Traps With No Vent Are Common

We inspect modern and old plumbing systems, and all plumbing traps should have a vent, right? We check for non-vented fixtures every day and document them as defects and amateur work. We know a poorly vented trap or an S-Trap will gurgle as it sucks air and water down the trap. Some create a strong suction as the water flows down the trap and pipe. With no water in the trap, we have lost our trap seal, and we have a sewer smell and a safety issue.

All Plumbing Systems Depend on Vents
All modern drainage waste and vent (DWV) systems are designed to allow air into the top of the system to help water drain. I like to think of a DWV system as a straw. You can fill a straw with water and stand it up, but if you block the upper end with your finger, no water will drain. When you remove your finger, air enters the straw and the water flies out.

Re-venting
When a trap or fixture is more than a few feet from the main drain stack, there is a potential for the horizontal run to be filled with water, creating suction on the trap. To prevent this, air is provided to the horizontal run with an additional vent pipe. An air admittance valve also may be used to allow air into the drain pipe. This is a check valve that allows air in and seals against sewer gas leaking out.

Kitchen Island Sink
For a kitchen island, there is no convenient way to vent the horizontal run. You may see a vent pipe that runs below the island to a vent stack in the framing below. With more modern systems, you may see an air admittance valve below the island sink.

But Every Home Has a Non-vented Trap
But…here it comes… every modern toilet is a non-vented trap. Have you ever thought about that? A modern toilet depends on no vent and a rush of water to create suction and to clear the bowl and the trap of waste and water. A toilet also depends on a trap full of water to prevent smell and safety issues related to sewer gas.

The key with a toilet is that, after every flush, the toilet automatically refills its trap with water. After the water in the tank is used to flush the bowl, water begins to fill the tank. Part of the tank fill system uses a small tube that directs water into the overflow tube in the tank. This water bypasses the tank and flows directly into the bowl and refills the trap.