Correct Plumbing for Garbage Disposals

Single-Basin, No Dishwasher

• The easiest installation process occurs beneath a single-basin sink without an adjacent dishwasher. The disposal's mounting assembly attaches to the sink basin's strainer and the disposal's upper opening threads onto the mounting assembly's screw-like threads. A specially designed drain outlet pipe, included with the disposal unit, attaches to the side of the disposal via a screw-mounted plate. Curved at a 90-degree angle, the drain outlet pipe extends downward to meet and join with the sink basin's existing P-trap.

Single-Basin With Dishwasher

• The presence of a dishwasher adds several steps to the single-basin installation procedure. Most dishwashers drain directly into a garbage disposal unit via a dedicated drain inlet. Before connecting to the disposal, dishwasher drain hoses attach to a ventilation device, called an air gap, that protrudes from the sink basin's rim. To attach a dishwasher drain hose to a garbage disposal, plumbers must punch the knock-out plate, a thin metal barrier, from the dishwasher drain inlet at the disposal unit's side. They then run a short drain hose from the disposal to one side of the air gap's Y-shaped fitting and a longer hose from the air gap to the dishwasher. Dishwasher hoses typically connect to fittings via hand-operated spring clamps.

Dual-Basin, No Dishwasher

• The separate branches of a dual-basin sink connect at a Y-fitting before draining into a shared P-trap. Installing a disposal beneath a dual-basin sink is nearly identical to installing a disposal beneath a single-basin sink. However, whereas the disposal drain pipe beneath a single sink connects directly to the P-trap, the disposal drain pipe beneath the double sink connects to an extension pipe. Like a P-trap, the extension pipe joins with the disposal drain pipe via a slip-joint compression fitting.

Dual-Basin With Dishwasher

• Installing a disposal beneath a double sink that adjoins a dishwasher resembles installation beneath a single sink with a dishwasher. The disposal unit accepts the air gap's short drain hose from a side inlet and a long drain hose runs from the air gap to the dishwasher unit. Additionally, the disposal's drain outlet connections resemble a standard dual-basin installation arrangement. However, to reduce drain hose length and potential leaks, plumbers generally install the disposal beneath the basin closest to the dishwasher.