Imagine washing your dishes while wastewater from the drain flows backward into your dishwasher or clean water line. Gross, right? That kind of contamination—called backflow—can cause serious health risks and plumbing damage. Fortunately, modern plumbing systems include specific safeguards to prevent it. One of the most important is the air gap.
In both residential and commercial plumbing systems, air interval plays an important role in protecting potable (potable) water from contamination. They are not only alternative components – they are often required by plumbing code to ensure safe operation of devices such as dishwashers, water softeners and reverse osmosis systems.
In this article, we’ll explore exactly what an air gap is, where it’s used, how it works, and why it’s such a crucial part of keeping your plumbing system—and your family—safe from polluted water.
What Is an Air Gap?
An air gap is a physical space between a water outlet and the highest level of a potential water source that could contain contamination. In simpler terms, it’s a vertical separation—often between a drain line and a fixture—that ensures wastewater can’t siphon or flow back into the clean water supply.
The air gap acts like a barrier of space—not a mechanical device—that prevents dirty water from ever touching clean water.
Imagine holding a glass under a tap. The water puts in the glass, but the end of the tap never touches the water. That place between tap and glass? This is an aerial difference. If the water returns to the drain or sink, the tap outlet remains safely on it, preventing contamination.
How It Prevents Contamination
Air gaps are designed to prevent:
- Backflow – when contaminated water reverses direction into the clean supply
- Backsiphonage – caused by pressure drops that create suction, pulling dirty water back into the clean line
By breaking the connection between the supply and the waste system, air gaps eliminate any chance for waterborne pathogens, grease, bacteria, or debris to travel backward into the potable system.
They’re non-mechanical, passive systems, meaning they work with no moving parts or maintenance—just good old gravity and distance.
READ MORE: Which Plumbing Device Helps Prevent a Backflow?
Where You’ll Find Air Gaps in the Home
Kitchen Sink and Dishwasher
One of the most common residential uses for an air gap is between a dishwasher and the sink drain. In many U.S. states, it’s code-required to install an air gap fitting on the sink or countertop when installing a new dishwasher.
Here’s how it works:
- The dishwasher pumps wastewater up into a small air gap device mounted above the sink (usually with two hose connections).
- From there, gravity allows the dirty water to flow downward into the sink’s drain or garbage disposal.
If the sink drain clogs, water may back up into the sink—but it won’t reach the dishwasher hose because of the air break provided by the gap.
Without an air gap, if your sink backs up or drains slowly, water could siphon back into your dishwasher—contaminating your dishes and possibly ruining the appliance.
Water Softeners and Reverse Osmosis Systems
Air gaps are also important in water softener discharge lines and reverse osmosis (RO) filtration systems. These systems often require drainage after cycles of filtering or regenerating.
If those drain lines are directly connected to a waste pipe or sink tailpiece without an air gap, there’s a risk of:
- Sewer gases entering your filtration system
- Contaminated drain water flowing back into your clean filter system
Manufacturers often include air gap fittings or recommend routing drain lines to a standpipe or specialized air gap adapter to ensure code compliance and sanitary operation.
How an Air Gap Works
Gravity and Vertical Separation
The air gap works on a very basic principle: water can’t flow upward without pressure. Since an air gap introduces open air between a drain line and an waste pipe, it prevents any reverse flows. Let’s break a basic dishwasher air gap landscape:
- The dishwasher pumps water through a discharge hose.
- This hose connects to the air gap mounted on the sink or countertop.
- Inside the air gap, water flows upward, enters an open chamber (exposed to air), and then flows down another hose to the sink drain.
- If the sink is blocked or full, water might back up into the air gap—but the air chamber prevents it from reaching the dishwasher.
The vertical space—usually a few inches—ensures nothing can siphon back into the appliance or clean water line.
READ MORE: What Does NPT Mean in Plumbing?
Preventing Cross-Connection
In plumbing terms, a cross-connection is any point where clean and dirty water might mix. These are dangerous because they can lead to contamination in the water supply.
Air gaps eliminate cross-connections by physically separating the lines. It’s a key reason air gaps are used in:
- Commercial kitchens
- Medical facilities
- Homes with backflow-sensitive appliances
Because the air gap is open to the atmosphere, there’s no way for waste to re-enter the clean system unless there’s a complete structural failure of the plumbing.
Air Gap vs Air Break: What’s the Difference?
Definitions and Use Cases
Many people confuse air gaps with air breaks, but they are not the same thing.
- Air Gap: A vertical space between the water outlet and the receiving drain or fixture. Required by code in critical systems.
- Air Break: A configuration where a drain hose discharges into an indirect waste receptor (like a floor sink), but the hose may extend into the receptor below the flood level.
Air breaks are common in laundry rooms, utility sinks, and some commercial setups. They still offer backflow protection but are not as foolproof as air gaps.
Why an Air Gap Offers Stronger Protection
Because air gaps provide a complete, physical separation, they offer the highest level of protection against backflow. Unlike air breaks, there’s no chance the drain line will become submerged or contaminated—because the vertical gap remains open to the atmosphere at all times.
For this reason, air gaps are required by plumbing codes (UPC, IPC) for dishwashers and water filtration systems in most jurisdictions.
