Classic Air Gap Design. The RO drain water goes up to the faucet in the red tube, then down to the sink’s drain pipe in the black. The permeate tube connects to the threaded metal stem. The hole in the faucet body serves as an overflow drain if the black tube is obstructed.
Air gap faucets are used with undersink reverse osmosis units. They are never used with filters because the air gap is not needed.
The purpose of the air gap faucet is not only to deliver drinking water from the RO unit, as all faucets do, but also to provide an “air gap” for the RO unit’s drain system.
An “air gap” is a siphon break to assure that drain water cannot flow backward from the household drain system into the reverse osmosis unit.
Note in the picture above that the tubes are of different diameters. The smaller tubes are 1/4″ and the larger tube is 3/8″.
The tube on the left in the picture delivers the “permeate” or purified water to the user via the tall spout when the faucet’s handle is pressed. The other two tubes are parts of the drain systems.
Reverse osmosis units produce waste water when they are in operation. This waste water comes up the center tube in the picture and is released into a small open trough inside the base of the faucet. The drain water flows along the open trough until it falls through a hole into the top of the larger tube on the right in the picture. The large tube connects to the undersink drain pipe. Water falls by force of gravity down the large tube and into the undersink drain pipe.