How to Install a Dishwasher Air Gap: Code and Steps

What an air gap does

An air gap is a backflow prevention device. It creates a physical separation between the dishwasher drain line and the home drain system. If the sink drain or garbage disposal becomes clogged, waste water can rise up the drain line. Without an air gap, that water could flow back into the dishwasher and contaminate clean dishes or flood the appliance.

The device is a small metal or plastic cylinder with two hose barbs on the underside: a larger inlet that connects to the dishwasher drain hose and a smaller outlet that connects to the disposal or drain tailpiece. A vent hole on the side allows air to enter, which breaks the siphon that would otherwise pull water backward.

Air gaps are required by plumbing code in many states and municipalities for built-in dishwashers. Check with your local building department before starting. For a general overview of how dishwashers connect to your home's plumbing, see dishwasher drain options and what is a food disposer dishwasher.

Tools and materials

Gather everything before you begin. You will need the air gap kit, which typically includes the body, the chrome or plastic cover, a mounting nut, and sometimes a rubber gasket. You will also need a length of 7/8-inch drain hose, two or three hose clamps, a screwdriver, plumber's putty or silicone sealant, and a drill with a hole saw if you are cutting a new mounting hole.

If the countertop is stone, granite, or tile, use a diamond hole saw and keep the surface wet while cutting. For laminate or solid surface, a standard hole saw works. The mounting hole is typically 1-1/2 inches in diameter, but check the instruction sheet that came with your air gap kit.

Step 1: Mount the air gap on the countertop

Choose a location on the countertop near the sink, usually within a few inches of the backsplash or at the rear corner of the sink deck. The air gap must be above the sink rim and in a spot where a small amount of water can escape without causing damage if the drain ever backs up.

Apply a bead of plumber's putty or silicone sealant around the underside of the mounting flange. Insert the body through the hole from above. From below, slide the rubber gasket and then the mounting nut over the threads, and hand-tighten. Finish with a gentle turn with pliers. Do not over-tighten; the body should be snug and oriented so the two hose barbs point toward the back of the cabinet.

Place the decorative cover on top. It should sit flush against the countertop. If it rocks, the mounting nut is uneven. Loosen, reposition, and retighten.

If you are running the drain line through a wall or under the floor rather than to a disposal, see dishwasher drain hose high loop for how the routing differs.

Step 2: Connect the dishwasher drain hose

Locate the drain hose exiting the dishwasher. It is the corrugated hose at the back, usually on the left or right side. Route the hose up from the dishwasher and into the cabinet below the sink. It should rise as high as possible before connecting to the air gap; this upward path is part of the backflow protection.

Push the dishwasher drain hose onto the larger inlet barb on the air gap body. It should slide on firmly. Secure it with a hose clamp and tighten the screw with a screwdriver or nut driver. Do not use the factory zip tie or a twist tie in place of a proper clamp.

The dishwasher will push water through this hose during the drain cycle. Any kink or pinch in the hose will cause water to back up in the tub. For tips on keeping the line clear, see dishwasher drain hose cleaning and how to prevent dishwasher hose kinking.

Step 3: Route to the disposal or sink drain

Run a second length of 7/8-inch drain hose from the smaller outlet barb on the air gap down to the garbage disposal or the sink drain tailpiece.

If you have a disposal: remove the knockout plug in the disposal inlet. If you skip this, water cannot enter the disposal and will back up into the sink. This is the most common cause of a new installation backing up. Push the hose onto the disposal inlet and secure it with a hose clamp.

If you do not have a disposal: connect the hose to a Wye fitting or a dedicated inlet on the sink drain tailpiece. Many tailpieces have a small inlet stub for this purpose. If yours does not, install a dishwasher Wye adapter between the tailpiece and the trap. Clamp the hose in place.

The drain hose from the air gap should slope continuously downward. It must not dip or form a low loop, because water would pool there and eventually grow mold. For related guidance on keeping the system clear, see dishwasher air gap leaking and dishwasher sewer smell drain.

Step 4: Test the installation

Turn the water supply back on and run a short rinse cycle. Watch the air gap while the dishwasher drains. A small trickle of water from the vent holes on the side of the air gap is normal during the drain cycle. A steady stream or water spilling from the top cover indicates a restriction downstream.

Check every hose clamp for drips. Wrap a paper towel around each connection and look for moisture after the cycle finishes. Tighten any clamp that shows a leak.

Listen for a gurgling sound from the sink drain. A little noise is normal. If water backs up into the sink, the drain line is restricted or the disposal knockout was not removed.

If you are installing a new dishwasher at the same time, see dishwasher first time use setup for the steps to prepare the appliance itself. For the broader connection picture, see dishwasher plumbing connection leak check.

When you don't need an air gap

Some local codes allow a high-loop installation instead of an air gap. In this arrangement, the dishwasher drain hose is routed up as high as possible under the counter, secured near the underside of the countertop, and then dropped down to the disposal or drain. The loop traps air and prevents siphoning. It is considered less reliable than an air gap because the loop can be accidentally pushed down, but it is accepted in some jurisdictions.

Check your local plumbing code before choosing this option. Even if your area allows a high loop, an air gap is the more protective choice. Many homeowners install one at the same time as a new dishwasher. If you are shopping for one, see best built-in dishwashers and best 24 inch built-in dishwashers for models that pair with a standard air gap installation.

What to pick for your installation

Questions

Do I need an air gap for my dishwasher?

It depends on your local plumbing code. Many states and municipalities require an air gap for built-in dishwashers. Some allow a high-loop drain instead. Check with your local building department before installing. An air gap provides better protection against backflow.

Where does the air gap mount?

It mounts through a hole in the countertop or sink deck, usually near the back of the sink. The body sits below the counter, and a decorative cover sits on top. The position must be above the sink rim and visible so you can spot a leak.

Which hose connects to which barb on the air gap?

The larger inlet barb connects to the dishwasher drain hose. The smaller outlet barb connects to a second hose that runs down to the garbage disposal or the sink drain tailpiece. Mixing them up will cause a leak at the vent holes.

Do I have to connect the air gap to the disposal?

No. You can connect the outlet hose to a Wye adapter on the sink drain tailpiece if you do not have a disposal. If you do have a disposal, the knockout plug in the disposal inlet must be removed or the water cannot flow through.

Why is water coming out of the air gap?

A small trickle during the drain cycle is normal. A steady flow or water spilling from the top cover means the drain line is restricted, the disposal knockout was not removed, or the outlet hose is kinked. Clear the blockage and straighten the hose.

Can I install an air gap on a countertop dishwasher?

Countertop dishwashers do not use air gaps. They connect directly to a faucet adapter and drain into the sink. Air gaps are for built-in dishwashers that tie into the under-sink drain. See what is a countertop dishwasher for more.