siphoning

[mikept]

When a drain is designed improperly and one fixture siphons the water out of another fixtures trap, when does the siphoning happen? While the drain water is quickly flowing past the pipe or immediately following it?

[rudytheplbr]

When a drain is designed improperly and one fixture siphons the water out of another fixtures trap, when does the siphoning happen? While the drain water is quickly flowing past the pipe or immediately following it?

This reads to me that you need a relief vent between the fixture that is causing your problem, and the siphoned fixture. A vent will add air to your drainage system and prevent any other fixture traps from being siphoned.
Good Luck,
Rudy

[Winslow]

When a drain is designed improperly and one fixture siphons the water out of another fixtures trap, when does the siphoning happen? While the drain water is quickly flowing past the pipe or immediately following it?

as the water flows through the drain it creates a vacume effect that sucks the water out of the trap. This could happen immediatley with a large volume at once as when you flush a toilet, or gradually as a tub drains. So theoretically it could be either way.

[mikept]

This reads to me that you need a relief vent between the fixture that is causing your problem,
Rudy

I dont have a problem in my house, if I did I could probably test it and then asnwer my own question for one type of improper dwv'ing.

[hj]

High flow can create a venturi vacuum, but when the water flow stops, the inertia of the final water can also create a low pressure area behind it. So you answer is either or both times can cause the problem. But those same conditions will make a drain flow faster if it does not have a vent.