I was recently checking out the plumbing for my dishwasher and noticed an unusual drain setup. The dishwasher drain pipe feeds into a DWV pipe that is open at the top. Is this compliant with plumbing code? I thought the dishwasher drain was supposed to connect directly to a fitting with a high loop, not just feeding to an open pipe?
Washing machines discharge the same way.
The standpipe in this case is high and has its own dishwasher P-trap. A clogged sink P-trap will not cause an overflow of sink water through the stand pipe into the cabinet.
The concern is that if the dishwasher empties and its P-trap is clogged, the open pipe will overflow and there will likely be no annunciation (no error code) by the dishwasher because water will flow, just not where intended.
If there is a direct connection, a clogged P-trap will either cause backup into the sink or trigger the backflow / pump error in the dishwasher. But not flood the cabinet and kitchen.
Also, if there is a clog downstream from the kitchen (past both P-traps), the dishwasher discharge will fill the sink before overflowing through the tall standpipe. This arrangement provides some protection.
However, the direct connection often poses a problem of its own. At the connection point of the discharge flex pipe to the drain there is a narrowing often causing fruit pits or seeds to jam the flow.