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.
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.
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 countertop as a small cylindrical fixture mounted parallel with the faucet. Below the countertop, the drain pipe of 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. 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 plumbing repair.
To further illustrate the air gap, consider what could happen if the air gap were eliminated by attaching a hose to the faucet and lowering the hose into a sink full of contaminated water. Under the right conditions (if the water supply loses pressure and the sink is higher than the point at which the water supply enters the house, for instance), the dirty water in the sink will be siphoned into the water pipes through the hose and faucet. The dirty water then will be dispersed throughout the drinking water system.All plumbing codes require backflow prevention in several ways. The plumbing fixture manufacturers build the fixtures to meet these codes. A plumber must not build cross-connections in his daily work practices, and Plumbing Inspectors look for improper designs or connections of piping and plumbing fixtures. A common misconception is that a “high loop” (routing a drain line above a sink’s flood level, for instance) will provide the same function as an air gap; this is not true, because the continuous connection in such a case still will allow backflow through siphoning.
An air gap must meet the requirements of being two times the effective inner diameter of the pipe (2*D) in order to be sufficient.