Just move into a new apartment which is on the upper floor of three. The bathrooms of each apartment are directly above one another. I believe that the exhaust vents of the bathrooms in the apartments below are connected to the main soil waste pipes, as is the exhaust vent in my own bathroom. I know they should be separately vented but these are old Victorian properties with no other vent options for the flats below. I can easily install separate venting for my own bathroom fan.
I have noticed a drain smell since moving in. I have just discovered that:
The 4" waste pipe is connected to the 3" roof pipe with gaffer tape. The connection is under my bath and effectively unsealed with the result that the waste pipe vents into my bathroom.
To make matters worse, there is another break in the 3" pipe behind the tiled rear wall of the bathroom. It is impossible to reconnect it to the roof vent without breaking into the wall.
So even if I fix the connection under the bath, the waste pipe will vent behind the wall.
My question is, can I just cap the waste pipe where it terminates under the bath with an air admittance valve, bearing in mind that the exhausts from the bathroom fans below vent into the waste pipe?
My concern is that the fans could cause a build up of air pressure in the waste pipe. Would the air just escape down the sewer?