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Double Traps


msteger
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I know the plumbing code forbids double traps, but why exactly? Does having two traps in series allow for easier siphoning or what? I am drawing a blank.

This morning, the dual kitchen sink had a trap in the drain line that both sinks drained to, but under one sink was also a garbage disposal. The drain line coming out of the disposal immediately went into a trap, then ran laterally to meet the other sink's drain, and then they went into the common trap.

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I know the plumbing code forbids double traps, but why exactly? Does having two traps in series allow for easier siphoning or what? I am drawing a blank.

This morning, the dual kitchen sink had a trap in the drain line that both sinks drained to, but under one sink was also a garbage disposal. The drain line coming out of the disposal immediately went into a trap, then ran laterally to meet the other sink's drain, and then they went into the common trap.

The idea is to allow the waste products to drain, not to make them play a round of miniature golf.

Two traps in series makes for an unnecessarily tortuous drain path.

- Jim Katen, Oregon

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If you don't install a vent between two traps then the air cannot get behind the first trap.

In AC, I sometimes put a 3/4" trap on the condensate line near the evaporator (cooling coil) to keep air from navigating the condensate drain line. If a plumber's trap is also installed on the house's plumbing for the AC condensate line then it's necessary to open the condensate line with a 'T' and a short vertical section of pipe between the two traps, otherwise you won't get the effect you wanted. It may also result in problems and a service call later on.

Marc

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  • 2 weeks later...

Well, for the first time in a long time I saw a sink drain with 2 traps. This sink was on the 2nd floor bath.

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Lo and behold, I went to the first floor sink and no trap at all.

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tn_20102710932_MineHill%20030.jpg

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My question is which sink did he install first?

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