Y plan question on blockage

[howy1]

hi all, just a cauriosity question , why is it on y plans the feed and vent close coupled section blocks ! just a friend has blockage cleared in this area and asked question.
Cheers
Howy.

 

[KingDoey]

Because that's generally where the crap from the feed and expansion cistern gets stuck as it is drawn down.

 

[JamesG92]

Most of the time its only 15mm too so can block easily on a dirty system.