a small-diameter pipe going skywards ? It doesn't have to provide the air inlet to stop the toilet traps emptying when you flush (you still have the AAV), it's just to protect against pressure bubbling through the traps.

I don't know how kosher this is for water byelaws - I might try and have a look in Butterworths,

[The Natural Philosopher]

I think you can use a 75mm pipe as a vent.

Yup. Looked it up - That's permitted.

Is there room to run a 75mm pipe somewhere?

I ended up with a 30' run of 75mm pipe and a fan vent on a gable end here. On top of a standard stack going up into the loft space.

Just make sure there is a run downwards at both ends to avoid condenstion build up. Mine used to drip from that in winter. But we raised the middle a lot, and now the bulk runs down the stack to the sewer.