Horizontal Vent Line Question

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Old 03-23-10, 08:18 AM
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Horizontal Vent Line Question

I have a question about providing adequate venting for a pedestal sink installation in a second floor bathroom. As planned, the sink would be beyond the "critical distance" from the main vent stack, even if using a 2" drain line (about 9 feet total drain run). The drain will run from the wall, under the floor through the floor joist area (running parallel to the joists) to the main stack. Is it possible to attach a vent line (1-1/2") off the top of the drain line under the floor (within the critical distance to the sink) and then run this vent horizontally through the joist area and then up within the wall to connect to the vertical stack (provided that the connection is above sink flood level)? The horizontal run would be about 4-5 feet. I would pitch the vent line to allow any drainage back into the drain line. Just wondering if this OK.
 
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Old 03-23-10, 01:30 PM
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I don't quite get what you are trying to do, if you are hooking up a pedestal sink where are you running the drain? if in the wall you just continue the pipe as a vent and tie it into the stack, if you plan on having an s-trap what you propose is going to be of little value, you will still end up with a fixture prone to lose its trap seal.
 
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Old 03-23-10, 07:22 PM
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The P-trap for the sink would be right below the sink tailpiece, and then it would go into the wall to a 90 fitting that would drop it down below the floor to run back to the main stack. The run from P-trap to inside the wall should be enough to eliminate the S-trap effect. There is a recessed medicine cabinet immediately above the sink, so running a vertical vent up would be difficult. I was hoping that by taking a vent line off the top of the line running back to the main stack I could overcome the long distance and get within the correct vent distance between sink trap and vent.

The setup left by the previous owner that I'm replacing is a true S-trap (drops immediately from sink trap down to the floor) with no venting at all for the long run.
 
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Old 03-24-10, 04:07 PM
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It doesn't matter that you run the trap arm in the wall like you propose, you still have an s-trap!
 
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Old 03-24-10, 06:19 PM
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....because of the fact it would not have a vent coming off a tee fitting in the wall.

You might be able to offset an in the wall vertical vent pipe to miss the medicine chest (might have to open the wall and go thru one stud), and then put a Studor vent in the wall, covered by a grill. A master plumber did that very thing for us for us in a rental where an unvented bedroom(obviously) was transformed into a spacious bathroom. And in ANOTHER rental where we turned an old homes entry sitting room into a kitchen, and he vented the kitchen sink, with such a grilled-over Studor vent, high up in the wall.
 
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Old 03-25-10, 03:20 PM
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No part of the vent can be horizontal ( less than 45) if it is below the flood rim of the sink ( actually, 6" above that, or 42" above the floor.)
 
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