How do overflows prevent sewer gases from entering the dwelling?

Section 4.8.2 of the text states the following but doesn’t elaborate:

The overflow is installed so as to prevent sewer gases from the drainage system from entering the dwelling.

Can someone please explain how and why this is the case? How does having an overflow prevent gasses from entering the building?

If an overflow is installed in the sink basin, how does gas not travel through it? Why is the gas stopped at the tailpiece because there is an overflow?

4.8 Overflows

An overflow is designed to prevent fixtures from flooding (or overflowing into) the area when the water-tight stopper is closed or the drain is obstructed. A stopper must not interfere with the overflow opening or the water overflowing from the fixture.

If you read the context of the section you quoted it states that the inlet should be on the house side of the trap.

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Sewer gas is stopped at the P-trap. They could have written the section a little bit better. The intent was to explain to you that the overflow does not go to the sewer side of the P-trap but rather the occupant side of the tailpiece.


OK, that makes a lot more sense! Thank you!


It doesn’t.
What prevents sewer gas are plumbing traps.

In plumbing, a trap is a device which has a defined shape by utilizing curves or/and angles along a pipe path, to abduct gray or waste water, and prevent backflow or gases, vermin or insects from entering a building, while allowing any gray or waste water to continue flow downstream.


Plumbing Traps, P-Trap

Just below a Sink or Wash Basin is the Tail Piece. An Inlet drain pipe mounted just above the Trap Adaptor connected to the Plumbing Trap.

The Plumbing Trap below the Trap Adapter in this image is referred to as a P-Trap, which in of itself, is a Plumbing Fixture and device.

P-trap consists of the Inlet, Trap Adapter, Top & Lower Dip, P-Trap, Cleanout, Trap Weir Seal or Crown Weir Seal below the Trap Crown, Outlet and Trap Arm.

A Plumbing Trap, by definition, is a device that retains a measured amount of water. When, in this case, lavatory fixtures are utilized, an amount of water is retained in the P-Trap while the rest of the water is drained. The water that remains in the P-Trap device is referred to as, The Trap Seal.

P Trap serves several functions.
1: Backflow prevention.
2: Trap Debris.
The Trap Seal serves several functions.
1: Prevent Sewage Odors & Gases, a complex mixture of toxic and nontoxic gases produced and collected in sewage systems, from escaping/dispersing into the room.
2: Impeding Vermin, Mice and Insects from exiting the drain pipe into your home.

The above image or a P-Trap was provided by InterNACHI, International Association of Certified Home Inspectors. The world’s largest inspection trade association founded by Nick Gromicko.

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