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BillyD 03-15-2008 06:35 PM

main line vent
 
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If you have a 3" sewer line and extend it up to the roof which means from the roof to the street sewer line why are more vents required? Looks like that would let plenty of air in to keep the traps protected.
Thanks
Billy

USP45 03-15-2008 06:50 PM

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called "Wet" venting. Figure if code states you need 1/4" per foot fall and there is 12 quarters in 3", minus 1/2" for air gap that leaves 10 quarters left so you may only run a 3" line wet vented maximum of 10 foot before it needs to be vented again. if you go beyond that point you have created an "S" trap. I will see if I can post a diagram of this. Good, it worked. Now the arrow is pointing to the water which is level, the slanted lines are the pipe. Notice how over the span we lost the air gap? Now if you use 4" pipe as a main you can gain another 4', and if you want to use 6" then you can gain 12'

BillyD 03-15-2008 07:45 PM

I see what you are saying but what is not clear is that the water is draining into the sewer so why would the pipe fill with water if there is air venting behind it. If the sewer were stopped up I could see it.
Also what is the purpose of a "S" trap vs a "P" trap.
Thanks for the education.

USP45 03-15-2008 07:51 PM

"S" trap not legal because it acts like a siphon and can suck the water out the trap.

Its not the water running into the sewer thats the problem. You need to look at it this way. You bathing in tub and empty the tub, at same time wife drains kitchen sink, the washing machine hits the spin cycle at the same time, then one of your kids flushes the toilet. All this water running out at the same time "May" fill the pipe momentarily to capacity which causes a negative pressure (Vacuum) behind it. This causes air to want to be drawn into the system via the traps. Thats why at times in old homes you hear the toilet bubble when the washer machine or the tub drains out.


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