<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 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 , 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 does the air gap do on a dishwasher
prevents drain water from the sink from backing up into the dishwasher , possibly contaminating dishes