What protect s a trap from syphoning when the drain is clogged and the vent is filled up to the frl with water and you clear the clog and that slug of water drops past the trap arm and trap.

Normally, nothing, because the vent's air way is opened to the trap arm as the water drops below it. A toilet is a different matter, because the water DOES siphon the trap as it drains away.

I built a model with 1 1/2 pvc using 2 ft vertical piece with a female adpt, plug, a ty, a short trap arm,a trap with a stand pipe,and a piece for the vent above the stand pipe. I plugged the bottom of the 2ft vertical piece and filled it to the top and the air bubbled out of the standpipe. When I unscrewed the plug and the water dropped a little in the p trap. the stand pipe was not excess drop and the trap arm was 2 times the pipe diameter long

That drop was due to the momentum of the trap riser's water column, which had nothing to do with the water in the vent.

Thanks!



Edited 1 times.