Air gap (plumbing)


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. Air gaps of appropriate design are required for water safety by legislation in many countries. A related term in computer security is Air gap.
A simple example is the space between a wall mounted faucet and the sink rim. 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 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.
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, 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.

Standards and codes

All plumbing codes require backflow prevention in several ways. The fixtures are manufactured to meet these codes. Plumbers must not build cross-connections in their 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" 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.
According to the International Residential Code 2003, an air gap must meet the requirements of being two times the effective inner diameter of the pipe in order to be sufficient.
A standard widely use in the United States is:
In the United Kingdom, legislation is by statutory instrument and vary by country, but include Water Supply Regulations 2016
and Water Supply Regulations. The categorisation of air gaps is standardised by European standards, which cover the basic design and dimensions for appropriate to different uses.