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Old 12-17-2010, 10:12 AM   #1
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P trap location


I'm installing a drain for a dish and cloths washers. I had originally intended to position a P trap right below were the pipe goes into the floor. Because of how the house is built, that ended up not being an option.

I am left with the option of snaking pipes around above the floor, behind the washer/dryer stack, were it will eventually be covered by a secondary wall (to insulate and hide a few things; W/D drain pipe, dryer vent, water supply, electrical, etc.) This seams like a bad idea. It will look like crap until its covered , then there would be no access after its covered. Alternatively, I can install it the basement a fair distance (about 7') from the drain. My gut reaction is that sounds like its too far, but the purpose of the P trap is to stop sewer gasses, and it will still do that just fine anywhere between the drain and the main sewer pipe.

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Old 12-17-2010, 10:18 AM   #2
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Originally Posted by forresth View Post
I'm installing a drain for a dish and cloths washers. I had originally intended to position a P trap right below were the pipe goes into the floor. Because of how the house is built, that ended up not being an option.

I am left with the option of snaking pipes around above the floor, behind the washer/dryer stack, were it will eventually be covered by a secondary wall (to insulate and hide a few things; W/D drain pipe, dryer vent, water supply, electrical, etc.) This seams like a bad idea. It will look like crap until its covered , then there would be no access after its covered. Alternatively, I can install it the basement a fair distance (about 7') from the drain. My gut reaction is that sounds like its too far, but the purpose of the P trap is to stop sewer gasses, and it will still do that just fine anywhere between the drain and the main sewer pipe.
What you're thinking of is called a running trap, and it is prohibited in every code i've ever seen. generally speaking, you have a max. of 24-30" tailpiece length between inlet and the trap, and you must install a vent within 3-5 feet after the trap. if you won't be able to see the piping, and its all installed properly, do it right and hide it with the sheetrock when you're done

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Old 12-17-2010, 11:33 AM   #3
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P trap location


well, after a little more digging, it looks like if I do it all above the floor, I would end up with an S trap, If I do it below the floor, I'll end up with a running trap.

So my choices are 2 traps that are both against code (or no trap that is dangerous and against code).

As this is a DIY in an old house already packed full of code violations, I am more interested in getting something that works and is safe than just hearing what is an is not code.

I understand the reasoning against an S trap is to prevent siphoning, and I can counteract that to some extent now that I know what to be wary of when I design my plumbing run.

what is the reasoning against a running trap?
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Old 12-17-2010, 11:50 AM   #4
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P trap location


a running trap will siphon under a heavy load, vented or unvented. they are prone to fail.

the code is there to ensure that your plumbing will work and be safe

install a vent to serve the trap, put in a pit, or run an indirect waste line could be an option
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Old 12-17-2010, 12:01 PM   #5
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I can't see why putting a P trap further out out from the fixture than 24" would make it more prone to siphon. These would be predominately horizontal runs, and should not be very prone to siphoning, especially with the relatively light flows it would see.

I will be looking into a vertical vent right at the wall, but I've run into so many stoppers, I don't hold out too much hope.

an indirect waste line isn't really an option. I'm not sure what you are talking about with a pit.

Last edited by forresth; 12-17-2010 at 12:05 PM.
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Old 12-17-2010, 12:26 PM   #6
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P trap location


perhaps an air admittance valve would work if a vent is impossible, make sure it is rated to handle the dischrage from a washing machine.

traps shouldnt be located far from the fixture it is serving, the more pipe between the the fixture and trap the more waste that can be left behind to decay and rot wich is a part of sewer gas smell.

a pit is not really an option here as it to would need to be vented.
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Old 12-17-2010, 12:50 PM   #7
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P trap location


I don't know why I didn't think of an AAV, I am planning to use one on another section of this project.

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