Why A Slow Drain Might Be Due To Insufficient Air Instead Of A Clog

29 August 2018
 Categories: , Blog


Plumbing noises that last for only a brief duration are easy to ignore. A plumbing noise accompanied by a slow drain, however, is more likely to warrant a closer look. A slow drain is not always indicative of a clog somewhere along the drainpipe system. A slow drain may be due to an inadequate intake of air into the drain.

As gravity pulls water through a relatively narrow drainpipe, there must be a supply of incoming air to equalize the pressure inside the pipe. Without adequate air intake, a slight vacuum builds behind the water and slows it down. Although most of the vent plumbing in a typical house is hidden from view, the vent system usually begins with an open-ended outside pipe.

Source of ventilation air

The outside vent pipe is connected to each water fixture drain through a network of pipes. Air freely flows into the drain system as needed. The outside vent can potentially become blocked, but a slow drain may be due to incorrectly sized vent pipes. If you are experiencing a slow drain accompanied by a siphoning sound, that particular water fixture drain may need an increased amount of ventilation air.

Ventilation for each water outlet

The drain for each of your interior water fixtures may have its own vent pipe connected to the central drainpipe. Professional plumbing codes specify the minimum size for individual vent pipes, based on the distance between the water fixture and the central drainpipe. If all of the vent pipes in a house are properly sized, only one outside air inlet is necessary.

A larger vent pipe diameter allows a greater distance between a water outlet and the main drainpipe. The diameter of the vent pipe for your troublesome drain may be too small. To solve the drainage problem, the undersized vent pipe can be replaced with a correctly sized pipe. Instead of pipe replacement, you might prefer to install a specialized intake valve near the affected water fixture.

Air admittance valves

Several manufacturers produce a plumbing product referred to as an air admittance valve. The one-way valve is installed directly on a water fixture drain, reducing the need for an incoming vent pipe. The valve opens in response to negative air pressure, drawing air into the drainpipe. The valve closes when the air pressure in the drain is stabilized.

The siphoning force caused by a large volume of water could potentially pull the water from a sink trap, defeating the purpose of the trap. Contact a plumbing contractor for more information about plumbing services or visit websites like http://www.rkknightplumbing.com.