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I realize this question is not about "home" improvement but as I had to deal with drain and vent lines recently, I was curious to note this installation at the Costco.

There is what it seems a condensate drain, with a P-trap and an air-gap at the bottom. Why the P-trap? What is the purpose, it should be AFTER the air gap otherwise I do not see how it helps anything:Note the P-trap on the top right cornerNote the P-trap on the top right corner

Detail of the drain on the base, see the airgapDetail of the drain on the base, see the airgap

  • I would think it's so that whatever's on the other end of that pipe can't have gasses flow down the pipe and out the air gap near the drain. Perhaps the other end is outside on the roof to drain rainwater, and they don't want a draft of outside air? – Nate S - Reinstate Monica Mar 22 at 20:02
  • I’m going to guess that the trap is part of the kit that comes with the walk-in refrigerator, and the floor level air gap was a local requirement. – Tyson Mar 22 at 23:57
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Assuming that it's a condensate line for rooftop HVAC equipment, the trap is required to prevent contamination of the habitable space by exhaust gases. It's also possible that an inducer fan could draw air through an un-trapped drain.

  • The line comes out of the refrigerator section you see in the picture, it does not come from above the roof. – Alessio Sangalli Mar 25 at 21:01
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It is called an "air gap" and it is installed on drain lines when required by a building code to insure that sewage or other drain water will not back up a drain line and contaminate a drinking water source or a system that is used to provide an atmosphere of conditioned air to a space.

  • Op is asking about the trap high up on the wall not the air gap at the bottom – Kris Mar 23 at 17:34
  • OK, the "P" trap above is probably overkill except a long vertical pipe rising to near the ceiling could act as a chimney and draw sewer gases up that pipe. With the "P" trap filled with water, that will not happen. All this stuff is in the code book, – d.george Mar 23 at 20:13

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