How Vents and Traps Work
At first glance, drains seem to work even without traps and vents. Plenty of do-it yourselfers leave out these key components, without understanding their importance.
• Traps hold enough water to form an airlock against sewer gases.
• The trap in each fixture, when properly vented, provides a liquid seal that prevents sewer gases from entering the building.
• Vents allow sewer gases to flow up the soil stack and exit the DWV system without coming into your home through plumbing fixtures.
• Vents equalize pressure to aid drainage and allow sewer gases to escape to the outer air.
• Without a properly functioning trap and vent, high or low pressure in the drains may create unsafe conditions.
• The vent system allows outside air into the drain system to keep vacuums from forming in the drains. Think of vents as upside-down drains with gasses rising in the pipes in the same way that water flows down them.
• Each plumbing fixture should also have a vent that allows odors and sewer gases to escape and atmospheric pressure to enter, thus preventing backpressure when water fills the pipes.
High pressure
• Without venting, high pressure in the drains may force sewer gas out through traps and toilets.
• You’ll probably hear gurgling and bubbling in the fixtures.
• You may also smell the vile bacteria odor mixed with sewer gas.
Low pressure
• Without proper venting, lower pressure in the drains may cause siphoning in the traps whenever you drain fixtures.
• If the traps are dry, sewer gas vents directly into the house.