An air gap faucet looks like an ordinary tap with a slightly thicker base, and inside it is doing something an ordinary tap does not: carrying the system's drain line up through its body, across an open break, and back down. Understanding that internal shape explains the noise, the larger deck hole and every complaint people have about them.
The three channels
One carries treated water up from the storage tank to the spout, which is the only channel an ordinary faucet has. The second carries the system's reject line up into the faucet body. The third takes water back down from the body to the drain saddle. That is why the base is thicker, why the deck hole is larger, and why there are three tubes under the sink instead of one. Each connects to a different part of the system and they are not interchangeable.
The break itself
Inside the body, the reject line discharges into a small open chamber and the water falls across an air space into the third channel. Because the two sides never physically connect, nothing in the waste pipe can be siphoned or pushed back towards the system, which is the whole purpose. The chamber is vented to the room, which is why you can hear the water crossing it, and why a blockage downstream shows up as water appearing at the base rather than backing up invisibly.
Why the third channel is the fragile part
Water leaving the gap falls by gravity rather than being pushed, so that channel and the tube below it are unforgiving. Any restriction downstream, a sag in the tube, a kink behind the cabinet, a partly blocked drain saddle or debris in the channel itself, means water arrives faster than it can leave and escapes at the gap. That is the origin of nearly every complaint about these faucets, and the fault is almost never the tap.
Living with the design
Expect a gurgle during production: that is water crossing the break and is normal. Clean the channel at the annual service, since reject water splashing and drying leaves scale, particularly on hard water. Keep the tube below short, steep and in the diameter specified. And if the design is unwelcome, ask whether local code will accept an approved in line air gap device instead, which puts the same break under the sink. Our whole house RO guide covers drains at larger scale.
Questions people ask about reverse osmosis faucet air gap
Why does an air gap faucet have three tubes?
One for treated water to the spout, one carrying the reject line up, and one taking water back down to the drain after the break.
Why can I hear it?
The chamber where water crosses the open break is vented to the room, so the water falling across it is audible during production.
Why does water appear at the base?
The gravity channel below is restricted, so water arrives faster than it can leave. The fault is almost never the faucet.
Can I avoid the design?
Often, with an approved in line air gap device under the sink, if local code accepts it. It provides the same break.