Plumbing Vents

• The plumbing vent pipes that extend through the roof of a typical residence allow sewer gases to escape into the air outside the house, but that is not their main purpose. Their primary function is to allow air into the waste system to replace the vacuum created by flowing water. Without venting, the flow of water creates a vacuum in the otherwise airtight pipes, and that vacuum can develop enough force to slow the flow of water or even stop it. When the force is strong enough, it sucks air through the only available openings, which are the P-traps of sinks and toilets.
  
  

The Force of a Toilet Flush

• When you let water out of a sink into an unvented waste line, it will flow slowly and may even stop altogether, giving the impression of a clogged pipe, but it probably won't affect other fixtures connected to the line. Flushing a toilet or emptying a bathtub, however, releases a large amount of water at one time, and as the water drops through the vertical waste stack, it creates considerable suction behind it. In the absence of venting, this suction can easily have enough force to empty P-traps connected to the same waste system, including the one inside an adjoining toilet.
  
  

The Remedy

• You can stop one toilet from emptying when you flush another by clearing the vent lines. It isn't unusual for vent openings to become obstructed by leaves or other debris, especially in the presence of overhanging trees, and the problem is easily corrected by climbing on the roof and clearing the debris. Debris can also collect inside the lines, and you can usually clear it by spraying a jet of water into the vent opening with a garden hose. If the water backs up, however, you may need to clear the lines with a plumbing snake.
  
  

Considerations

• Before the vents get blocked to the point where the suction is great enough to empty a toilet, you'll usually be able to notice other symptoms of restricted air flow into your waste system. Such symptoms include gurgling drains when you flush a toilet or drains that appears to be clogged despite your efforts to clear them. It's a good practice to regularly check your vent openings for obstructions and clear them or, if they get frequently blocked, to affix a cap or screen to the openings to keep them clear. Besides emptying P-traps, poor venting can also cause actual clogs that may be difficult to remove.