It is not all that unusual to turn the dishwasher on and have water come out of the air gap device---such as this one where the hoses were hooked up backwards.
This condition is immediate and is obvious as soon as the dishwasher starts to drain the first time. It usually only occurs in new construction or newly installed dishwashers.
The other day I had one in a newly remodeled kitchen back up out of the air gap device at the middle of the draining cycle instead of at the beginning. It was as if the drain could take some water for a while and then it would gradually back up the hose and out the air gap device.
The reason for this was because the drain between the air gap device and the disposal connection had a BIG loop in it. This loop held water and continued to hold water even after the washer was done.
Water can only drain by gravity through this section of pipe because the air gap device represents a break in the drain. Prior to the air gap device the drain is under pressure. Because the water cannot drain by gravity, eventually the amount of water being pumped into the air gap device exceeds what can drain by gravity through the drain that is full of water.
This is a pretty simple fix and is accomplished by installation of a drain from the air gap device to the disposal that does not trap water.
Charles Buell, Real Estate Inspections in Seattle.
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WA State, Home Inspector Advisory Licensing Board