An air gap is typically defined as the vertical space between a water outlet (such as the end of a faucet) and the highest point (flood rim) of a fixture (such as a sink). In many home inspection reports, the air gap is commonly defined as an opening on the waste line, above the flood rim of the sink, intended to keep an overflowing sink’s dirty drain water from possibly backing up into the dishwasher.  Through a situation where a siphon is created, waste water from the sink and drain line can flow against gravity and contaminate the dishwasher.  Air gaps help to inhibit a siphon from being created, and therefore protect your dishes (in the dishwasher) from potential contamination.

There are two different versions of codes which address the issue of dishwasher drain safety.  The IRC (International Residential Code) and the UPC (Uniform Plumbing and Mechanical Code).  Depending on the area where you live, the local code enforcement authority may enforce either the IRC or the UPC.

The UPC requires that a dedicated air gap device be installed at the top surface of the kitchen sink.  This device acts as an air break, keeping a siphon from being created.  The IRC allows the use of a high loop (in the dishwasher drain line) to perform the same function.  The high loop is created by simply attaching the drain line to the underside of the counter top.  This also creates an air break, keeping a siphon from forming in the event of a back up.

Please consult with your local code official, or a qualified licensed plumber, to determine which method of protection is best for your situation.

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Joe