An air gap faucet is a reverse osmosis drinking water faucet with a second set of passages built into its body. Drain water from the system is lifted up to the faucet base, poured across an open break of air inside it, then dropped into the drain line, so nothing from the sink waste can ever be siphoned back into the filtered side.
The third tube and what it does
A standard reverse osmosis faucet has one tube: treated water up to the spout. An air gap faucet has three. One brings treated water up, one brings reject water up from the system, and a third, larger tube carries that reject water back down to the drain saddle. Between the second and third, inside the faucet body, the water falls freely through open air. That gap is the entire safety device, and it is the reason the faucet has a wider base and a small vent slot.
Why plumbing codes ask for it
Drain lines are not always under positive pressure. A blockage downstream, or a strong siphon somewhere in the system, can pull waste backwards along a connected pipe. An unbroken tube between a drain saddle and a treatment unit is a potential path for that to happen. A physical gap of air cannot be siphoned through under any circumstance, which is why many jurisdictions specify one on point-of-use treatment units and why manufacturers ship air gap faucets as an option on nearly every model.
How to spot one and what installing it involves
Look under the sink: three lines to the faucet shank rather than one means an air gap. The mounting hole usually needs to be larger than a plain faucet requires, so a spare sink hole is not always enough, and the drain saddle has to sit above the trap on the vertical tailpiece. Occasional gurgling as the system produces water is normal, not a fault. Our best faucet water filter comparison covers the simpler alternatives if a full reverse osmosis unit is more than you need.
Questions people ask about what is an air gap faucet
Does an air gap faucet make noise?
A gurgle or trickle while the system is producing water is normal, because the reject water is falling through open air inside the faucet body before it reaches the drain.
Can water leak out of the air gap faucet?
It can if the drain line is partly blocked or the saddle is installed badly, since the water backs up and escapes through the vent slot at the base. That leak is the warning, not the fault.
Do I have to use an air gap faucet with reverse osmosis?
It depends on local plumbing code and the installation. Many systems are offered with both options, and the air gap version is the one that satisfies the strictest requirements.