<P> An air gap, as it relates to the plumbing trade, is the unobstructed vertical space between the water outlet and the flood level of a fixture . </P> <P> A simple example is the space between a wall mounted faucet and the sink rim (this space is the air gap). Water can easily flow from the faucet into the sink, but there is no way that water can flow from the sink into the faucet without modifying the system . This arrangement will prevent any contaminants in the sink from flowing into the potable water system by siphonage and is the least expensive form of backflow prevention . </P> <P> A common use of the term "air gap" in home plumbing refers to a fixture that provides back - flow prevention for an installed dishwasher . This "air gap" is seen above the sink as a small cylindrical fixture mounted parallel with the faucet . In the base cabinet under the sink, the drain hose from the dishwasher feeds the "top" of the air gap, and the "bottom" of the air gap is plumbed into the sink drain below the basket, or into a garbage disposal unit . When installed and maintained properly, the air gap works as described above, and prevents drain water from the sink from backing up into the dishwasher, possibly contaminating dishes . Water flowing from the fixture into the sink indicates a need for maintenance or repair . </P>

What is an air gap for a faucet