View Full Version : close venting????

11-23-2008, 05:01 PM
I was wondering. I have always been told in venting
that air should follow water to protect the trap. Why is it I see vents up to the allowable distance after the trap in some cases. Does it still protect well? Is close still good?


Obama the Plumber
11-23-2008, 05:10 PM
The length between the trap and the vent can be longer if the pipe is bigger.
Small pipe, means a shorter distance between the trap and vent.
The vent is to break the siphon effect and to prevent the water in the trap from running up and out, leaving an empty pipe, which can be very smelly.

Have you ever siphoned water before?
If you lift the hose out of the water for even a second, it quits siphoning.
The vent does that for you,
A siphon is bad, it pulls the water out.
So yes, a vent a little close is better at that.
You can run the max distance, but you are also at the end of what consistently works.
Consistent because the distance is based on a clean pipe.
A gooey, gunky pipe, reduces the effectiveness by decreasing the diameter of the inside pipe area. That is why cleaning and snaking a pipe helps when it has slimmy gooey stuff in it, that reminds me, Is it dinner yet?

11-24-2008, 12:16 AM
You must be careful not to move the vent to close to the trap or it will be considered crown vented which is against most codes. The rule in most codes is the vent should be at least twice the diameter of the trap away. For example if you have a 2" p-trap then vent must be at least 4" away. Also codes do limit how far away a vent can be too.Again code around here usually says the vent must be no more than 5 foot away from the drain.