Do You Need an Air Gap for Your Dishwasher?
When It’s Required by Code
In many U.S. states and cities, an air gap is not optional when installing a dishwasher—it’s the law. The Universal Plumbing Code (UPC) requires air gap in all new dishwasher installations, while the International Plumbing Code (IPC) provides for alternatives under some conditions, such as high loop configuration. Provisions have been brought by states to require mandatorily the air difference:
- California
- Washington
- Minnesota
- Wisconsin
- Oregon
These states often include the air gap mandate in building permits and inspections, so if you’re replacing or upgrading a dishwasher, you’ll likely need to install an air gap on your countertop or sink.
Even in areas where it’s not legally required, home inspectors, plumbers, and appliance manufacturers often recommend using one for safety and warranty compliance.
READ MORE: What Does FIP Mean in Plumbing?
What About the High Loop Alternative?
Some homeowners opt for a “high loop” instead of a physical air gap. This method involves routing the dishwasher drain hose upward—looping it as high as possible under the countertop before connecting it to the drain.
While a high loop:
- Takes up less visual space
- Requires no countertop drilling
- Is easier to install
…it’s not as effective or code-compliant as a true air gap in many areas. A high loop can reduce backflow risk—but only an air gap fully eliminates it by design.
How to Install an Air Gap on a Kitchen Sink
Installing a dishwasher air gap is a simple process that most DIYers can handle. Here’s how to do it:
Step-by-Step Installation Guide
- Purchase an air gap kit
These are available at hardware stores in chrome, brushed nickel, and other finishes to match your faucet. They usually come with the air gap body, cap, and mounting hardware. - Drill a hole in your sink or countertop
If you don’t already have a knockout hole, use a hole saw (⅞ to 1¼ inch) to create one near the back edge of the sink. - Insert the air gap body into the hole
Feed the threaded section through the hole from above. Secure it from underneath using the washer and locknut. - Connect the dishwasher drain hose
Attach the smaller hose (usually ⅝ inch) from the dishwasher to the smaller barb on the air gap using a hose clamp. - Connect the drain hose to the sink or disposal
The larger hose (usually ⅞ inch) goes from the air gap’s larger barb to the disposal’s dishwasher inlet or a branch tailpiece. Secure both ends with clamps. - Secure the decorative cap on top
Snap or screw the cap onto the air gap housing. - Test the system
Run the dishwasher and watch the air gap. A small amount of water may flow from the gap briefly—this is normal. Check for leaks or hose disconnections.
Pro Tip: If water sprays out of the air gap during dishwasher use, the lower drain hose may be kinked or clogged. Remove and clean both hoses if needed.
Maintenance and Troubleshooting Tips
Although air gaps are mostly maintenance-free, they can occasionally develop issues, especially in high-use kitchens.
Common Air Gap Problems
- Water spurting from the air gap
Likely caused by a clogged or kinked drain hose. Disconnect the hose and flush it with water to remove debris. - Gurgling or sucking noise
Caused by improper hose slope or poor venting. Check that all hoses have a downward angle and smooth interior walls. - Leaking around the countertop
This is usually a sign that the gasket or locknut is loose. Tighten loosely and hold in place using the provided washer. - Odors from the air gap
Clean both of the hoses with warm water and baking soda, or clean inside air gap room with a bottle brush.
READ MORE: What is Sewer Gas in Kitchen Plumbing?
Benefits of Using an Air Gap
Still debating whether you need an air gap? Here’s why it’s a smart choice:
Health and Safety
- Prevents cross-contamination between clean and dirty water
- Keeps dishwashers and water filters free from bacteria and sewage backup
- Reduces risk of gastrointestinal illness from waterborne pathogens
Plumbing Code Compliance
- Required by many local codes and often necessary to pass home inspections
- Ensures that installations are warranty-approved by manufacturers
Peace of Mind
- Passive system—no moving parts to fail
- Easy to install and maintain
- Works even during power outages or pressure drops
Conclusion
An air gap in plumbing is one of the simplest yet most effective tools for protecting your home’s water supply. Whether you’re connecting a dishwasher, water softener, or filtration system, the air gap creates a crucial barrier between clean and dirty water—using nothing but gravity and space.
In many areas recommended by experts, this code is necessary, and gives you peace of mind that your family’s drinking water is protected from contamination. If you are upgrading your kitchen or replacing equipment, installing the air gap is not just smart – it is often necessary.
FAQs
1. Is an air gap required for dishwashers in all U.S. states?
Not all, but many—especially in places like California, Oregon, and Minnesota. Check your local plumbing code or consult a plumber.
2. Can I install a high loop instead of an air gap?
In some areas, yes—but a high loop is not as effective or universally accepted as a true air gap.
3. Why is water coming out of my air gap during dishwasher use?
This usually means there’s a clog or kink in the lower drain hose. Disconnect and flush the hose to clear the blockage.
4. How often should I clean the air gap?
Check it every few months, especially if your dishwasher smells or drains slowly. Use warm water and a bottle brush to clean the chamber.
5. Do garbage disposals affect air gap installations?
No, but you’ll need to connect the air gap’s drain hose to the dishwasher inlet port on the disposal. Make sure the knockout plug is removed first.
