An air gap is open space that keeps drain or sink water from being pulled back into clean water. On a regular faucet it is the distance from the spout to the sink's rim. On a reverse osmosis system it is the small spout on the faucet where waste water drops before it runs to the drain.
The air gap at every faucet
The 2024 International Plumbing Code (IPC) that Phoenix accepts defines a water-supply air gap as "the unobstructed vertical distance through the free atmosphere between the lowest opening from any pipe or faucet supplying water to a tank, plumbing fixture or other device and the flood level rim of the receptacle." In everyday terms, that's the space between the end of your spout and the top edge of the sink bowl.
Section 608.16.1 sets the minimum, and the size depends on the outlet:
| Outlet | Away from a wall | Close to a wall |
|---|---|---|
| Lavatories and fixtures with openings up to 1/2 inch | 1 inch | 1 1/2 inches |
| Sinks, laundry trays and gooseneck faucets with openings up to 3/4 inch | 1 1/2 inches | 2 1/2 inches |
| Over-rim bath fillers with openings up to 1 inch | 2 inches | 3 inches |
The same section says that openings equipped for hose connection must be protected by means other than an air gap. So a threaded outlet needs another backflow device, not just a gap.
The air gap on a reverse osmosis system
The RO air gap does the opposite job. It sits on the waste side. An RO unit sends rejected water to the drain, and code doesn't want that drain connected straight to the unit. Section 611.2 says "The discharge from a reverse osmosis drinking water treatment unit shall enter the drainage system through an air gap or an air gap device that meets the requirements of CSA B483.1 or NSF 58."
Many residential kits do it with an air gap faucet. The RO unit's drain tube connects to the faucet, the gap is built into the faucet body, and a second tube carries the waste water from the faucet to the drain saddle. That's why these faucets have a small hole at the back.
Manufacturers give you numbers to follow:
- Pentair says "If discharging into the utility sink or standpipe, an air gap of greater than 1/2-inches above the flood rim must be provided."
- Pentair's installation diagrams for a faucet without an air gap are labeled "1.5" Air Gap Required."
- Watts Premier says the black 3/8-inch drain tube "must be as SHORT and STRAIGHT as possible to the drain saddle," with a downward slope from the faucet to the saddle.
The IPC's general rule for drains that end at a receptor (Section 802.3.1) is a gap of "not less than twice the effective opening" of the waste pipe. Check the code and your model's manual, and follow the stricter number.
Why an air gap faucet leaks
A leak at the back of the faucet base usually means the drain line isn't draining. Watts says a bend or dip in the tube keeps the rinse water from flowing, and "Water may back up and come out the air gap hole in the back of the faucet."
The drain connection matters too. Pentair says not to connect the system drain line to the dishwasher drain or near the garbage disposal, because "Back pressure from these units may cause the air gap to overflow." Watts adds that if you have a disposal, the drain saddle shouldn't go downstream of it.
What to check
- 1Measure the gap at your kitchen faucet if you've changed the sink or faucet.
- 2For RO, confirm that your kit either has an air gap faucet or a device listed to CSA B483.1 or NSF 58.
- 3Keep the drain tube short, straight and sloped downward.
- 4Keep the saddle away from the disposal and dishwasher connection.
Phoenix allows plumbing plans to follow either the IPC or the Uniform Plumbing Code, so ask your city which one applies to your job. If your air gap faucet leaks or your RO drain sounds like it's gurgling, HQ gives free estimates and upfront pricing. For more on backflow protection at other fixtures, see what is a dishwasher air gap.