That small cap beside the faucet is a dishwasher air gap—a backflow stop that vents above the sink to keep drain water out.
You’re not alone. That thumb-size cylinder by the faucet puzzles lots of homeowners.
It isn’t a soap dispenser or a spare sprayer. It’s the dishwasher air gap.
This simple fitting keeps dirty sink water from siphoning into a clean wash cycle.
It does the job passively, with no power and no moving parts in the cap you see on the deck.
Below the counter, two short hoses complete the picture.
One carries waste water from the dishwasher up to the air gap.
The second drops that water from the air gap down to the garbage disposal or sink tailpiece.
If the sink or disposal backs up, the air gap breaks the suction and vents at the counter instead of letting waste flow backward into the machine.
What Is The Thing Next To My Kitchen Faucet Called?
The proper name is “dishwasher air gap.”
It’s a backflow barrier for the drain side of a dishwasher.
By raising the discharge to a point above the sink’s flood rim, the air gap prevents any siphon from pulling waste into the appliance.
Think of it as a one-inch-tall hill that sewage can’t climb.
How An Air Gap Works
The body of the fitting has two chambers separated by open air.
The dishwasher sends water into the first chamber.
That stream passes across the air break into the outlet chamber, then down the second hose to the drain.
If the drain blocks, water spills out at the deck cap instead of traveling backward into the dishwasher tub.
That visible spill is a safety signal and a prompt to clear the blockage.
Why Dishwashers Use It
Dishwashers connect your potable water supply to a spray system and a heat cycle.
Every plumbing code treats that mix as a high-risk cross connection.
An air gap is the most reliable barrier for this scenario.
Many regions require it outright, while some allow a high-looped drain hose if local rules permit and the maker’s manual allows it.
Dishwasher Drain Safeguards At A Glance
| Method | What It Does | Where It’s Accepted |
|---|---|---|
| Deck-mounted air gap | Two-hose fitting breaks suction above the sink rim; vents at the cap during a blockage | Required by many jurisdictions under the Uniform Plumbing Code; also used where makers specify it |
| High loop | Drain hose is fastened high under the countertop to reduce siphon risk | Allowed in some areas that follow the International Plumbing Code and when listed in the appliance manual |
| Check valve only | Spring or flapper resists reverse flow | Usually supplemental; not a substitute for an air gap where one is required |
Identifying The Dishwasher Air Gap Beside The Faucet
The cap is usually a short cylinder or a sloped “button” near the faucet.
Caps pop off by hand. Under the cap sits a plastic tower with two hose barbs below the counter.
One barb points to the dishwasher; the other points to the disposal or a branch tailpiece.
Most dishwashers use a 5/8-inch or 7/8-inch drain hose, secured with worm gear clamps.
Parts You Can See And Touch
Cap: The trim piece you notice on the deck. It lifts off for inspection and cleaning.
Body: The molded core with the air window. This is the part that passes through the sink or countertop.
Locknut and washer: These hold the body in place from below.
Hoses: A short run from the dishwasher up to the inlet, and a second run from the outlet down to the drain.
Where The Hoses Go
The dishwasher hose climbs from the cabinet to the air gap’s inlet barb.
From the outlet barb, a second hose drops to the disposal or a branch tailpiece with a dishwasher port.
If your dishwasher drains into a disposal, that port ships with a molded knockout plug from the factory.
The plug must be removed before clamping on the hose; otherwise the air gap will erupt at the cap the first time you run a cycle.
Code Basics And When You Must Use One
Many cities that adopt the Uniform Plumbing Code require a listed dishwasher air gap.
Other regions using the International Plumbing Code sometimes allow a high loop if the dishwasher maker lists it.
Either way, the safe approach is simple: follow local rules and the installation manual.
For a quick reference, you can read the current language in the
Uniform Plumbing Code
and check a sample appliance guide such as the
Bosch installation instructions.
If you’re connecting to a disposal, makers like InSinkErator spell out the step to
remove the dishwasher knockout plug
before attaching the hose.
Installation And Replacement Tips
Basic Steps
Shut off power to the dishwasher and disposal.
Loosen the locknut on the old air gap from inside the sink base.
Lift the body out of the deck, move the hoses to the new fitting, and tighten the new locknut with the rubber washer in place.
Aim the outlet barb toward the drain run to avoid sharp bends.
Finish by snapping the cap back on.
High Loop Or Air Gap?
If your area requires a deck-mounted device, install the air gap.
If your area allows a high loop, fasten the drain hose to the underside of the countertop at the highest point you can reach.
Many manuals call for a loop about 32–33 inches above the floor.
A loop improves performance but doesn’t equal a true air break at the deck.
Working With A Disposal
New disposals arrive with a solid plug in the dishwasher inlet.
Pop that plug with a screwdriver and hammer, then fish the loose slug out of the grinding chamber before you attach the hose.
Run water and the disposal for a few seconds to ensure no fragments remain.
Maintenance And Quick Fixes
When Water Spits From The Cap
That splash tells you the outlet hose or the disposal port is blocked.
