Open the sink cabinet next to almost any dishwasher and you'll find one of two things: the drain hose arching up and clipped under the countertop, or a small chrome or white cap on the sink deck with hoses running to it. The first is a high loop. The second is an air gap. Both exist to stop the same problem, dirty water from the sink or disposer finding its way back into the dishwasher, and installation guides require one or the other.
The problem they solve
A dishwasher drain hose that runs low and straight to the sink drain can behave badly in two ways.
- •Siphoning. If the hose runs downhill to the drain connection, it can act like a siphon and pull water out of the dishwasher mid-cycle.
- •Backflow. When the sink is full or the disposer is grinding, waste water can push back up a low hose and into the dishwasher tub.
A high loop makes water climb to a point near the underside of the counter before it can flow either way, which defeats both. An air gap goes further: it physically opens the drain line to the air above the sink, so there's no continuous pipe for dirty water to travel back through.
What the manufacturers specify
GE GLD6500
GE's instructions offer two drain methods, an air gap or a "high drain loop," and say an air gap must be used when required by local codes and ordinances. They also require an air gap if the waste tee or disposer connection is less than 18 inches above the floor, to prevent siphoning, and remind installers to route the hose through the required 32-inch minimum height when no air gap is used. Total drain hose length must not exceed 10 feet.
Whirlpool
Whirlpool's drain loop page says the loop should be a minimum of 20 inches above the floor, and to use an air gap if the hose connects lower than 20 inches from the subfloor or floor. It also says to connect the hose above the drain trap. The WDT705PAKZ dimension guide repeats the 20-inch rule and recommends that the hose either be looped up and fastened to the underside of the counter or connected to an air gap.
Bosch SHE53T56UC
Bosch's spec sheet says to raise a portion of the drain hose between a minimum of 20 inches and a maximum of 43 inches above the cabinet floor, notes that plumbing installations vary and to refer to local codes, and shows the disposer plug being removed.
The differences that trip people up
GE's high loop figure (32 inches) is much higher than Whirlpool's and Bosch's (20 inches minimum). Bosch also sets a maximum and measures from the cabinet floor, while GE and Whirlpool measure from the floor. Follow your model's diagram, and measure from the reference point it uses.
The disposer plug
New garbage disposers ship with a knockout plug in the dishwasher inlet. GE puts it bluntly: if the plug is left in place, the dishwasher will not drain. It's a classic explanation for a brand-new installation that fills, washes and then sits full of water.
What applies in DC
Manufacturers defer to local code, and in the District that's the DC plumbing code, administered by the Department of Buildings. Whether a high loop is acceptable for your installation, or an air gap or other connection is required, is a code question. A DC-licensed plumber will know the current requirement. The Department of Buildings' Instant Permits page lists plumbing permits for installing or replacing a fixture and says they can only be issued to licensed contractors.
Two practical notes for local kitchens:
- •Stone countertops. Adding an air gap means drilling the countertop or using a spare sink hole. On granite or quartz that's a fabricator's or plumber's job, not a cordless drill job.
- •Older sink drains. Rowhouse kitchens that have been renovated several times sometimes have a waste tee set low in the cabinet. Below 18 inches, GE calls for an air gap regardless.
Checking your installation
- Open the sink cabinet and trace the dishwasher hose from where it enters the cabinet to where it connects.
- Find the highest point of the hose. Measure from the floor (or the cabinet floor, for Bosch) to the top of that loop.
- Make sure the loop is fastened up high, under the counter or to the cabinet wall, not sagging onto the cabinet floor.
- Look at the sink deck for an air gap cap. If there is one, the dishwasher hose should run to it, and a second hose should run from it to the disposer or tee.
- Measure the height of the waste tee or disposer inlet. Compare with the 18-inch and 20-inch thresholds above.
- If the disposer is new, confirm the knockout plug was removed.
Symptoms of a bad drain route
- •Dirty sink water appears in the dishwasher tub.
- •Standing water after a cycle, and a sour smell.
- •Poor cleaning or fill errors, because the machine drains while it's trying to fill.
- •An air gap that spits water onto the counter. That usually means the line from the air gap to the disposer or tee is clogged.
The hose and loop have to fit inside the cabinet opening, too. See dishwasher cutout dimensions.
Who should do the work
- •Adding an air gap, re-piping a low waste tee, or replacing a sink drain is plumbing work.
- •Clearing a clogged air gap line is often simple, but if the clog is in the disposer or sink drain beyond it, a plumber should handle it.
- •A dishwasher that won't drain with a correct route needs an appliance technician.
If the hose is routed properly and the tub still holds water, or sink water keeps getting in, the drain pump, check valve or filter is the next place to look. Our dishwasher repair technicians handle those calls across Washington, DC.
Sources
- •GE: GLD6500 installation instructions (air gap per local code; air gap below 18 inches; 32-inch loop; 10-foot hose; disposer plug)
- •Whirlpool: Checking the Drain Loop Height (20-inch minimum; air gap below 20 inches; connect above the trap)
- •Whirlpool: WDT705PAKZ dimension guide (loop under the counter or air gap; 20-inch rule)
- •Bosch: SHE53T56UC spec sheet (20 to 43 inches above the cabinet floor; local codes)
- •DC Department of Buildings: Instant Permits (plumbing permits issued to licensed contractors)