What Are Dishwasher Air Gaps and Are They Necessary?

Posted by
John Woodard on March 08, 2024

If your plumber has told you that you must install an air gap alongside your brand new dishwasher, you may be left wondering why. What exactly is that little fixture that juts up above your countertop, and do you really need an air gap to keep your dishwasher safe from contamination? The truth is an air gap is the most effective method to protect your dishwasher from flooding with wastewater. Stopped-up sinks and clogged tubing can send dirty water from the drain back into your dishwasher. Many plumbing codes mandate dishwashers drain via an air gap fitting. But an air gap isn’t just a code-compliant annoyance, it is a smart way to shelter your dishwasher from backflow.    

What is a dishwasher air gap? 

A dishwasher air gap is a fitting mounted about two inches above the sink that prevents contaminated water from re-entering the dishwasher from the drain via backflow. An air gap is a simple way to ensure wastewater and contaminants never re-enter your clean water supply. When you are running your dishes through a wash cycle, the last thing you want is for them to emerge streaked with grime pumped in from your clogged garbage disposal. Air gaps entirely separate the hose running dirty water from the hose running to the drain. Since these two paths never intersect, there is no risk of wastewater creeping back into your clean appliance. 

The space between your kitchen faucet and the rim of your kitchen sink is a perfect demonstration of an air gap. If your garbage disposal is clogged and your sink overflows with filthy water, the water will spill over the rim of the sink and onto the counter. There is enough distance between the mouth of the faucet and the sink’s rim to ensure that dirty water will never backflow into your kitchen faucet and pollute the clean water supply. This literal gap of air protects the potable water from contamination.   

How do dishwasher air gaps work? 

A dishwasher air gap separates two branches of hose with a physical gap of air to guarantee that cross-contamination between the dishwasher and drain is impossible. One branch of the air gap fitting connects from the dishwasher to the air gap. The other branch descends from the air gap to the garbage disposal. The hose running the dirty water from the dishwasher exits the dishwasher and curves upward. At its peak, the branch ends, and the water pours out of the open mouth of the tube, through the air gap, and down into the second branch. The second branch carries the wastewater down toward the designated drainage site. 

The two branches of the air gap are installed beneath your sink or countertop. Air gap fittings have a decorative heading that extends above the counter, usually next to your faucet or soap dispenser. This heading houses the vertical air gap. Air gaps are notched with holes that will release water into your sink if the drain becomes clogged or the tubing is obstructed. An air gap leak is an indication that your drainage tubes are clogged and need to be cleaned out. 

Learn more: How to fix air gap leaks

dishwasher air gap diagram

What is backflow?

Backflow is the unwanted reversal of water flow, ushering tainted water and contaminants back into a clean water supply. Dishwasher air gaps are a form of backflow prevention. In plumbing, this point where wastewater could potentially pollute potable water is called a cross-connection. Cross-connections can lead to backflow when there is an alteration in pressure. For example, when a sink drain becomes clogged, the hose leading to the drain will siphon the wastewater back into appliances. Without an air gap or other means of backflow prevention, your dishwasher will flood with contaminated water.

Learn more: What is a backflow prevention device?

What is a dual inlet air gap?

Dual inlet air gaps allow you to run two dishwashers to one drain line through a single air gap or drain both a dishwasher and a water filtration system concurrently. Some dishwashers require separate drain lines from the top and bottom compartments of the machine, both of which can be run through one dual inlet air gap. If you have a reverse osmosis system under your counter, you can run the wastewater produced by the system to the same drainage site as your sink and dishwasher without worrying about cross-contaminating any other appliance. Dual inlet air gaps are a versatile innovation on the standard air gap that allow you to simplify your under-sink plumbing setup.

Do dishwashers need an air gap?  

Many plumbing codes across the United States require all dishwasher installations to include an air gap. Most plumbing codes stipulate that all commercial food and beverage preparation sinks must possess an air gap. Multiple states, including California, Washington, Minnesota, and Hawaii, have all adopted dishwasher air gap installation as a mandatory residential plumbing procedure. In these states, a plumber installing a new dishwasher will require you to have an air gap connection. If your system does not have an air gap, your system will not be up to code. When you sell your house, the inspector will require you to install an air gap. 

Air gaps are the most effective means of preventing your drain from contaminating your dishwasher with waste. If you want to protect your dishwasher from flooding with contaminated water, you need an air gap. While air gaps are not the only method of preventing your dishwasher from backflow, they are the only method that is proven to work under every circumstance. Air gaps have no moving parts, so they cannot fail. They work on the principle of simple physics. Water cannot siphon back through an unpressurized gap of air. If the drain tube becomes clogged the discharge water from the dishwasher will simply spill out of the holes in the air gap’s heading. While other backflow prevention techniques can discourage cross-contamination, only an air gap guarantees this protection. 

Dishwasher air gap alternatives 

If you live in a part of the country where plumbing codes do not mandate an air gap, there are other backflow prevention options available to you. Some homeowners dislike the air gap’s pronounced appearance on their counters and prefer to revert to other methods of separating wastewater from potable water. 

