How does the loop vent work?

[newhomeowner]

I am doing my first project in my basement. I need to put in a sink, but I am about 15 feet away from the waste-vent stack. I was just going to run a pipe along the walls and tie into the existing one(where I have another sink connected to). I did some research and found out that if I am more than 3.5 ft from the stack I can't just tie in. I need a loop vent. Two questions:
Do I really need a loop vent?
How does the loop vent work? I am looking at a drawing, but can't quite get the benefit of the loop vent. What does it help prevent? How does it work? I understand how to build it, but can't quite see the benefit.

Thanks

 

[hj]

loop vent

You do not need what is conventionally called a loop vent, but you do need a true circuit/loop vent, which is a vent connected to the new drain within about 3 feet of the trap and then run back to the vent for the existing sink, (at a height at least 42" above the floor), and then connected into the vent for the existing sink.

 

[Sponsor]

 

[newhomeowner]

Building a true loop vent isn't really an option for me. More work then I can do, my basement is fully finished.
Do I have to have it?
What if I connect directly into the stack that is 13 feet away? Why the 3 ft rule?
Also can anyone explain what a loop went supposed to prevent/help?

Thanks again

 

[hj]

vent

The "loop" part is immaterial. The purpose of a vent is to prevent the sink from losing its trap seal when it drains, in certain cases it will also allow the sink to drain properly even if there is a problem with the drain further downstream. The 3 feet is arbitrary, but codes typically restrict the dimension from a trap to the vent to between 3 and 5 feet depending on the size of the pipe. You are going to have the drain pipe running to the sink, so what would be the problem with the vent pipe coming back.

 

[e-plumber]

Sink Installation

Is it a possibility to tie in the (new sink) drain in as low as possible to the existing drain line, (15' away) & run the drain line above ground along the wall then double back with the vent for a proper tie in?

 

[newhomeowner]

How far above the the drain line should the vent line be?

First of all thanks a lot for everyone that replied. I appreciate your time and opinions.
I can run a second line as the vent. I was just not sure why the "loop" part. But from the responses it sounds like I don't really need the loop (I am talking about that "U" you put on top that should be above the sink drain level). My question now is, do they connect through the same connection point(do I just put in a T or a Y at the stack and connect the drain and vent together?)
If no, how much above the drain connection do I need to have my vent line. Could it be right on top of my other (drain) connection?

 

[hj]

vent

That "U" shaped "loop" is for a very special application and it does not apply to your installation. Your vent will rise 42", or more" above the floor and then travel back to the vent stack, with a constant upward slope towards it and then have its own connection to the stack at that point, or higher.

 

[jmariechambers]

I have to agree with HJC...

I have to agree with HJC, a vent is essential. I am not sure about whether you should tie into the existing stack or make a new one. I would shoot for the existing stack, with the upward slope.

A point of caution -- I just went through this in my master bath and want to pass along what I learned. MAKE SURE WHEN YOU CUT THE STACK FOR THE NEW DRAIN, YOU CUT IT LOW ENOUGH FOR THE CORRECT SLOPE. It took me three tries before I figured it out. For your 13-foot run your slope should be about 3 1/4" over the distance of the run. Personally would increase that figure to at least 3 1/2". Just a friendly word to the wise, from another weekend warrior!

 

[Deb]

Deb

It really should be a fairly simply straightforward install. It must be vented correctly--venting is never optional. As already mentioned, cut into your existing stack as low as possible. Install a san tee or a wye and 1/8th bend (I prefer a wye and 1/8th bend though UPC allows a san tee going from hortizontal to vertical). Code requires a base of stack cleanout. You might want to install a test tee on the stack above the sink tie in and on the horizontal run to the new sink. Run a horizontal line with 1/4" per foot grade to the sink location (you can be up to about 3' away-UPC), use a long sweep 90 or 2 45s to turn up. Install a 2 x 1-1/2 x 1-1/2 san tee at the correct height for the sink you are installing. Run a horizontal line off the san tee to the center of the sink location and 90 out of the wall. If you ran the drain to the center of the sink location, you can simply face the horizontal hub of the san tee out. Out of the top of the san tee rise straight up at least 42", 90 and run a horizontal line back to the vent. Tie in with a san tee pointing up. Maintain a little grade on the vent.
Deb
The Pipewench

 

[Terry]

In some situations, not yours, you can loop a vent down and tie in farther away.
The looped vent will drain back into the waste and wye off to a vent that does go up.