Why is water leaking out of my air gap onto the counter?

Water overflowing from the air gap cap means the outlet side of the drain — the hose running to the garbage disposal or drain — is partially blocked. The air gap is functioning correctly by releasing water rather than letting it back-siphon into the dishwasher. Clear the blockage at the garbage disposal knockout, check the outlet hose for kinks, and clean both ports on the air gap body.

Do I need an air gap for my dishwasher?

It depends on your local plumbing code. Many jurisdictions require a dedicated air gap device at the sink rim. Others accept a high-loop drain configuration as an alternative. California requires an air gap by code. Check with your local building department before using only a high loop — an inspector can fail a dishwasher installation that lacks a required air gap.

How do I clean a clogged air gap?

Remove the decorative cap (twist or pull off), then unscrew the plastic body cap beneath it. Use a small bottle brush or pipe cleaner to clear debris from both the inlet and outlet ports. Flush with water to confirm both sides drain freely. Reassemble and run a dishwasher cycle to verify the overflow has stopped.

Does replacing an air gap require a permit?

No permit is needed to replace an existing air gap in kind. It is a maintenance repair. Installing an air gap for the first time as part of a new dishwasher installation may be covered under the appliance installation permit or a minor plumbing permit, depending on jurisdiction.

What is the difference between an air gap and a high loop?

An air gap is a physical device that creates a true air break — the two water paths never connect. A high loop routes the drain hose up as high as possible under the counter before dropping to the disposal, relying on gravity rather than a physical gap. Air gaps offer stronger protection and are required by code in many areas; high loops are a code-permitted shortcut in jurisdictions that allow them.

Dishwasher Air Gap — Backflow Prevention at the Sink

An air gap is a backflow-prevention device mounted at the sink rim that creates a physical break in the drain line between the dishwasher and the household drain, preventing contaminated water from siphoning back into the dishwasher.

Air Gap (Dishwasher) diagram — labeled parts and installation context

What It Is

An air gap is a small chrome or decorative fitting that sits on the sink deck or countertop, typically next to the faucet. Inside the body, two separate ports are separated by a physical air space — the inlet receives the drain hose from the dishwasher, and the outlet connects to the garbage disposal or drain tailpiece. Because the two water paths never touch, a backflow event at the drain side cannot push contaminated water back into the clean dishwasher tub.

Air gaps are the only backflow prevention method for dishwashers that is accepted in all US jurisdictions. Some codes allow a high-loop alternative (routing the drain hose in an elevated arch under the countertop), but many local plumbing codes — including California under CPC Section 807.4 — require a dedicated air gap device at the rim. When an air gap is working properly, it is invisible and silent. When the drain is slow or partially blocked, water can overflow out of the air gap cap and onto the counter. This is the device doing its job — it is telling you the drain is backed up. The overflow is a diagnostic signal, not a malfunction, and clearing the blockage downstream will stop it.

Types

Standard countertop air gaps are the most common residential type. They mount through a standard sink-deck knockout hole — typically 1-3/8 inches in diameter — and rise about two inches above the counter surface with a chrome, brushed nickel, or oil-rubbed bronze decorative cap.

Undersink air gaps mount beneath the countertop for installations where no deck hole is available or where the homeowner prefers a clean countertop surface. These units attach to the cabinet sidewall and accomplish the same physical break between the dishwasher drain hose and the outlet hose.

Dual-inlet air gaps accept drain hoses from two appliances — typically a dishwasher and a water softener or reverse-osmosis system — combining both into a single fitting with separate inlets and one shared outlet. They are slightly larger than single-inlet units and require a larger mounting hole.

Where It Is Used

Air gaps are installed on kitchen sink decks, countertops, or the sink rim, directly in line between the dishwasher pump drain hose and the garbage disposal or drain tailpiece. The inlet hose from the dishwasher is typically 5/8-inch inside diameter, and the outlet hose running to the disposal is 7/8-inch inside diameter. They are a standard fitting in dishwasher drain systems wherever local plumbing code requires them.

In new construction, the sink deck knockout for the air gap is pre-punched by the sink manufacturer. In retrofit installations, a hole must be drilled through the countertop or the sink rim if one is not already present. Granite and quartz countertops require a diamond-core bit for this hole.

How to Identify One

Look for a small chrome or brushed-metal cap on the sink deck near the faucet — often mistaken for a soap dispenser or side sprayer. The cap is typically 1-1/2 to 2 inches in diameter and rises about two inches above the counter. Lift the decorative cap off and you will see a plastic body with two hose barb ports of different diameters. Tracing the hose from the dishwasher under the sink will also lead directly to the air gap inlet.

If you see only a single hose looped up high under the countertop without a fitting at the deck, the installation uses a high loop rather than an air gap. A high loop is not equivalent to an air gap and may not satisfy code in your jurisdiction.

Replacement

Air gaps are inexpensive ($10–$30) and easy to replace without tools beyond a basin wrench for the mounting nut. No permit is required. Replace when the plastic body cracks, when the internal baffle is clogged with debris and cleaning does not restore drainage, or when the cap no longer stays seated. Clear blockages first by removing the cap and cleaning both ports with a small bottle brush before assuming the unit needs replacement.

When installing a new air gap, connect the 5/8-inch dishwasher drain hose to the smaller inlet barb and the 7/8-inch outlet hose to the larger barb running to the disposal knockout or drain tailpiece. Secure both hoses with stainless-steel hose clamps. Run a full dishwasher cycle after installation and check under the sink for leaks at both connections.