Originally Posted by
Mikey
The main reason for venting (combined with fixture traps) is to prevent the infiltration of sewer gas (it's as bad as it sounds) into the living space.
Imagine a fixture like a sink with a straight pipe going into a sewer. With no water in the drain pipe, the gases generated in the sewer have a clear path into the house. This would be bad :(. If you've ever used an outhouse or port-a-potty on a hot day, you have only a slight inkling of how bad it can be -- like fatal :eek:.
So, the sink (and all other fixtures as well) have those "trap" thingies in their drains to keep (trap) a small puddle of water in them, sealing the pipe from the sewer. Toilets have a trap built in to the bowl -- a large part of the water used in the flush goes to refill the trap with nice clean water after all the poop has been washed downstream.
However, if an upstream neighbor or fixture let go a large discharge, that frontal air pressure you mentioned might push the trapped water aside and burble sewer gas up into the sink. Bad :(. Or, when you drain the sink, or when any large slug of water going goes down the drain downstream from the sink, a low-pressure situation could be created and siphon the trap, draining enough water to destroy the seal. Bad :(.
The vent system allows all the traps to remain in a pressure-neutral state, and not be siphoned, maintaining the seal. Good (big-grin smiley intended here, but prohibited by the system, which I just found out is limited to 4 images per message).