Do You Have to Have Dishwasher Venting?

Why Vent

• In addition to detergents, the sanitary sewer carries bits of food, saliva and other organic compounds away from the dishes and flatware in your dishwasher. As the washing and rinsing take place, chemical reactions create gases, including methane, that bubble through the wastewater and rise as the water travels through the drains. Vents provide escape routes for these gases while allowing fresh air to enter the waste pipe and equalize the negative pressure created by the passage of water. Any appliance that uses and discards water in your home plumbing requires venting.

Building Vents

• Water supply lines run from one source to fixtures in homes. Drains join vertical stacks that extend up to the roofs as well as downward to sewers. The number of vents depends on the number of water-using appliances and their placement. To minimize the number of vents breaking through the roof, stacks are centrally located in relation to the fixtures. The trap, a U-shaped pipe in the drain, must be no more than 5 feet from the vent stack so that water remains under pressure, sealing sewer gases in the vent.

Venting Dishwashers

• Dishwashers must attach to vents because they expel water laden with decaying organic compounds. Because dishwashers are commonly installed next to a sink for ease of loading, they can share water sources and waste outlets. Wastewater travels through the garbage disposals or to the sink drainpipes above the traps. Your dishwasher’s drain line loops higher than the drain connection on the sink or garbage disposal, marrying to the sink drain and blocking any inflow of waste from the sink or disposal. The sink trap serves both the sink faucet and dishwasher.

Air Gap

• Some dishwashers use devices called air gaps that act as a backflow preventer before the waste lines enter the sink drain lines. The drain lines rise from the dishwasher through the kitchen counter. They are capped by a metal vent and then drop down below the counters and connect with the disposers or main sink drains. The devices may vent steam in some models. If they pour water into sinks, they may signal blocked inlets in disposal or clogged traps.