Site of durgo valve

as long as it is above flood level of wc and basin if you have one that it serves this ok, it may be the automatic air admitting valve,( durgo or aav ) is defective or wrongly fitted,it may be that the drain it goes into is blocking causing back pressure that is pushing foul air back through wc and basin traps.

The purpose of a Durgo valve is to allow air into the soil and waste stacks, this is to eliminate negative pressure caused by large volumes of water/waste moving through the pipework. When working correctly they should self-seal whenever there is no vacuum and should remain sealed when there is any back pressure in the stack. Negative pressure in waste and soil pipework can cause slow drainage and induce self syphoning of traps, Durgos are usually installed where an open vent can not be used. If smells are escaping from the Durgo, I would imagine it is not functioning correctly and would need checking to see if it has been installed correctly or is a defective unit, To eliminate the problem of a defective Durgo from any future mechanical faults, I would instead suggest running a vent pipe up externally to rejoin one of the stacks or to its on high-level terminal. Hope this helps

The surge valve is normally a very reliable piece of kit if fitted as per manufacturers instructions but can cause a smell if not fitted correctly to the soil pipe, these can cause a smell if there is no external vent on the foul drainage system or if the system has a partial blockage which can increase back-pressure in the system and force foul air past the diaphragm in the Durgo valve or bubble back through the basin trap or WC.

There are two types of AAV's. Internal and external, if an external aav has been fitted internally then that could be the problem as they open more often than internal. It has to be 600mm above any waste inlet into stack.