S-traps can lead to siphoning

Plumbing · Floor Drains · printed page 291 (PDF page 292)

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S-Traps Can Lead to Siphoning Plumbing

Text in the illustration

no vent pipe or mechanical vent provided as the last bit of water flows down the drain, a vacuum flows at the top part of the trap air pressure from the basin pushes the water out of the bottom of the trap, allowing odors into the home S-traps can lead to siphoning
no vent pipe or mechanical vent provided
as the last bit of water flows down the drain, a vacuum flows at the top part of the trap
air pressure from the basin pushes the water out of the bottom of the trap, allowing odors into the home

Generated explanation

The illustration shows a basin draining through an S-trap into a vertical discharge pipe. A dashed outline marks the location of a missing vent pipe or mechanical vent. Arrows trace flow through the trap and down the discharge leg, illustrating siphoning as the last water drains. The annotations explain that a vacuum at the top of the trap and air pressure from the basin can remove the trap's water seal, allowing drain odors into the home.

Components

basin basin drain tailpiece S-trap vertical discharge pipe slip-joint connections trap cleanout plug

Defects shown

Unvented S-trap Missing vent pipe or mechanical vent Siphoning of trap water seal Drain odors entering the home through a depleted trap seal