Mystery Sewer Smell in House after Extension

[pnburdon]

Hi, new poster hoping someone might be able to help me with an issue that has me at the end of my tether! For almost a year we have been experiencing intermittent sewer gas smell in our house. 2 years ago we had a rear extension done, including a downstairs WC, which involved re-routing some of the waste pipes under the house. The smells that we have had tend to be from the area around the downstairs WC, but mainly they seem to just come from underneath the floorboards.

Things I have already done: -

Checked all traps and joints and repaired/replaced where necessary (some improvement made but smell not eliminated)
Had CCTV survey of pipework to exclude blockages and /or damaged pipes (all clear except for identifying a buried inspection chamber we didnt know about)
Smoke test (nothing)
Water pressure test (found a couple of small leaks which were fixed)
Removed inspection chamber that was buried under extension (no smell from chamber when uncovered, took it out anyway)
Had plumbers from 4 different companies come and look

The smell comes and goes, usually worst morning and evening - seems like weather/atmospheric conditions determine weather the gas is flowing away from or toward the house?

Any advice or ideas welcome - the plumbers I have had out all basically said I had already done everything they would have tried..... :-(

 

[SimonG]

Is the new toilet vented to atmosphere, air admittance valve or nothing?

 

[pnburdon]

Is the new toilet vented to atmosphere, air admittance valve or nothing?

Vented to AAV - that was the first thing I did was replace the AAV, but the smell is definitely coming from underneath the floorboards, and the AAV is of course above ground behind a panel.

 

[Chuck]

If four plumbers have checked the drains, it may be worth spreading your net wider and considering other possible sources of nasty smells.

What does this 'sewer gas' smell like, exactly?

Did building the extension put any air vents out of commission?

Is it possible that the smell is coming from the toilet pan itself? It's rare, but sometimes a faulty casting allows sewer gas to cross into the void that supplies water to the rim jets.

If your drains are okay, what about your neighbours' drains? How far away are they? Is somebody discharging grey water into a soakaway?

Have you considered the possibility that you have the corpse of a dead animal (cat, squirrel, etc.) under the house somewhere? A dead mouse pongs (like a full dustbin in a warm summer) for a several weeks, something bigger can be (many) months.

 

[moonlight]

Do you have a washing machine trap behind a cupboard that is not capped off? and is now a dry trap

 

[JCplumb]

Air brick near a drain allowing foul smells under the boards is my guess.

 

[Christopher buzzard]

How’s the seal on the pan connector? had it before where 1 was leaking smells intermittently but not leaking water. it wasnt sealed to the shape of the outlet of the pan on the top part.