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Bow Vent This vent gets its name from the loop of drainpipe that runs above the entry point of the sink trap. A bow vent can be located ahead of the vent stack or behind the vent stack (Figure 4).

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Figure 4. A bow vent may be approved by the plumbing inspector, although it gobbles up cabinet space.

Both arrangements require a cleanout. A bow vent gobbles up considerable cabinet space, and when a large multibowl sink is placed in a small base cabinet, the only alternative may be to install the piping in an adjacent cabinet. You should get approval from your local inspector before installing a bow vent.

Drum Trap

Although they’re not commonly used now, some inspectors will allow a drum trap (Figure 5), but only if it is installed with an accessible cleanout cover.

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Figure 5. A drum trap may be approved for island drainage as long as the cover is accessible from the living area.

A drum trap can save room within the island cabinet because the drainpipe drops straight down to the drum trap. Since the drum trap must be located near the sink, the access cover will end up in the cabinet floor. Another option is to provide a floor hatch that allows access to the trap, but most clients will balk at this approach. I’ve seen cases where plumbers install the drum trap upside down to allow access from the basement. The result is an inverted cover that will ooze gook for years until it finally fails.

Automatic Vent

Sometimes referred to as a bladder vent, this type of vent functions as a one-way check valve that lets makeup air into the system as the sink drains, then closes automatically, sealing out sewer gases. Automatic vents are typically glued to the end of the short vertical vent pipe that rises above the sink trap. I’ve yet to run into a plumbing inspector that thinks much of these contraptions. If an automatic vent were to fail, sewer gas could enter the living area (a great conversation starter at a party).

Don’t Forget the Dishwasher

Installing a dishwasher drain is easy: Just fasten the flexible drain line to the projecting stub on the sink, right? Not so fast. Most plumbing codes require that a portion of the flexible drain line be located at least 20 inches above the discharge outlet on the dishwasher pump. Some codes specify that an air-gap vent be installed to prevent the sink or disposal from siphoning the water seal out of the dishwasher pump. The trouble with an air-gap vent is that it must be installed above the rim of the sink. There are versions available that will screw into a blank knock-out in the sink rim, but if you’re installing an exotic one-piece counter-and-sink, you may be forced to drill an unplanned hole in an expensive countertop. The venting strategies mentioned above are supplements to proper system vents — not substitutes. Before you say yes to a customer’s request for an island sink, check the distance to the nearest vent, and get approval from your local plumbing inspector for your plan of attack.