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How does an air gap work?

An air gap is an efficient way to prevent dirty drain water from flowing back into your clean drinking water. Whenever a drain connects an appliance to a drinking water supply, plumbing codes require backflow prevention. The air gap creates a separation in the line to the drain. If the drain were to back up, dirty water would not flow backward past the air gap.

air gap to RO faucet

The size of your air gap depends on the flow rate of the water going through it. You can use an air gap for your dishwasher, washing machine, water softener, or reverse osmosis system. Either install an air gap faucet or attach a dual inlet air gap on your appliance if you prefer a faucet with a special design. 

Browse Tomlinson air gap faucets or Watts designer faucets.

What causes an air gap leak in a reverse osmosis system?

Reverse osmosis separates contaminants from water molecules and flushes them to the drain. Water leaking from an air gap in an RO system may result from improper installation or a restriction preventing water from flowing to the drain. Gravity is the only force that moves water from the air gap to the drain, so any resistance causes the water to back up and leak from the air gap hole. A leaking air gap reintroduces contaminants back into the filtered water.

An air gap leak may be the result of present feed water conditions. The more contaminants and dissolved solids in the feed water, the greater the chance for accumulated debris. Air gaps back up when something obstructs water flow. A slow-running sink drain may cause debris to collect in your RO system air gap. Where may blockage occur?

  • In the drain saddle where the drain tube connects
  • In the drain tube at the saddle connection
  • In a clogged drain pipe
  • In a drain saddle clamp that has rotated away from the drilled hole
  • In a clogged air gap hole

Tip: Prevent build-up by cleaning your drain during your annual filter replacement. Scheduling maintenance is better than reacting to a leak.

How to fix an air gap leak in an RO faucet:

              air gap under sink

Proper installation prevents restrictions in the air gap.

Here are a few tips to use when adding an air gap to an RO faucet:

  • Place the drain saddle as far away from a disposal as possible. A disposal can shoot debris into the drain line and block the air gap. Locate a place where tubing can run as straight as possible without looping or extreme bending.
  • Adjust excess tubing that dips below the saddle location that causes water to back up the air gap and leak out.
  • Keep the drain saddle from rotating away from the drilled hole. Here's how: apply the clamp, tighten it, then drill the hole. Be careful not to damage the tube connection fitting. Rotation can also occur over time if your system under the sink gets bumped. 
  • Use a horizontal mount to minimize the gurgling noise your RO system makes while filtering water. The location of the saddle drain may also help. 
  • Clear accumulated debris by taking the tubing apart from the saddle. During the cleaning process, make sure the hole is properly lined up with the clamp. Insert a pencil or small screwdriver through the fitting to ensure proper alignment. 

installing an air gap

Download PDF instructions to help you repair your air gap leak.


How can you prevent an air gap leak?

You may not be able to completely eliminate the occasional air gap leak, but you can minimize the frequency with which they occur. Avoid rinsing large chunks of food or other objects that might cause an obstruction down the drain. Treating your drain with MegaMicrobes, an all-natural drain cleaner, keeps biofilm and other organic waste build-up in your drain under control.


7 comments

  • Joe Z,

    Black sludge can be an indication of buildup occurring in your drain itself. A periodic dose of a drain cleaner, like MegaMicrobes, can go a long way in keeping the line and RO discharge free from debris and accumulation.

    John Woodard, Master Water Specialist on

  • Ryan,

    If you had enough of the tube (hose) attached to the drain saddle to cut it and still reach the fitting, you may have had too much tubing to begin with. The extra tubing may have contributed to the clog. Making sure your sink drain pipe stays clear and the tubing from air gap to drain pipe is as straight as possible may reduce the clogging. Also, the location of the saddle can contribute to the problem (such as being too close to the disposer).

    John Woodard, Master Water Specialist on

  • Leak detection is not that easy job as it seems as identification is the most important thing. There are so many instructions that need to be taken care of while detecting leak whether it is about water leaks, roof leaks or any other kinds of leaks. It was quite informative thing for us to know about. I would really like to see more good posts on air gap leak detection.
    Manual leak location surveys

    Manual leak location surveys on

  • John Brown,

    The fitting or connection where the tubing connects to the drain pipe is the most usual place for debris to accumulate. Just disconnecting this periodically to check for and clean the accumulation is great maintenance. As long as you are careful with what goes into the sink drain, you shouldn’t have to worry about taking apart the drain. If you see cracks, however, you may want to consider having the drain pipe replaced. Hope this helps!

    John Woodard, Master Water Specialist on

  • We had a vent hole that was leaking. I disconnected the vent line, at the sink drain pipe. I took compressed air (60 psi) and blew the line from the faucet vent side. Black sludge came out. Looks like this was our problem. 3 days later no leak

    Joe Z on

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