An air gap faucet keeps the drain line of a reverse osmosis (RO) system physically separated from the sink drain by a short open space. The open space stops drain water from flowing back into the RO unit. Some plumbing codes require it, and the fitting needs either a second faucet or an extra sink hole.
The air gap question comes up with RO systems, which have a drain line. For the wider category of units, see under sink water filter.
Code requirements vary by jurisdiction, and installation steps come from the maker's manual. Follow the maker's manual for those.
How the Three Connections Fit Together
An air gap faucet setup uses three connections. Treated water runs from the RO system to the faucet spout. The RO drain line runs into the base of the faucet. A second line runs from the faucet down to the drain saddle, which clamps onto the sink drain pipe.
The spout delivers the treated water. The RO drain line carries drain water from the unit into the faucet base. The line below the faucet carries that water to the house drain.
For the unit itself, see reverse osmosis system.
What the Air Gap Prevents
The air gap prevents backflow. Backflow means drain water moving the wrong way, back toward the RO unit. The gap is a short open space between the RO drain line outlet and the drain connection, so drain water cannot flow backward through it.
If preventing backflow is the reason you are looking at this fitting, understand the gap first. The rest of the fitting connects the lines around that gap.
Plumbing Codes Decide Whether an Air Gap Is Required
Some plumbing codes require an air gap on an RO drain. The requirement is not the same everywhere, because codes vary by jurisdiction.
Ask your local plumbing authority which rule applies.
If your code requires an air gap, a setup without one does not meet it. If your code does not require one, the factors below (the extra sink hole, the drain saddle, and the leak risk) are the ones to weigh.
The Second Faucet or Sink Hole
An air gap faucet needs either a second faucet or an extra sink hole. The fitting has a spout for treated water and a separate drain port, so the setup needs room for both functions in the sink deck.
Count the holes in your sink deck before you choose a fitting. A sink with a spare hole and a sink without one lead to different installs.
Where the Drain Saddle Clamps
A drain saddle is a clamp-on fitting that taps the sink drain pipe. It usually sits above the trap. The line from the faucet runs down to the saddle.
Because the saddle taps the drain pipe, it is part of the drain path. A clog at the saddle blocks that path, and the vent symptoms in the next section follow from that blockage.
The saddle usually sits above the trap.
Why Water Leaks from the Faucet Side Vents
The air gap body has small vent holes on its sides. When the drain line or the drain saddle is blocked, drain water can come out of those holes instead of draining away. A clogged drain saddle or a clogged drain line can cause that.
The two symptoms below have different causes, so match what you see to the row that fits.
Look at the drain side first, because every cause of vent leaks sits there. Work through the saddle, then the drain line from the faucet.
| Symptom | Cause |
|---|---|
| Water out of the side vent holes | Clogged drain saddle or clogged drain line |
| Gurgling sound during a refill or flush | Water moving through the drain line during a refill or flush cycle |
Gurgling During a Refill or Flush
Water moving through the drain line during a refill or flush cycle can make gurgling sounds.
The gurgle and the vent leak have different causes. Gurgling is tied to a refill or flush cycle. Water out of the vents is tied to a blocked drain path. Seeing one does not confirm the other.
Air Gap Setups Compared with Setups Without One
An air gap setup has the open space described above. A setup without an air gap does not have that space between the drain line outlet and the drain connection.
If your code requires an air gap, the code has already chosen that setup.
The choice then comes down to the sink holes, the drain path, and the leak and noise points covered above. For the broader installation picture, see under sink water filter installation cost.
Choosing a Fitting Once the Code Question Is Settled
Settle the code question first. The steps below follow the order of the decisions.
NSF/ANSI 58 is the standard for point-of-use reverse osmosis systems. Check the product documentation to confirm certification. For specific system comparisons, see best reverse osmosis system. For what reverse osmosis treats, see what does reverse osmosis remove.
- Settle whether your local code requires an air gap.
- Count the open holes in the sink deck, or plan for a second faucet.
- Plan the drain side, including the drain saddle above the trap and the drain line from the faucet.
- Choose an RO system built to NSF/ANSI 58 if that certification matters to your decision.
Common questions
Do I need an air gap faucet for a reverse osmosis system?
Your local plumbing code decides that. Some plumbing codes require an air gap on an RO drain, and codes vary by jurisdiction. Ask your local plumbing authority to confirm.
What does an air gap faucet prevent?
It prevents drain water from flowing backward into the RO unit. The short open space between the RO drain line outlet and the drain connection breaks that path.
Why is water coming out of the side holes on an air gap faucet?
Drain water comes out of the vent holes on the air gap body when the drain line or drain saddle is blocked. A clogged drain saddle or a clogged drain line can cause it. Start on the drain side, since those causes sit there.
Why does an air gap faucet gurgle?
Water moving through the drain line during a refill or flush cycle can make gurgling sounds.
Does an air gap faucet need its own sink hole?
It needs either a second faucet or an extra sink hole. The fitting has a spout for treated water and a separate drain port, so the setup needs room for both.