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How & Why Air Gaps Are Used

Learn why air gaps are needed for some common plumbing fixtures

What is an Air Gap?

An air gap is the most simple and effective means of preventing contamination of a potable water supply.

Once potable water leaves the outlet of a fixture, it's either used or sent down the drain. In the event of a drain back up or flood, the contents of the drain line should never be able to make contact with the water supply - the two plumbing systems (for potable water and for waste) should always remain separate. Should they come into contact (a "cross-connection"), the potable water supply can be seriously compromised. Contamination can occur in a backflow event, where either siphoning or a difference in pressure in the supply line can draw the contents of a directly-connected drain line into the clean water supply.

Air gaps prevent cross-connections and contamination by discharging water and waste across an unobstructed space (a gap of air). That space is between the fixture outlet and the "flood level rim" of a receptacle (be it a sink, floor drain, standpipe, or other approved unit). Think of a faucet (outlet) and sink basin (receptacle): water that leaves the faucet flows across empty air into the basin. Even if the basin were to stop draining and back up to its flood level rim (the top edge or the overflow), dirty backed-up water can never reach the faucet thanks to the space between.

The faucet/sink setup is a perfect example of the concept, but when we're talking about air gaps it's usually in reference to a fixture-specific device. Many are familiar with dishwasher air gaps, but there are other situations in which a direct drain connection might be prohibited, and an air gap necessary.

Pro Tip: Most plumbing codes require a minimum 2" air gap. For discharge/supply pipes that are larger than 1" in diameter, the minimum gap distance is usually twice the pipe's diameter (e.g. a 2" pipe requires a 4" air gap).

Air Gap Devices

For Dishwashers

To many, a dishwasher air gap is merely the thing next to the kitchen faucet that water sometimes spills out of. But as explained above, its function is incredibly important: the rare backflow event that has the potential to move wastewater back through the drain hose and into the dishwasher is instantly neutralized with an air gap.

For those with dual dishwashers (it's a thing!), a dual dishwasher air gap has twin inlets that can conveniently connect both units to the sink tailpiece or garbage disposal for draining.

To learn more about dishwasher air gaps and how to deal with one that's leaking, click here!

For Reverse Osmosis Systems

Reverse osmosis (RO) is one of the most effective and popular water filtration methods around, but produces a good amount of wastewater (or "brine"). Most plumbing codes require an air gap on the RO drain line to prevent the possibility of contamination.

Since RO systems must dispense water through their own faucet, many opt for an air gap faucet that incorporates the air gap inside the body of the faucet itself.

For those with dishwashers in the kitchen, a combination air gap can be installed to service both fixtures, allowing you to use an (often less expensive) standard RO faucet.

RO systems with an air gap often make gurgling noises during the regeneration cycle as the water spills across the gap and into the drain. Annoying as this may be, we still recommend using an air gap with any RO system, even when not required by code. This is not only a safety concern: a backflow event could also damage the RO system, adding insult to injury.

For Water Softeners

Water softeners are another fixture that regularly discharge brine, and are usually prohibited from having a direct drain connection. Because they're usually installed in garages, basements or utility rooms, softeners are typically drained into a floor drain, laundry sink, standpipe or a washing machine outlet box. When floor drains or sinks are used, the softener's drain hose/pipe can simply be secured above the receptacle following local guidelines for air gap distance. Securing the line is important, as the softener discharge does exit with some force.

For draining into standpipes or outlet boxes, we offer a specialized water softener air gap. This device features a 1/2" inlet for the softener's discharge line as well as a 1-1/4" opening for standard washing machine drain hoses, allowing both fixtures to utilize the same drain.

Indirect Waste

Other fixtures and equipment can be prohibited from having a direct connection to the drain - things like emergency eyewash stations, commercial ice makers, food-handling equipment, condensate drainage and some others. Code typically requires they be discharged "through an indirect waste pipe by means of an air gap". Whereas the other fixtures we've discussed have uniquely designed air gap devices, these do not. Instead, the discharge pipe/hose itself is merely secured at the proper height (likely 2x the diameter of the discharge) above the indirect waste receptacle to create the air gap.

To securely support such indirect waste pipes, we offer the Tru-Gap, which can be connected to floor drains or glued to PVC pipe, and accommodates up to 6 drain pipes/hoses.

Always check with local authorities if you're at all unsure of the air gap requirements in your area, as they are not the same everywhere. We highly encourage the use of air gaps, even when one is not required by code (as can often be the case with dishwashers). When it comes to preventing contamination, there's nothing more simple or reliable.


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