I have a problem with occasionally getting a sewerage whiff in our conservatory. I'll try my best to describe the set-up without resorting to ascii art.
The conservatory is adjacent to a single story kitchen extension. The kitchen has an inaccessible roof space, which probably permits air to flow from it into the conservatory. Through the kitchen roof space runs an off-horizontal soil pipe from the upstairs bathroom. The soil pipe then runs about 40ft to a vented cesspit.
If I remove one of the kitchen ceiling light fittings I catch a strong smell of sewage coming from the roof space.
There used to be a soil stack vent, which was replaced (not by me) with an air admittance valve inside the kitchen roof void. That air admittance valve is a couple of feet lower than the WC and sink.
There is no other vent, other than on the cesspit.
Could an air admittance valve sited below the WC, a long way from the cesspit and effectively inside the building be a problem?
There are other possibilities, involving rats, but I'd like to understand how likely it is that this set-up is a problem before causing excessive work, expense, mess and tears.
Thanks in advance to anyone patient enough to read through this.