Every dishwasher drain needs a way to stop dirty water from the sink or disposal flowing back into the tub. The two common approaches are a high loop and an air gap. They look different, work differently, and come with different trade-offs. This guide compares them so you understand what's under your sink and what to ask your plumber.
The problem both solve
A dishwasher drain hose usually connects to the sink drain or the disposal. If that drain clogs or backs up, or if a siphon forms in the hose, wastewater can move the wrong way: from the sink into the dishwasher. That can leave dirty water standing in the tub, contaminate clean dishes, or create smells.
Option 1: The high loop
What it is: The drain hose is routed up as high as possible under the counter, usually fastened to the underside of the countertop or the back of the cabinet, before it drops down to the drain connection.
How it works: Water backing up from the sink has to climb over the high point of the loop to reach the dishwasher. The loop raises the bar for that to happen.
Pros: - Nothing to install on the countertop. - Inexpensive, usually just a strap or clamp. - Many installation guides show a high loop as an acceptable method.
Cons: - It's a barrier, not a break. A severe backup or a strong siphon can still push water past it. - Hoses sag over time, especially when items stored under the sink push against them. - It depends on the hose staying properly routed.
Option 2: The air gap
What it is: A small fitting mounted on the sink deck or countertop, usually a chrome or colored cap near the faucet. Two hoses connect below it: one from the dishwasher and one going to the drain or disposal.
How it works: Wastewater from the dishwasher rises into the air gap, then falls through an open space into the hose that goes to the drain. Because there's a physical break open to air, water from the sink can't be siphoned or pushed back into the dishwasher hose. If the drain side clogs, water spills out of the air gap into the sink instead of going back.
Pros: - A true physical break, which is the most reliable protection against backflow. - Gives a visible warning: water spilling from the air gap into the sink means the line beyond it is clogged.
Cons: - Takes up a hole in the countertop or sink deck. - Can clog and overflow onto the counter if the line to the disposal isn't maintained. - Some homeowners don't like the look.
Which does your area require?
Plumbing rules about dishwasher backflow protection vary by jurisdiction, and some areas require an air gap. We're not going to state a code requirement here. If you're installing a new dishwasher or remodeling, ask a licensed plumber or check with your township or borough code office (in Pennsylvania) or your town's construction official (in New Jersey) about what applies to your project. Your dishwasher's installation instructions will also show the methods the manufacturer accepts.
The disposal knockout plug
When a dishwasher drain connects to a garbage disposal for the first time, the disposal's dishwasher inlet has a knockout plug that must be removed. If it isn't, the dishwasher can't drain at all. This is one of the most common problems after a new disposal or dishwasher installation, and it's an easy one to miss because everything looks connected.
Signs your drain protection isn't working
- •Dirty water in the dishwasher tub after using the sink or running the disposal.
- •Standing water in the bottom of the dishwasher between cycles.
- •Food particles or grease on dishes after a wash.
- •Water bubbling from the air gap into the sink when the dishwasher drains.
- •Sour or sewer-like odor from the dishwasher.
What to check under the sink
- Find the dishwasher drain hose. Follow it from the dishwasher to where it connects to the drain or disposal.
- If there's no air gap, check whether the hose rises to a high point before it drops to the connection. If it sags along the cabinet floor, re-secure it as high as possible.
- If there's an air gap, remove its cap and clean out any debris.
- Check the hose for kinks, especially behind the dishwasher where it may have been pushed back.
- Run the disposal with water to clear the connection.
A note on older kitchens
In older kitchens, the sink drain may have been modified many times, and it's not unusual to find a dishwasher hose connected into the drain below the trap, or a drain that's partially blocked by years of grease. If you find something like that, have a plumber look before you replace the dishwasher. Our article on replacing a dishwasher in an older kitchen covers the other things to check.
When it's not the drain path
If the high loop or air gap is set up correctly and the dishwasher still won't drain or keeps backing up, the problem may be in the dishwasher: a clogged filter, a failed drain pump, a stuck check valve or a blocked drain hose. These usually come with symptoms like humming without draining, or error codes for drain problems.
When to call a technician
If your dishwasher holds water, drains slowly or keeps getting dirty water back into the tub after you've checked the hose routing, a technician can test the drain pump and check valve, clear blockages, and confirm the hose is routed correctly. Our dishwasher repair technicians handle these calls throughout the Philadelphia suburbs and South Jersey; see our service areas. Book a visit online and mention whether your setup has an air gap.