Sewer smells

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Hi I've got an issue with my soil stack. It happens once every few months.
Suddenly we get horrible smells of sewer coming into downstairs room where the soil stack is. There is no toilet in that room.
The soil stack has an aav outside. However years ago the plumber broke the soil stack and had to join it. They used a rubber coupler with a bit of pipe inside. I can't help feel that is the cause of my issues.

I wonder if air escaping or coming in has a knock on effect. That join us external and the smell comes from the base of the soil pipe which is about 3m aways from the join.

Is it possible this could cause the issue? I don't notice issues with the toilets or plumbing.
 
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It could be where it joins at the base of the stack? Photos may help.
 
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Your soil stack has an AAV outside? Does the stack terminate too close to a window?

Spraying pipe joints with soapy water may help visualize any leaks..
 
Break open the plasterboard, it will have to go anyway to make the repair.

Blup
Well, it doesn't have to be a break, necessarily; cutting a good amount out in a one-er with an oscillating multi tool usually means it can be patched back in reasonably easily with a bit of filler..
 
Multi tool ideal for this, one of the best tool inventions ever.

Blup
 
Your soil stack has an AAV outside? Does the stack terminate too close to a window?

Spraying pipe joints with soapy water may help visualize any leaks..
It is outside but the smell is no where near those rooms.

I think the cause could be a faulty aav. The 1st floor bathroom is not used much and the bath trap is dry. I think this is being sucked out. I put water in 3 days ago and the smell returned today!
 
This is the outside and the aav is at the top. No idea how I'm going to get to that mind
 

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I think the cause could be a faulty aav. The 1st floor bathroom is not used much and the bath trap is dry. I think this is being sucked out. I put water in 3 days ago and the smell returned today!
As you know the bath trap will let smells in if dry, start with why the trap is dry.
 
Perhaps indeed the aav is faulty and not admitting air, so flushing the toilet is sucking all the water out of the bath trap too.

I'd get to that aav with a ladder. It isn't actually necessary to have the soil pipe extend all the way up to the soffit like that; they could have stopped it say half a metre above the bend and put an aav on it. As such, you could just cut it off anywhere you like in the vertical (maybe near the top if you like the aesthetic of a big black poo pipe running up the house, or nearer the bend if you don't) with a standard wood saw to give yourself some working room

To start with I would maybe cut it off at the top and wouldn't bother refitting an aav; if you seldom open that window next to the pipe you may not even need an aav to go back on


All this said, a faulty aav could explain why the bath trap is dry but it wouldn't seem to explain why the downstairs room with no drain appliances in is experiencing sewer smells; that is more likely coming from a broken pipe or failed seal, and a working aav won't prevent it. AAVs admit air into the sewer system when pressure within it is low. They do nothing to alleviate increased pressure situations, where smells find their way out via other means such as escaping through open holes, breaks or unsealed joints. You may find that removing the AAV diminishes the problem because air that is escaping into the downstairs room instead escapes out the top of the stack as an easier route
 
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The other option you have is no external work, just fit a waterless trap to the bath internally if you determine that the bath trap is always going dry and admitting smells. You wouldn't need to remove the water trap from the bath either, if it's awkward; just find a length of waste pipe run under the bath that is long enough to accept something like one of these: https://www.toolstation.com/viva-magna-self-sealing-waste-valve/p16545
- they're about 8 to 10 inches long, shorter ones exist from other brands

They have a silicone flap inside that is pushed open by escaping water and then seals up when the water has passed, preventing smells coming back up
 
Thank you all for the help. I will try the aav and if that does not work I will move to a plumber as I think I would have reached the end of my skill. I think fitting the dry trap would be masking the issue.

The main soil pipe is brand new but I have no idea what the fitting into the drain is like.
 
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