what could happen is (see graph)

if it is an old house with cast iron drain the Y tie junction may become corroded and just start to leak when there is significan pressure when the toilet is flushed.  

所有跟帖： 

• 再谢波大, it makes sense! 这Condo就两层，楼上y型连接上头有pipe(通气?)么，还只用一个盖盖上 -山地野沟- ♂ (20 bytes) () 08/07/2022 postreply 20:35:42

• the vent goes through the roof (typically without cover). -borisg- ♂ (0 bytes) () 08/07/2022 postreply 20:38:48

• 再次谢波大！所以大概是y管上接通风管的联接处，flush短时冲力大时，水就从那里逾出来了 -山地野沟- ♂ (0 bytes) () 08/07/2022 postreply 20:50:39