Does every sink in a house need a vent?

A vent is a necessary part of the drain system for any plumbing fixture. Its purpose is to equalize pressure in the pipes and prevent a vacuum from forming as the fixture drains.

thespruce.com - How to Install an AAV (Air Admittance Valve) for a Sink - The Spruce

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A vent is a necessary part of the drain system for any plumbing fixture. Its purpose is to equalize pressure in the pipes and prevent a vacuum from forming as the fixture drains. Without venting, the negative pressure caused by the flow of draining water can potentially suck water out of the drain trap and allow sewer gases to enter the home. The vents allow air into the drain pipes to help keep the drain flowing properly. In some instances, though, properly venting a drain in the traditional way—with pipe that connects directly to an outdoor air supply—can be difficult to do.

How a Traditional Vent Line Works

In most houses, the drain line for each sink extends horizontally back into a wall, where it fits into a sanitary tee. One outlet on the tee extends downward into a vertical drain line, and the other extends upward into a vent system that is open to the outside air. This exposure to fresh air releases sewer gases and allows fresh air into the system to help water drain quickly. Sometimes, though, it is difficult to connect fixture drains directly to an external vent pipe. In mobile homes, for example, there may be no external vents at all. And in homes with island sinks with no cabinetry above the island, it can be difficult to find a route for linking the sink drain to an external vent. In these instances, an alternative method of equalizing air pressure is required. This is where the AAV comes in. An air admittance valve (AAV), sometimes called an auto vent, is a device attached to the fixture drain line. It has a mechanism that opens up to admit room air into the drain under the force of the negative pressure caused by water flowing through the drain. With this negative pressure dissipated, the water in the drain trap cannot be siphoned off. Once the water stops flowing and the negative pressure ends, the AAV device automatically closes to prevent sewer gases from entering the room. Tip Rules on the use of AAVs vary widely from state to state. While the trend seems to be toward gradual acceptance of these devices, some state building codes still do not allow air admittance valves as a replacement for traditional direct vents. Other states limit the number of AAVs that a single home can use. Always check with your local building inspections office for advice on what venting strategies are allowed.

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When to Install an AAV

Where allowed by code, AAVs can be used with any type of sink or fixture. Some sinks will give hints that an AAV is necessary. A sink that gurgles loudly, for example, or one that drains very slowly even though there are no clog issues, might be doing so because of negative air pressure in the lines. Adding an AAV often resolves these issues. Especially where there is no direct connection to the vent system, as is often the case in sink island situations, installing an AAV may improve the function of the drain. If the drain already has an AAV, such symptoms might indicate that the valve needs to be replaced.

Before You Begin

Installing an AAV to vent a sink is fairly easy, whether you are replacing an old existing vent or putting one in for the first time. It basically involves tapping into the existing drain trap configuration to install an AAV via a new sanitary tee fitting and a short upward extension pipe. While it is possible to install an AAV hidden inside walls during remodeling work, it is more common to install them right under the sink cabinet as an adaptation of the pipe joining the drain trap to the branch drain. With each sink and each drain configuration slightly different, you should be ready to adapt while installing an AAV. As every home plumber knows, it's not uncommon for additional adapter fittings and other parts to be necessary to fit unique circumstances.

thespruce.com - How to Install an AAV (Air Admittance Valve) for a Sink - The Spruce

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