Double Trapping

[Jerry Simon]

Why would this trap be here? It's downstream from already-trapped kitchen sink and slop-sink waste lines. What problems does double-trapping cause? Siphoning-out both?

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[hausdok]

That's not going to siphon any trap dry, it's too far away from the first trap to do that.

Don't know why anyone would put a trap there.

OT - OF!!!

M.

[Bain]

Plumbers call that a "running trap," and it isn't allowed here. It's shinier than the other PVC; probably a well intended improvement by a Bubba.

[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

[fqp25]

Somebody could have installed that running trap to serve a floor drain or condensate drain they added "after the fact". (Post Construction). I've seen that a lot.

Is that Tee a vent or another drain line?

[Jerry Simon]

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

Yes.

[Jerry Simon]

Originally posted by fqp25

Somebody could have installed that running trap to serve a floor drain or condensate drain they added "after the fact". (Post Construction). I've seen that a lot.

Is that Tee a vent or another drain line?

Drain.

[Chris Bernhardt]

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

[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(?).

[Jim Katen]

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: MissingOffset .JPG

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[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?

[Jim Katen]

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: FlowguardGoldpg8.pdf

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[hausdok]

Hi Jim,

You forgot the attachment, no? By the way, isn't the Floguard manual somewhere in the TIJ library?

OT - OF!!!

M.

[kurt]

That explains why I've never seen one. No PVC pipe in commercial structures around here; it's all iron & copper.

[Chris Bernhardt]

From the 2006 Uniform Plumbing Code Section 705.3 Special Joints

705.3.2 Expansion joints. Expansion joints shall be accessible, except when in vent piping or drainage stacks, and may be used where necessary to provide for expansion and contraction of the pipes.

BTW I have never seen one.

Chris, Oregon

[AHI in AR]

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...

[msteger]

The reason the trap was installed where it is located is because Bubba ran the pipes from either end and cut the pipes before realizing that he was a little short. All he had left over was some small fittings and a trap, so Bubba primed and glued them to make the two PVC lines meet.

[hausdok]

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.

ONE TEAM - ONE FIGHT!!!

M.

[Tedlawson]

The house trap was on no account planned to take the place of individual fixture traps, and does not achieve the same job as they do. It was simply to keep sewer main sewer gases out of the house structure.