High Loop

A high loop is a method of backflow prevention where the drain line runs from the dishwasher to the highest point beneath the sink. The loop is affixed to the underside of the counter with a bracket, then drains down into the garbage disposal or sink drain. In a high loop set-up, the drained water from the dishwasher is forced to travel upward before it can flow to the drain. All high loops should measure at least 32” above the floor of the kitchen. If there is not at least 32” of clearance between the floor and the peak of the high loop, you will have to install an air gap. Since the tubing is running at a drastic slope, wastewater is unlikely to backflow through a properly installed high loop. This is known as an air break. If the drain becomes clogged, the waste should back up through the garbage disposal and into the sink rather than back into the dishwasher. 

Though a high loop is a classic and effective method of backflow prevention, it is not as safe as an air gap. A high loop cannot assure back siphonage prevention. When the water pressure on the supply side drops significantly, water flow can reverse and the dishwasher drain line can suction dirty water and bacteria into the appliance. For example, if you have a double sink in your kitchen and they are draining simultaneously, this could cause a pressure differential that could lead the dishwasher to siphon water back into it through the drain line. There’s also the risk of the high loop becoming loose and sagging or poor installation rendering it ineffectual. However, high loops are popular, inexpensive to install and have proven to be reliable backflow prevention methods. If you opt not to install a dishwasher air gap, a high loop is the best alternative.

Dishwasher standpipe

A standpipe is a length of vertical pipe above a P-trap that water drains into. P-traps are plumbing fixtures that prevent sewage and odors from wafting from the drain back into your home. Standpipes must be vented and at least two inches in diameter. They are most often used to rapidly drain washing machines between fill and drain cycles, but some homeowners install standpipes under the sink to circumvent installing an air gap. 

Generally, it is unwise to install a standpipe beneath your sink for a dishwasher. It is overly complicated and less effective than both air gaps and high loops. Standpipes must be taller than the sink’s flood level, or else water can overflow into your cabinet. Unlike air gaps and high loops, they also require excessive plumbing work. In most instances, it is unlikely that a plumber would ever recommend a dishwasher standpipe as the ideal solution for draining your dishwasher.

How to install a dishwasher air gap 

Installing an air gap is a simple undertaking that a homeowner can easily do without the assistance of a plumber.

  1. Locate the air gap hole on your counter.Many counters will have a precut hole already present on their counter. If this opening is not already housing an air gap, it is probably covered by a flat disc-shaped cover. Remove this disc and set it to the side. If you do not have an air gap hole on your counter, you must drill one yourself. Bore a 1-3/8” hole in the countertop using an electric drill and hole saw. Be sure the hole is close to the rim of the sink so the air gap will have ample room to drain in the event the tubes overflow. You do not want an air gap flooding your counter. Before drilling, tape off the section around the hole with masking tape to protect your counter from scratches. If your countertop is granite or marble, this task will be more difficult, and you should consider bringing in professional help. 
  2. Connect the air gap to the dishwasher drain hose.The dishwasher drain hose connects to the smaller leg of the air gap. Attach the 5/8" tube to the 5/8" leg of the air gap. Using hose clamps to secure the tube tightly to the air gap.
  3. Connect the air gap to the drain hose or garbage disposal.Measure and cut a length of 7/8“ tubing to connect the air gap to either your drain or your garbage disposal. Secure the tube to the air gap with a hose clamp. If you are running the air gap directly to the drain, join the tube to the Y-branch tailpiece connecting the sink to the drain and secure the tubing with a hose clamp. Make sure your Y-branch tailpiece is compatible with 7/8" tubing. If you are connecting the air gap to a garbage disposal, locate the tube protruding from the side of the garbage disposal unit. If an air gap has not previously been installed on this disposal, you will need to remove the metal plug inside the tube. This allows disposals to be compatible with both air gap and non-air gap set-ups. Check to ensure there are no kinks in the tubing and attach the 7/8" tubing to the disposal, securing it with a hose clamp. 
  4. Push the air gap up through the hole on the counter.Remove the vanity covering on the air gap heading and insert it through the hole in the counter from below. Tighten the air gap against the counter by threading the nut along the air gap’s threads. You may want to have an assistant hold the air gap in place to prevent it from spinning as you install it. Once the nut is threaded on by hand and the air gap is stabilized, use a wrench to make sure the air gap is securely fastened to the counter. Place the vanity cover back on the air gap.
  5. Run your dishwasher on a fill cycle.Check the air gap and tubing connections for any signs of leaking. Make sure there’s no water escaping from the garbage disposal or air gap and dripping into your cabinet. 

Can I install an air gap under the counter? 

There is no approved air gap that can be installed entirely beneath the counter. The air gap must be installed at a higher elevation than the dishwasher for it to function correctly. A dishwasher air gap must extend above the countertop and drain into the sink. An air gap installed under the counter could flood your under-sink area. If the drain were to become clogged, water would overflow out of the air gap and into the cabinet, damaging the floorboards. 

Some homeowners object to air gaps because they find the fixture to be unsightly in their kitchen. If you wish to hid your air gap, you have some options, such as a hybrid air gap soap dispensers. These disguise the air gap within a functional hand soap dispenser, making the air gap heading less conspicuous on your counter.


If you have any additional questions, please do not hesitate to contact us.
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