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From documentation left with our house we know that our sump pump used to empty into the sanitary sewer. In the early 90’s the home owners took advantage of a city grant that paid for moving the drain to the storm sewer. We assume that the pictured drain pipes were installed at that time. We have a receipt that indicates the work was done by a professional plumber and we suspect that it was probably inspected by the city at the time.

In the lower right of the picture, the drain leaves the front of the house toward the storm sewer. Our front yard slopes downhill, so this level should be comfortably above the level of the storm sewer itself. From the upper left corner of the picture the pipe continues through the ceiling to the back of the house where it drops fairly directly into the pump.

Question 1 - What is the purpose of the upside-down U bend and the split into two separate pipes which then rejoin halfway down the wall?

Question 2 - If I were to simplify this pipe, is there something that might go wrong?

I've asked two different professional plumbers who were in the house for other reasons and neither had any ideas.

Strange bend and split in sump pump drain pipe

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  • The answer to the second question is dependent on the answer to the first question. – Alaska Man Oct 31 '20 at 18:11
  • "split into two separate pipes which then rejoin halfway down the wall?" I would guess that to be some kind of venting. The upside down trap is a real head scratch-er, possibly to keep water from flowing back to the pump but why there and not where it turns down to the pump? – Alaska Man Oct 31 '20 at 18:15
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    Why do you want to simplify it? Is there a problem you are trying solve? – Alaska Man Oct 31 '20 at 19:34
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    I’m insulating the rim joist of the house. Despite appearances, the joist behind that plumbing is exterior facing. In order to put insulation there, I was thinking of moving the drain pipe over to the next joist bay. – codegardener Oct 31 '20 at 20:27
  • You can move it one bay over and not reconfigure it. If you do not know why it is configured that way then best not to change it, move it but don't change it. ( If it taint broke, don't fix it ) – Alaska Man Nov 1 '20 at 17:45
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"Professionals" can be idiots too. That serves no purpose and, accordingly, has no code requirement. You can cut it out, move it and all you need to do is ensure you have a check valve near the exit from the sump pump and a long 90 (avoid a hard 90 with a drop like this) to the entrance to the lateral. You should install a clean-out near the exit from the building as well, i can't tell if it's there or not. Make sure all joints are properly glued, and the clean-out threads are taped, when done right you'll be protected from any potential back flow.

I'm certain this was done after the initial install. My guess is that a previous homeowner remodeled the original work with the idea of some kind of a water hammer arrestor or perhaps they had some backflow and didn't know how to actually prevent it... is there a check valve on it now?

I can tell from the various parts, like the different-colored/cheapo-corner-hardware-store wyes, the coupling above the lower wye, the flexed upwards pipe before the upside down trap, and the not to code opposing 90s at the entrance to the lateral, that whoever installed this had no idea what they were doing but apparently had some money to throw away on a little m.c. escher inspired plumbing project. I highly doubt this is original work or the work of a plumbing journeyman... unless they were tripping.

Rip it out, run it straight, level, plumb, and include the components noted above.

You do not need a backflow preventer as long as the joints are correctly sealed, the pipe runs above the level of the highest point in the system serving your area, and there is no reason to suspect backflow forces in excess of the joint strength. For the uninitiated

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