Soil Pipe Smelling Do I Need an AAV?

8 May 2013
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United Kingdom
Well I'm a complete laymen, know nothing about plumbing whatsoever so excuse my dreadful attempts at trying to explain what is happening.

Ever since we bought our house we've occasionally had bad smells coming, we believe, from our downstairs toilet. The smells have been your typical sewage type smell but also on occasion of white spirits (we've assumed one of our neighbours poured some down the drain). Most of the time this smell isn't there but often is after there has been rain.

Then last weekend the smell has gotten stronger and is almost always there. The drains outside the house are clear so I know there isn't a blockage and it seems that the fault is within the house.

Our downstairs toilet is very small, just wide enough for a toilet and a small corner basin. The layout of of our house is that you have the front door, then very small floor area where we hang coats. Directly in front is a sliding door to the downstairs toilet and to the right is a door into the living room. The toilet itself is side on to the front of the house so the cistern is against the dividing wall to next door.

The soil pipe from the toilet, instead of going towards the front door actually goes right (as you look at it) into a boxed in unit. In there it then joins onto what I believe is a short stack where the soil pipe from the basin also connects to the stack.

Hopefully this picture will make things clearer.

The top of the soil stack simply unscrews and I can see all the way down into the pipe that then goes under the floor and out into the drain and this all looks completely clear, clean and dry.

In doing reading up on this problem smell it sounds like an AAV could be the answer except that I don't have one currently fitted and the top of the soil stack is tightly screwed on so any smells in there seemingly shouldn't get out anyway but also, if that is air tight, how is the toilet still flushing? I did buy an AAV but I can't seem to get it to fit because the stack is too close to the wall for one to fit on, also even if I could get one to sit on top I can't see a way of securing it to the stack.

Any ideas as to where the smell is coming from, what I need to stop it and if an AAV is needed?
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Bit of an update.

The smell is definitely coming from the soil pipe. I just unscrewed the cap again and as soon as I took it off I felt a draft coming up the pipe (still perfectly clear) and the smell quite literally hit me in the face!

If it's capped off though, why/how/where would the smell be getting out?
Ahh just realised I was being an idiot about fitting on the AAV. Didn't realise that the one I bought came with an additional bit of plastic 'pipe' that isn't needed and that by unscrewing that it reveals the actual vent which sits neatly inside the soil pipe :oops: :oops: :oops:

Well it's fitted now and the toilet still flushes so will see if that gets rid of the smell. Still can't see how this is any different from just having a screw cap fitted like the one in my photo.
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Spoke too soon, it's still there :evil:

I think it's not quite as bad as it was but it's still there.

It's definitely not a blocked drain as I've lifted the manhole covers and they are completely clear.
The trap in the toilet has water in it so it's not getting out there.
The trap in the sink has water in it so it's not getting out there.
The toilet flushes absolutely fine and the sink drains away perfectly so I don't think there's an obstruction in the internal pipes.
The smell seems to go away for a bit when the toilet is flushed so don't think the soil pipe is damaged
There is no sign of any leak around any pipe joint and the soil pipe, as far as I can see, seems secure with no sign of any bad joints.

As I say, if I take the cap off the top of the soil pipe I can sometimes feel an upward draft and the smell certainly seems to be carried on that draft.
Have you got a proper vent pipe somewhere on the property . Where are you in Sussex
The problem is your traps are not be getting pulled with the right water still in them, but what is most likely happening is your traps are being "blown" The opposite if you like. As soon as the air blown back finishes the water will settle back in the trap.

It sounds line there is insuffient open venting at the head of the drain (per Nigels thoughts I think) and perhaps a partial blockage in the pipe that causes a temporary block when a toilet (maybe not even yours) is flushed. First as Nigel suggests, you should check that the head of the drain has a clear open vent. i.e. one of the manhole branches your stub stack goes to should have an open vent on it.

An AAV only lets air into a drain - no good to you. An open vent lets it out as well.
Can see from your pic that the basin waste is also connected to the stub stack - check depth of the water seal in the basin trap. The flushing of the wc may be sucking the water from your basin trap and allowing the pongs into the house through there. If this is the case, the simplest option may be to fit an anti-siphon or self-sealing trap.
You should hear gurgling/bubbling if your traps are being blown through.

We had a blocked toilet once which leaked into downstairs. The reason being is the finned pan connector had been pushed directly into the socket, so the seal was rubbish and water leached past. In your case it could be air.

Therefore if you have a finned pan connector, check that there is a length of pipe in the socket of the junction (might just be a short stub) and the pan connector is pushed into this. If not, buy a new short section of pipe (you can get white 1m lengths to cut to size) plus a new pan connector, and recommission the toilet.
Sorry to Hijack the OP. I have a similar sounding problem. I removed a natural air stack leaving an AAV in the loft. I took some advice from a few people before I did this. Now on occasion we have a strong sewerage smell. I have put another 40mm AAV into the system. Which seems to have reduced the occurrence but this morning it was back again. My question is 3 fold.
1)will the removal of the natural airvent have caused this, albeit the smells never really arrived for about 2 months after.
2)Will another AAV possibly fix it?
3)If not will putting the natural air vent back in place, although without major disruption to decor it will need to have a few bends and a horizontal run and be spigoted into the main stack below the AAV?
any help would be much appreciated.
You should never block the naturally vented soil pipe unless you are certain there is another vented pipe that will serve the same drainage function for the affected drains. It is all part of the building regulations. Putting an AAV on a vent is a partial block as it allows air movement one way only and is essentially to prevent only syphonic action

In response to the other poster about gurgling traps - When you get a trap "pulled" by syphonic action you tend to be in the locality and hear it. When the problem is a "blown" trap it is not usually the because of the WC or basin in that room but somewhere else even a neighbour, so you probably won't hear it when the air is blown back into your room.
In reply to DANTMAN, I echo what Blagard has said, AAV's have their uses but replacing a vent pipe with an AAV can lead to the problem you describe. An AAV will open if the system is under negative pressure, however if positive pressure builds in the drainage system, the AAV will remain firmly shut... The positive pressure has no natural escape to atmosphere if the vent is removed, so will take the path of least resistance, usually into the building..... Fitting another AAV will have no effect!

The drainage system was probably designed with ventilation provided as required. Altering it can lead to problems as you've found out, so the vent really needs reinstating, if it's not part of the 'wet' section of the stack then bends wont matter and could be reduced to 82mm (3") if need be. Important factor is the provision of a path to atmosphere for any excess pressure in the system.
Cheers hugh jalek I think it was 82mm anyway but in a far to large a boxing. I will reinstate the natural vent. Thanks for tips.
Hi, did you ever get to the bottom of this? I'm having a similar issue

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