An air gap faucet has a second passage cast into it that breaks the drain line open to atmosphere before the water reaches the sink drain. That break is what stops dirty drain water siphoning back into a reverse osmosis system if the sink ever backs up. It is a plumbing code feature in many jurisdictions, not a filtration part.
How the air gap works inside the faucet body
Drain water leaves the RO unit, travels up a small tube into the faucet base, spills across an open air space inside the casting, then falls into a larger tube that runs down to the drain saddle. Because the two tubes never touch, there is no continuous column of water for a siphon to pull on. The trade-off is that the passage is short, tight, and easy to block with the scale and slime that grow in any drain line.
Gurgling, spitting and water in the sink
A healthy air gap makes a brief gurgle while the system produces. Loud sustained noise, water spitting out of the small hole at the base, or a puddle around the faucet almost always means the outbound drain tube is restricted. Common causes are a kinked line, a drain saddle whose hole was never fully drilled through, a downstream slope that traps water, or debris packed into the saddle port itself.
Clearing a blocked air gap step by step
Shut the feed valve and open the RO faucet to depressurize. Pull the larger drain tube off the faucet shank and flush it, then check the drain saddle by removing the clamp and looking for a clean hole in the pipe wall. A pipe cleaner or a length of stiff trimmer line clears most blockages. Reroute the tube so it falls continuously to the saddle with no dips, and shorten it if you can: slack collects water.
Air gap or not, and what changes if you skip it
Some local codes require the air gap, so ask before you choose. Non air gap faucets are quieter, simpler, use one fewer tube and are far less prone to nuisance leaks, but they rely on the drain saddle being above the trap for protection. Countertop systems dodge the question entirely because they do not connect to the drain at all, which is one reason households in rentals choose them.
Questions people ask about faucet air gap
Is an air gap faucet required for reverse osmosis?
It depends on your local plumbing code. Some jurisdictions require an air gap on the RO drain line, while others accept a high loop into a properly placed drain saddle.
Why is water coming out of the hole at the faucet base?
The outbound drain tube is restricted, so drain water backs up in the air gap chamber and overflows. Clear the tube and the drain saddle port before replacing the faucet.
Can I convert an air gap faucet to a non air gap setup?
Yes, by swapping the faucet and running one drain tube straight to the saddle, provided your local code permits a non air gap installation on the RO drain.