By Rick Andrew

Many professionals in the water treatment business are familiar with air-gap faucets—what they are, how they work and their pros and cons. These faucets, of course, dispense drinking water, just as non-air gap faucets do. But they also contain a small air gap in the base, with the reject or waste line of the RO connected to the inlet side of the air gap and then the outlet side of the air gap connected to the drain under the kitchen sink. The outlet connection is usually a larger diameter than the inlet connection, 0.375- versus 0.25-inch (9.525- versus 6.35-mm), for example (see Figure 1). The main advantages of air-gap faucets, as many know, are the protection from back siphonage of wastewater into the RO system and conformance with plumbing code requirements. The primary disadvantages include noise while the storage tank is filling and additional tubing connections.

You may have wondered exactly how the effectiveness of an air-gap faucet is established. After all, the air gap is relatively small compared to typical plumbing air gaps. You might wonder, is it possible that wastewater could be siphoned back into the RO system under typical operating conditions? Is there some kind of test performed to verify that the air gap works?

NSF/ANSI 58 air gap requirements
Fortunately, NSF/ANSI 58 includes a straightforward yet rigorous test for the air gaps in these faucets. The test requires a simple test rig, described in the schematic diagram presented in Figure 2. For testing purposes, the air-gap faucet is fully submerged in water in a vessel with a vacuum hose attached to the outlet side of the air gap. A vacuum pump is then used to draw the water in the vessel down through the outlet side of the air gap, with a vacuum of 85 kPa, or 25 inches of mercury. The water draws down to the point where it is at the level of the air gap on the outlet side. This level is considered to be the critical level for the air gap. The critical level is technically defined by NSF/ANSI 330 Glossary of Drinking Water Treatment Unit Terminology as “a point on a backflow prevention device, which determines the minimum elevation of the device above the flood level rim of the fixture or receptacle served.” In practical terms, this is the point where the vacuum applied to the outlet side of the air gap will not draw the water in the vessel down farther.

At this point in the test, the vacuum to the outlet side of the air gap is terminated and a vacuum is instead applied to the inlet side. The same degree of vacuum as initially applied to the outlet side of the air gap is applied to the inlet side of the air gap for one minute. There can be no evidence of back siphonage during this one-minute period of application of vacuum, as identified through use of the sight glass installed on the vacuum hose. If no back siphonage is identified within one minute, the test has been passed.

Confidence in the air gap
Typically, plumbing codes require an air gap of two pipe diameters, or one inch (25.4 mm), whichever is larger. An air gap of this size will no doubt be effective regardless of vacuum conditions. In fact, in light of typical plumbing code requirements, NSF/ANSI 58 requires that reject-water connections for POU RO systems other than air-gap faucets must be designed and constructed for connection to the drain system through a vertical air gap of two pipe diameters or one inch, whichever is larger.

Air-gap faucets provide an alternative mechanism to protect the RO system from back siphonage of wastewater, instead of traditional air-gap plumbing approaches. Because these devices are an alternative to traditional approaches, often with a smaller air gap, there can be questions of confidence regarding the effectiveness of these typically smaller air gaps in air-gap faucets. By conducting a performance test with a significant degree of vacuum applied, this confidence can be gained. With confidence comes adherence to codes and acceptance by inspectors. This concept of confidence is one of the important foundations of standards and certifications. By requiring testing according to rigorous methods for aspects of products that could lead to questions of confidence, these standards and certifications serve this purpose well.

About the author
Rick Andrew is NSF’s Director of Global Business Development–Water Systems. Previously, he served as General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols) and Biosafety Cabinetry Programs. Andrew has a Bachelor’s Degree in chemistry and an MBA from the University of Michigan. He can be reached at (800) NSF-MARK or email:


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