Aav's -gotta have 'em-BC says so,forum says so time and again,they are used a bit randomly in the trade,often outside which we know is wrong-what I can't quite grasp is,you don't get a full bore condition when you operate a w.c.-2" flush valve,4" pipe-so do you get a partial vacuum,which might pull a trap,in which case why can't you use an anti-syphon trap,is it a case of being cautious,or just good practice,don't know of an anti-syphon shower trap for instance,always puzzled me this,they are,after all a mechanical flap valve which must have a service life? I sometimes wonder as I explain the unsightly boxing to the customer for the umpteenth time if that valve ever moves off its seat....reading this back I think i need to get out more 
