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What Are Dishwasher Air Gaps and Are They Necessary?

Posted by Brock Robinson on January 05, 2023

dishwasher air gap

If your plumber’s told you that you’re going to have to 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 do mandate dishwashers drain via an air gap fitting. But an air gap isn’t just a code-compliant annoyance, it’s 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 make certain 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 does backflow work?

Backflow is the unwanted reversal of water flow, ushering tainted water and contaminants back into the 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 start to siphon the wastewater back into appliances. Without an air gap (or other means of backflow prevention) your dishwasher will flood with contaminated water. 

How does a dishwasher air gap 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. 

dishwasher air gap diagram

Learn more about how to fix air gap leaks.

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 (known as brine) 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 allowing you to simplify your under-sink plumbing set-up.


Do I need a dishwasher air gap?  

Air gaps are the most effective means of preventing your drain from cross-contaminating your dishwasher with waste. If you want to protect your dishwasher from flooding with contaminated water, you need an air gap. Dishwasher air gaps are also required by plumbing codes in many locations. 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 on the air gap’s heading. While other backflow prevention techniques can discourage cross-contamination, only an air gap guarantees this protection. 

The reliability of air gaps is why 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. States including California, Washington, Minnesota, and Hawaii have all adopted a dishwasher air gap installation as a mandatory residential plumbing procedure. In these parts of the country, air gaps are certainly necessary. A plumber installing a new dishwasher will require you to have an air gap connection. If you do forgo an air gap, your house will not be up to code. If you were to ever attempt to sell your house, the inspector would require you to install an air gap. 

How do I 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 will have to 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 may 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 stainless steel hose clamps, 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 and secure 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 in order 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. There are some creative options available to those wishing to hide their air gaps, 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.

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 ideally would 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. Standpipes are most commonly used to rapidly drain washing machines between fill and drain cycles, but some homeowners install them 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. 

Air gaps for water filtration 

Just like your dishwasher, water filtration systems like reverse osmosis and water softening can back-siphon wastewater into their units. Plumbing codes require water softening systems to air gap their drain lines and it’s strongly recommended you include an air gap in any under-sink reverse osmosis system. Furthermore, the purpose of these systems is to deliver high-quality water to you and your home. It is prudent to protect your water filtration systems from preventable contamination. 

Water softener air gap

Water softeners require an air gap of at least two inches between their drain hose and the dedicated drain. After a water softener completes a regeneration cycle, it rapidly discharges a briny solution from its tank through its drain hose. If the hose is too close to the drain site, this discharge water could backflow into the softener and be distributed throughout the house. 

Depending on your drain set-up, you can use a physical gap of air to protect the softener from backflow. For example, if you are draining your water softener into a floor drain, you can connect the drain hose to a pipe two inches above the floor drain. Since the discharge exits the drain hose forcefully, you’ll want to make sure the drain hose is properly secured and incapable of slipping and submerging in the drain. 

You can also purchase a water softener air gap fitting to attach to the end of the water softener’s drain hose. If your water softener drains into a standpipe, you will need an air gap to separate the end of the hose from the mouth of the standpipe. This is to prevent the water softener from siphoning the brine back into the system and then into the household water supply.

Learn more about how water softeners work. | Explore 5 benefits of having a water softener

Reverse osmosis air gap faucets

Reverse osmosis systems (RO) utilize an air gap faucet to prevent dirty water from a clogged sinking siphoning back into the reverse osmosis system. Under-sink reverse osmosis systems require a dedicated faucet at their point-of-use, and many plumbing codes demand a reverse osmosis systems possess an air gap. While some people opt to run their RO drain lines straight to the drain, air gaps ensure that contaminated water from a stopped-up sink is unable to travel back into the RO system via the drain line. Wastewater can foul the reverse osmosis membrane, damage the system, and pollute the pristine water the system produces. 

An air gap faucet has three tubes connecting it to your under-sink plumbing. One delivers the crystal clear filtered water from the system to the faucet and into your glass. The other two work to drain the RO unit. One carries the brine from the reverse osmosis system to the air gap and dispenses it in a trough and the other collects the wastewater and funnels it to the drainage site. Just like with a dishwasher air gap, air gap faucets have a physical break between two lengths of tubing to eliminate cross-connections and prevent backflow. They also have a small hole in the body of the faucet that ejects water into the sink if the drain line becomes stopped up by debris. 

reverse osmosis air gap faucet diagram

The biggest disadvantage of an air gap faucet is their noisy performance. While the RO storage tank is filling, air gap faucets are known to make sputtering and gurgling sounds that you may find disruptive or unpleasant. They also cost more than a standard RO faucet and involve a more complex installation process. However, if you have invested in installing a reverse osmosis system and providing your home with high quality, contaminant-free water, it’s wise to take every precaution to protect your system. Most RO systems can be purchased with an air gap faucet. The air gap faucet can be installed alongside the rest of your unit. It is better to safeguard your system from the outset than deal with a ruined membrane and damaged unit in the event the drain line back-siphons.

Explore more on how reverse osmosis works. | Learn how to troubleshoot your reverse osmosis system.


Drinking Water Faucets

Most under-sink drinking water filtration systems require a dedicated drinking water faucet. These faucets are adjacent to your sink's primary faucet, and connect directly to your under-sink water filtration system. These faucets can provide fresh, filtered water on demand. Many under-sink systems, like ultrafiltration units or the popular Everpure H-300, don't require an air gap faucet to protect from backflow. These systems can be complemented with a drinking water faucet that provides direct access to the filtered water. These non-air gap faucets are available in a variety of finishes, like brushed nickel and chrome. These faucets come in a wide range of stylish designs that are suited to match your kitchen decor. 

Comments 1-10 of 2

Thanks for the write-up – it was a good overview covering all the plumbing use cases in the general sink area.

Rahul on September 12, 2023

Thank you for this explanation! It was clear, to the point, and the physics made sense. (Sorry, I’m not interested in leaving my email address)

Richard L on March 27, 2023
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