Mystery Sewer Gas Smell 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.....

 

[dilalio]

I appreciate that you're desperately "reaching out" for some suggestions, but having had 4 plumbers round, who've had "eyes on" (and noses) , you'll be hard pushed to get anything other than thought bubbles via this forum.

Here are a couple from me...

Can you introduce a soil vent pipe to the extension? I assume it's on an underground branch and a localised high level SVP may rid you of the odours by allowing them to escape up a vent pipe.

Can you retrofit air bricks into the walls of the extension (below floor level), again to create a constant supply of fresh air under the floor and maybe a membrane over the floorboards.

Some diagrams of the extension and pipe runs may help to inspire.

 

[Hugh Jaleak]

Sewer gases are naturally present in the system, given Morning and Evening are usual peak flows outside of heavy rainfall, (people showering, using the WC, preparing meals etc), it is quite possible the additional flow in the main runs is dissipating pressure into the house drains, and increasing the issue you experience.

However, given a smoke test should have easily pinpointed any leaks, it is difficult to add to what has already been said and/or done. My only thought is could there be a sticking Air Admittance Valve on the WC, (Hydrogen Sulphide is heavier than air so could initially drop to below floor level inside any boxing in), otherwise my only thought concurs with Dilalio above and I would also suggest fitting an external vent to atmosphere to relieve any positive pressure in the system. For the record, is there an open vent anywhere else on the property, on the same drain run do you know?

 

[Nige F]

As Hugh says about boxing - I worked @ a large old hotel, there was a blocked sink in the basement, I put some drain cleaner down . The resulting pong spread through the whole ground floor I discovered an old uncapped waste pipe hidden in a boxing, the pong travelled a long way through the boxing.

 

[oldbutnotdead]

You've checked your drains- are there someone elses drains in your back garden which have been breached by your extension dig? (I'm thinking maybe abandoned clay runs that are still linked to the main drain).

 

[dilalio]

If you can't cure it... Divert it

 

[gas112]

what about calling out national grid /cadent to make sure it isnt actually gas your smelling

 

[pnburdon]

I appreciate that you're desperately "reaching out" for some suggestions, but having had 4 plumbers round, who've had "eyes on" (and noses) , you'll be hard pushed to get anything other than thought bubbles via this forum.

Here are a couple from me...

Can you introduce a soil vent pipe to the extension? I assume it's on an underground branch and a localised high level SVP may rid you of the odours by allowing them to escape up a vent pipe.

Can you retrofit air bricks into the walls of the extension (below floor level), again to create a constant supply of fresh air under the floor and maybe a membrane over the floorboards.

Some diagrams of the extension and pipe runs may help to inspire.

thanks, here is my best shot at a diagram, hope you can make it out. Solid lines are pipework, dashes are walls

we have air bricks and pipes running between the air bricks and the cavity under the floor of the house, because the extension has a concrete floor

 

[dilalio]

thanks, here is my best shot at a diagram, hope you can make it out. Solid lines are pipework, dashes are walls

we have air bricks and pipes running between the air bricks and the cavity under the floor of the house, because the extension has a concrete floor

No drawing attached!
And we need one to understand the layout because you previously said...

but mainly they seem to just come from underneath the floorboards.

And then later said...

because the extension has a concrete floor

 

[mattylad]

the drawing is within the quoted text



Improved
Mod.

 

[jj4091]

With a similar problem to yours it turned out that the cavity below the dpc had not been filled properly & when the public sewer cracked on the boundary foul water seeped through the ground & into the cavity below ground level but the smell came up under the suspended wood floor. Took a long time to find as the smell was intermittent.

 

[Ian H]

How did you do the smoke test? Looking at your plan i’d put a pellet in a small open tin and push it slightly up the branch in the manhole.

You might not see the smoke but you should smell it so have a good sniff near everything inside.

 

[Nige F]

thanks, here is my best shot at a diagram, hope you can make it out. Solid lines are pipework, dashes are walls

we have air bricks and pipes running between the air bricks and the cavity under the floor of the house, because the extension has a concrete floor

View attachment 179404

Looking @ the diagram - there are 2 places the smell comes + goes - I would get those manholes lifted and look for any partial blockage/water not flowing quickly . Your Water co. should clean and cctv the run, The random occurence of the smells makes me think a partial blockage that gets worse as the main flow rises- @ random times . As said could be a broken pipe also.

 

[dilalio]

Check the AAV in the diagram.

 

[OvertonPlumber]

I may have the solution. Looking at the diagram i would be checking the two bath traps what depth of seal are they??
Its very common nowdays for plumbers to fit bath traps with extremely shallow seals as there easy to fit. However let the bath contents go and it will suck out the water trap.