We have an occasional noxious emanation from an upstairs bathroom. It’s in the en-suite to a spare room so little used, so we suspected the traps (sink, bath and toilet in that room) were drying out and allowing sewage gas back into the room. Regularly flushing the toilet and flowing some water into fixtures seems to have helped, but it’s not perfect.

In trying to figure out how to improve this, I’ve been reading about plumbing (very little DIY knowledge, so relying on wikipedia etc..). From what I have read – e.g. http://en.wikipedia.org/wiki/Plumbing_drainage_venting – there should be some means of venting the sewage stack to stop traps being siphoned. However, all pipework is behind plasterboard (none on outside of house) and there is no pipework extending to roof or loft. I can’t find any obvious locations for an air admittance valve either.

So.. is it possible that the air admittance is hidden somewhere and/or is there some other way that a system could be vented and – if not – what’s the best way of remedying this? It seems to me that the entire system might be drawing air through this bathroom – which is at the far end of the house from the outflow (it’s connected to a septic tank).

The property is a barn conversion from the early 90s and we know, from when we had it surveyed prior to buying it a few years ago that the developer had failed to comply with all 90s building regs – e.g. no extractor fans in bathrooms that have no other means of ventilation.

there probably isnt an air admittance valve/breather, lots of systems work well with out them… you could add a 40mm one under the bath or something.

An AAV for 4″ soilpipes is almost as large as a football! If there is one fitted it is supposed to be accessible for maintainence. The pipe would ‘T’ off behind the toilet and continue vertically; the AAV should be above sink height or in attic. Kind of usefull piece of kit as it stops the water being sucked out the traps when the toilet is flushed!

Difficult without seeing it. If it’s been installed badly and not ventd properly, it’s possible that flushing toilets elsewhere will suck the water out of your traps upstairs. I think it’s quite likely that it may not be vented correctly. They may have used slatevents, which are made to look like a slate rather than having a pipe sticking through the roof. AAVs should be accessible as chickenman said, and they do fail. I think the only answer is to fit one or vent the thing properly through the roof.

Stick loads of cheap thick bleach down showers basins bogs baths etc on a regular basis.