Pull the cap, wipe the window clear, and check the outlet hose for kinks or sludge.
Make sure the hose climbs up from the air gap and then drops cleanly to the drain without a sag that could collect debris.
If The Dishwasher Won’t Drain
Start at the air gap. Twist the cap off and look for seeds, pasta, or fiber around the window.
Rinse the tower under the tap.
Next, loosen the outlet clamp and look inside the hose.
At the disposal, inspect the rubber nipple for clogging, or the branch tailpiece for scale.
If the dishwasher hooks to a disposal and the unit is new, confirm the knockout plug was removed.
Keeping Debris Out
Run the disposal for a few seconds before starting a cycle to clear the nipple.
Avoid dumping fibrous scraps into the sink.
Every few months, pop the cap, spray the tower with warm water, and seat the cap again.
If the cap looks scuffed, swap it for a metal trim piece that matches your faucet finish.
Common Symptoms And Fast Fixes
| Symptom | Likely Cause | Quick Fix |
|---|---|---|
| Water spurts from the air gap cap | Outlet hose or disposal nipple blocked | Clear hose and nipple; run disposal; flush with hot water |
| Dishwasher tub holds water | Clog at air gap window or a kinked hose | Clean the tower and straighten the hose run |
| New dishwasher won’t drain | Disposal knockout plug still in place | Knock out the plug and remove the loose slug |
| Gurgling during drain | Hose sag forming a trap | Re-route with smooth fall; add clamps to keep the shape |
| Odor at the cap | Food film inside the outlet hose | Detach and rinse hose; sanitize the disposal nipple |
Air Gap Next To The Kitchen Faucet: Keep Or Replace?
If the device is required by your local code, keep it.
If your area allows a loop instead, you still gain insurance by leaving the air gap in place.
The part is inexpensive, easy to clean, and it gives a clear signal when the drain path needs attention.
Replacement is simple when the plastic body cracks, the cap goes missing, or you’re upgrading finishes.
You can choose low-profile caps, taller domes, or all-metal trims.
Some combination units add ports for reverse-osmosis waste and dishwasher on a single deck fitting.
If you add RO waste, use the correct adapter and maintain the air break between streams.
Safety Notes And Good Habits
- Never seal the vent slots under the cap. That opening is the relief path that protects the dishwasher.
- Keep the outlet hose run smooth and short. Avoid tight loops, crushed sections, and long sags.
- Before every cycle, run the disposal for a few seconds if your drain connects there.
- Use stainless worm gear clamps sized for the hose. Snug them without cutting the hose.
- If you swap sinks or counters, reinstall the air gap with the flood-level mark at or above the sink rim.
- When in doubt about local rules, check with the building department or read your dishwasher manual.
Simple Checks To Prove Yours Works
Quick Visual And Water Test
Lift the cap and look for the open window around the tower.
Start a dishwasher drain by running a rinse program, then watch the tower.
You should hear water rush through the fitting and down the outlet hose.
No gurgle at the cap means the outlet path is clear.
If water spills, clear the outlet hose and the disposal nipple. Keep a small towel nearby.
Sink Backup Simulation
To see the safety feature in action, set a rubber stopper in the sink and run the faucet until the bowl is nearly full.
Turn the disposal on for a second, then off, so the sink stays full.
Start a drain on the dishwasher.
Water should spout from beneath the cap.
That splash shows the air break is doing its job by venting instead of letting the dishwasher pull in dirty water.
Sizing, Heights, And Clean Routing
Deck Hole And Cap Fit
Most sinks include an accessory hole that accepts a standard air gap body.
If you’re drilling stone, use the size listed on the fitting and a diamond bit.
Hose Runs That Drain Smoothly
Short runs drain best.
Aim for gentle bends instead of tight turns.
The inlet hose should rise from the dishwasher up to the air gap without a dip.
The outlet hose should fall from the air gap down to the disposal or branch tailpiece in one smooth arc.
Secure both with clamps so cleaning under the sink doesn’t pull them loose.
Flood Level Mark
Many cores carry a small “FL” mark.
Set the locknut so that mark sits at or above the sink rim.
That keeps the break point above any standing water line in the basin.
Buying And Upgrading Tips
Materials And Finish
Most cores are plastic, which is fine for durability.
You can dress the look with a metal cap.
Accessories And Combos
If your kitchen also has a reverse-osmosis system, a combo air gap can route RO waste and dishwasher discharge in one body while keeping the streams separated.
Follow the diagram that ships with the part so each hose lands on the correct barb.
When To Call A Pro
If you see repeated overflows, can’t locate a clog, or find old rigid tubing that won’t clamp safely, schedule a visit with a licensed plumber.
A short service call beats water under the sink or a ruined cabinet floor.
The mystery nub by the faucet earns its keep.
It protects wash water from backflow, points straight at a clog when one happens, and asks only for an occasional rinse.
Treat it like a safety device, not an eyesore.
Keep the cap clear, route the hoses cleanly, and it will run for years with little attention.