DIY drain smoke test

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I have a sewage smell in the bathroom that has been there for weeks.

I have checked all the traps and none are dry. I have checked the drain from the bath and it is clean, no sludge at all. The seals that I can access all seem good around the toilet.

I want to try a smoke test to see if this helps locate the source. I was thinking I could open the manhole outside in the garden, block off all exits except the bathroom and drop a smoke stick in there. Would that work?

The main stack has a one way valve on the top, inside the boxing. It does not vent to the outside.

I read on one forum that smoke tests can be bad for plastic pipe but it didn't say why. Heat? The manhole has clay pipes so only the smoke would get to the plastic inside.

Thanks
 
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Does the overflow from the toilet cistern connect into the waste stack. If so then it could be coming from there.
 
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The toilet is a push button flush and the overflow just goes in to the bowl so it is not that.

The vent is in the bathroom, buried in the wall. It will be a major pain if that is the cause (plastering and retiling). The vent will be about 30 years old so it could well be that. I guess a smoke test will show that up? As everything is a pain to get to, I really want to see the source before I start hacking walls open. I can see all the pipes with the side of the bath removed. I just can't get to them.
 
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The vent is in the bathroom, buried in the wall.

A vent allows sewer gas to vent into free air outside the building.

An air admittance valve ( AAV ) allows air into the stack pipe to prevent a vacuum being formed when the toilet is flushed. This vacuum would suck water out os U traps.

I suspect you have a failed AAV. If it is 30 years old then overdue for replacement.
 
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Would a smoke test show up a failed AAV? Really don't want to have to pull the wall apart if I don't need to. I can get a camera in the wall so will see if smoke is coming from the AAV
 
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I put the smoke bombs on a metal tray sat in the manhole channel.

You might not see the smoke but you will smell it so it’s best if one person sets off the bomb and puts the lid back on the manhole, then someone else stays inside to check for the smoke/smell.

When I do this on my own I often think the smell is on my when I run into the house.
 
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I have the side of the bath off and can see the soil stack. I can just about see up the boxing to see the AAV, but there's no room to reach it.
 
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The AAV is just above the wooden batton at the top of the photo.

All the replies have been useful thank you, but my main question hasn't really been answered yet. I read elsewhere that smoke testing can be bad for plastic pipes but it did not say why. Does anyone have experience with testing plastic pipes with smoke? I can see why it would be bad for the pipes so long as the burning pellet is in the manhole and can't melt the pipes.
 

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Might be an issue with heat around the Bomb itself, but if that's let off in a masonry chamber I dont see the smoke causing an issue to plastic pipework upstream. I'd suggest an air test on the pipework first to prove any leaks, (rather than finding the pipework is sound and in fact you've a dead rodent in the floor void...)
 
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I haven't heard of an air test. Would I need to get someone out to do that? How does that differ from a smoke test except you can't see the leak?

I got some smoke pellets yesterday so I guess I may as well try that first.
 
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Can do an air test yourself, just need bung, a Manometer (U Gauge), and a Bicycle pump. Fit bung in inlet to manhole of the pipe you want to test, Fill Manometer with water to correct point. Add pressure with bike pump, watch water level in Manometer rise, note set point. If level then drops, you have a leak.
 

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