Drain smells - positive pressure in waste

[Michael Atkinson]

The en-suite of the house into which we recently moved has the pleasant aroma of drains. Having put up with this for a few months I now want to tackle this problem.

The outlet from a shower runs some 2m near-horizontal and then feeds into a soil pipe at much the same point as a WC (with an air-inlet valve above the join - more on this later), then drops one storey down to ground level. Halfway along this 2m length the outlet from a wash basin joins. What is happening is that when this basin disgorges its contents, water in the shower trap fires an inch or so into the air, at the same time introducing the aforementioned unpleasant smell. It may be that this shower trap is then siphoned empty, but I believe not, as this mini-volcano can be reproduced again and again.

We *did* have blocked drains shortly after we moved in, but this was much further down the line, and I believe this was cleared. I found at this time that removing the valve from the aav cured problems of the toilet backing up, but this time I note removing the valve has no effect on the shower trap when the basin is emptied. The shower trap also doesn't fire when the toilet is flushed. The lack of effect in removing the inlet valve has convinced me my problem is local to the shower/basin area.

I'm speculating that the effect of the basin waste water dropping vertically down the pedestal drain is causing a pressure build up, pushing water out of the shower trap some 1m away. If this is the case, what can I do? I'm considering replacing the shower trap with a HepVo (unless the positive pressure will be sufficient to reverse the internal valve of this device?) - is this reasonable? Presumably this will stop gurgling and any air coming back through the shower drain. But I'm currently uncertain that I can actually get to the shower trap without ripping out the shower - could I leave the trap in place but put the HepVo inline before the basin outlet joins?

Before I begin to believe that a HepVo is the answer to my problems, is there another solution that doesn't involve too much waste re-plumbing?

I'm indebted to a work-around in another thread - a large coin in the basin plug-hole does seem to help, presumably by slowing the flow out of the basin. But I'm looking for a more long-term solution.

Many thanks in advance for any advice offered!

Mike Atkinson

[Dave Liquorice]

The AAV lets air into the drains not out...

I'm assuming the AAV is on the top of the stack somewhere above the toilet connection. So the stack isn't pressurising. This means there must be a restriction in the section of pipe from the stack to the point where the basin connects. It's probably gunged up with a combination of soap scum, body fat, skin, and hair...

[The Medway Handyman]

Would caustic soda remove that....?

[GMM]

Drumming up a little re-selling business there Dave, now you've found a supplier? -)

[Michael Atkinson]

Correct.

Thanks. (Although I perhaps shouldn't have read that earlier whilst trying to eat my lunch. Tuna sandwiches will never taste the same again!). I'll have another go with chemical cleaners. I may also be able to get a wire springy thing (what's this unblocker called?) in from the basin trap.

But I was curious as to whether there was a design flaw with the vertical drop from the basin joining the show outlet.

Regards Mike

[Dave Liquorice]

They may work but I've know them to make things worse and completely block the pipe. If there is a fair bit of gunge in the pipe the chemicals loosen bits off near the basin that then get washed down into the still constricted part of the pipework...

I'd got for mechanical clearance working up from the stack rather than down from the basin/shower.

Provided there is enough fall on the horizontal section it shouldn't be a problem. To little fall or even negative means that the water is flowing fast enough to carry solid matter along properly, Similarly too much fall means the water runs way too fast and leaves the solids behind.