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Double Trapping

Jerry Simon

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I have seen the use of a running trap on the laundry drain alot here on older construction. When I see it done in ABS or PVC it means that Bubba replaced the line and was just copying what was there before.

The functional issue is that the velocity of water entering the trap is not enough to scour the trap and keep it clean. Ideally you want your traps at the end of a vertical drop so that turbulence occurs and scours the trap.

Chris, Oregon

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Originally posted by kurt

Originally posted by Jim Katen

Perhaps it was someone's attempt at constructing an expansion offset. Was the trap in the middle of a particularly long, straight run?

- Jim Katen, Oregon

Does anyone ever see expansion offsets in real life? Not sure I ever have(?).

I've seen a few in residential and many in commercial. They're the sort of thing that you don't notice unless it's missing.

Here's one where they forgot to install one. It's a hot water distribution line. It runs about 100' down a hallway in a straight line -- that is till they put hot water into it. Then it twisted into a snake-like shape.

- Jim Katen, Oregon

Download Attachment: icon_photo.gif MissingOffset .JPG

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Originally posted by Eric B

"They're the sort of thing that you don't notice unless it's missing."

Not for me! I'd notice it if it was THERE! The idea of providing for expansion would have never crossed my mind.

Jim, are the standards for expansion offsets?

It's almost never an issue in residential plumbing. Most houses are too small to need any kind of provision for expansion of plumbing pipes. The only thing I can find in the IRC is a general note in P2605.1. It says, "Piping shall be supported to ensure alignment and prevent sagging, and allow movement associated with the expansion and contraction of the plumbing system."

There are also manufacturer's requirements. I've attached page 8 from the Flowguard Gold manual. It contains the standards for offsets.

copper.org has similar standards for copper.

- Jim Katen, Oregon

Download Attachment: icon_adobe.gif FlowguardGoldpg8.pdf

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Just out of curiosity in trying to guess why that was done that way...is it possible that the drain just upstream of the trap is coming off a clothes washer? The photo doesn't show its full length. If that was a remodel, it may have seemed easier to the installer to put the trap there since he had to cut the drain anyway.

After all, when you see the drains strapped up with rope slopped over the duct work you have to wonder...

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Lot's of theories here but little fact. If there's no prohibition against it, I can't see spending mental effort trying to figure out why some unnamed person at some unnamed time did anything.

Come to think of it, I don't try to figure it out even when there is a prohibition against it. It is what it is - dwelling on it for more than a second doesn't seem necessary to me.



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