Air Gap vs. Non-Air Gap Faucets for Reverse Osmosis

What is an Air Gap Faucet?

Generally, Reverse Osmosis systems come standard with an air gap faucet, but many RO system manufacturers will also offer the non-air gap option.

Air gap faucets dispense water just like standard non-air gap faucets, but have an air gap built into the base of the faucet. Air gap faucets are designed to prevent backflow into your reverse osmosis unit by incorporating a "backflow prevention" method called an "air gap". In the event of clogging, a small amount of air, rather than dirty water, would go back into the RO system, thus protecting the system.

Similar to how a dishwasher air gap ensures dirty water never re-enters the clean water supply of a dishwasher, an RO air gap faucet prevents dirty water from backflowing into the reverse osmosis filtration system.

Both types of faucets have their advantages and disadvantages. And while there are many different makes and models of Reverse Osmosis drinking water systems, one common question we're asked is, "do I need an air gap or a non-air gap faucet?" We tackle this question below.

What is an “Air Gap” Faucet vs. a “Non-Air Gap” Faucet?Air Gap vs. Non air-gap RO faucets

All Reverse Osmosis drinking water systems send some water to the drain. This is due to the extremely fine nature of Reverse Osmosis filtration. To continually produce excellent water, the RO membrane repeatedly washes itself off and sends the impurities to the drain. 

The main difference between an air-gap and non-air gap faucet is that an Air Gap Faucet is designed to create a physical siphon break from the Reverse Osmosis (RO) system itself and the sink drain, by dropping water from one tube into another through the air. 

What's the Purpose of an Air Gap Faucet?

The air gap system routes the unit's drain water upward through a 1/4" flexible tube to a small trough built into the base of the faucet. The water then flows in the non-pressurized trough to the other side of the faucet stem where it falls into a hole and drops by force of gravity into a 3/8" flexible tube which drops it into the under sink drain pipe. 

This process requires that the air gap faucet have three tubes rather than one (one for the upward drain water, a second for the downward drain water, and the third for the unit's product water, which you drink). Consequently the air gap faucet must have a wider base and will require a larger hole in the sink.

With an air gap faucet, the drain line water from the RO membrane is routed up to the bottom of the faucet where it drops into a non-pressurized trough and then flows by gravity down to the sink drain. Whereas a non-air gap faucet sends the drain line water from the RO membrane directly into the sink drain. A non-air gap faucet only has the drinking water line connected to it.

Again, this air-gap faucet design came about so that in the event of a sink drain being clogged and backed up, the air gap installed in the RO discharge line would pull a small amount of air rather than the dirty water back into the RO unit.

Can I replace an Air Gap Faucet with a Non-Air Gap Faucet?

Yes, you can easily switch out faucets.

Switch Tubing: Keep in mind that an air gap faucet uses 1/4" tubing from the reverse osmosis system to the faucet and 3/8" tubing from the faucet to the drain. A non-air gap typically uses 1/4" tubing from the RO system directly to the drain. To convert, simply take the 1/4" tubing from the system directly to the drain, but may need to find a fitting to reduce the connection from 3/8" to 1/4" to fit the drain line tube now coming directly from the system.air gap faucet and non air gap comparison

Air gap faucets were designed so that in the event of clogging, a small amount of air (rather than dirty water) would go back into the RO system.

The Pros and Cons of an RO Faucet with Air Gap


Air Gap RO Faucet Advantages

Air Gap RO Faucet Disadvantages

System protection: In the event of a stopped up sink drain, the physical line break in an air-gap system ensures that water is not siphoned back into the RO system (which could damage the unit and contaminate the membrane).

Clogging: Small tubing and trough on the air-gap system can cause sediment clogging resulting in water backing up into sink or counter tops.

Plumbing Codes: In many areas the local plumbing code requires the use of an Air Gap faucet with an RO system (and air gap faucets only work with air gap RO systems).

Noisy: There's a physical air barrier of water dropping into the trough, Air Gap faucets can make a gurgling sound while the storage tank is being filled.


Difficult installation: Extra tubing and connections makes the installation cumbersome.


Location: An air-gap system won't work if not installed by a faucet since it must be connected to a sink drain.

The Pros and Cons of a Non-Air Gap (Regular) Faucet:



Non-Air Gap Reverse Osmosis Faucet Advantages:

Non-Air Gap Reverse Osmosis Faucet Disadvantages:

Lower Cost: Non-air gap systems are generally is less expensive than air gap systems.

Potential Damage: Chance that if the drain backs up, the RO system could be damaged or the membrane contaminated.

Less tubing/hoses: A non-air gap unit simply has one water line that goes up to the faucet.

Plumbing Codes: Considered non-compliant for some city/county plumbing codes.

Noise: No gurgling sound since the drain line from the RO membrane flows directly into the sink drain line.


Ease of Installation: Fewer connections and tubing allows for easier installation.


Air-Gap Faucet Installation Tips

Installation of an air gap faucet requires a couple of additional steps. The faucet itself has a small hole in its body so that water can flow out onto the sink or countertop should the drain tube become clogged. The hole is part of the air gap device. It must remain open and free of debris. (A faucet without a hole is not an air gap faucet. Inside this hole, waste water goes into the top and out through the bottom. Water drops past the hole into the 3/8" waste line. 

Some local plumbing codes allow you to install an in line check valve in the drain line for backflow prevention. This allows the use of a non- air gap faucet, thereby eliminating the noise associated with air gap faucets. When code allows, the 1/ 4" waste water line flowing from the system is connected to a 1/4" drain saddle installed on the drain pipe under a sink. This is also a very quiet installation. Air gap faucets can be installed as non-air gap faucets by installing a 1/4" drain saddle. Remove the 1/4" waste water line out of the reverse osmosis system from the base of the faucet and attach it to the 1/4" drain saddle. Remove the 3/8' drain line from the air gap faucet and discard. air-gap-instructions-.jpg

Why are Air Gap Faucets Noisy?

Generally, Reverse Osmosis systems are very quiet -- though you may hear a "gurgling" sound as the "concentrate" or wastewater flows from the membrane to the drain. Reverse Osmosis is designed so that "crossflow" water flushes the contaminated concentrate away from the system. This flushing keeps the unit from fouling.

You might hear additional sounds if you have an air gap system as these systems tend to make a gurgling sound as the storage tank is being refilled. Also, as the tank is slowly filling with water, water is also slowly flowing to the drain line under the faucet. This water falls into the "air gap" trough and then flows down to the sink drain. The slow trickle of water often causes a gurgling sound.

Note that if you hear "hissing" noises, you should immediately call your local water treatment professional as there may be a problem with water pressure, the air gap, or a leak.

Shop Faucets Now: When it comes to RO faucets, there are a variety of styles and finishes to choose from whether you select an air gap or a non-air gap faucet. 

Air Gap Faucets  Non-Air Gap Faucets


Air Gap Faucet Diagram

Non-Air Gap Faucet Diagram

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Installed at the kitchen sink: A small RO filtration system can be installed at the kitchen tap to provide top-quality drinking water. This is called a “point of use” (POU) water filtration system and can also connect to your fridge and ice machine. Water pressure is a consideration with some refrigerators, so be sure to refer to your RO system’s owner’s manual as the pressure from the Reverse Osmosis unit is about two-thirds of the incoming line pressure. Related content: Can I Install a Reverse Osmosis System Myself?

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