I live in a 23 storey block of flats and my apartment is on the 22nd floor. There is no other toilet above me, viz, the 23rd floor have their toilet in a different area of their floor space. The SVP appears to be vented to the roof but has not been capped off to prevent the ingress of birds or other debris. My w.c.pan is connected properly to the stack pipe within a distance of 40cm.
My problem is thus:-
for short periods of time, usually about a week, the water from the trap in my conventional toilet is sucked out. It can then go for a few months with no problem. During this period of being sucked out I have to put a plug of material into the S bend to prevent smells emanating into the room and we then cannot use the toilet, fortunately we have a second toilet.
I have had the toilet replaced with a low level suite and the problem still occurs. I believe it is because the open end of the stack pipe becomes blocked but I wonder if any other sanitary engineer or plumber can throw any other light on this problem please. i.e. some other cause.