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Why an Air Gap Leaks and How to Fix It

Posted by John Woodard on September 18, 2019

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. 

Learn more about how dishwasher air gaps work.

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.


Comments 1-10 of 13

My problem is different. My air gap is now leaking constantly. It’s not s clog. I’ve used pipe cleaners, and compressed air to blow it all out. Doesn’t matter, still leaks. Even if I disconnect drain tube from the saddle and let that drip into a bowl, water is still coming out the air gap… help!

Bill on March 28, 2023

Our dishwasher air gap spills water out into the sink and all over the counter. Isn’t it clogged somewhere?

Don Aguillard on December 30, 2021

Thank you so much for this whole article. It was very informative and easy to understand. I fixed the leak quickly. It was the drain clog in the saddle. Thx again

Thank You on December 07, 2021

I took off the drain tubing to the saddle and made sure that it was cleaned out, and it is lined up with a hole. For some reason water is coming out the air hole of fixture. Is there any way to flush air out of it so it doesn’t push water out that instead of down the drain?. I have made sure there’s no obstructions in the drain line from the faucet to the drain saddle and everything is how it should be.

David Fillmore on May 05, 2021

Installed brand new northstar system, and it is constantly seeping water out of air gap hole and no water will come out of spout when turned on.

Dyllan on September 01, 2021

We have a vent hole that leaks air constantly. The only way to silence it is to place something over the hole to smother it. It is still leaking air, but at least we can’t hear it. It will shut off momentarily and then just randomly start back up again. Any suggestions?

Fawn Wallace on April 22, 2021

I got a new system and water only drains and comes out of air hole. It doesn’t seem to fill the system or tank. All connections have been triple checked.

Doug on March 19, 2021
I put a new filter faucet in and now the water comes out “foggy”. Air is getting in. How can I fix this?
Deb on May 27, 2020

Great question, Seth!

Noise from an airgap is common. Depending on the location of the drain saddle connection to the drain, the noise of water dropping into the water in the ‘P’ trap can sometimes make that constant ‘gurgle’ sound as it slowly pushes water through the trap. The diagram shows how water enters the drain line. This sound occurs only when system is making water. If it seemingly never stops, turn the tank valve off and see if the system shuts off (gurgle stops). If after several minutes it continues, your system may have an issue causing the shut off diaphragm to not function.

Hopefully this helps!

John Woodard, Master Water Specialist on January 07, 2020
Just installed and hearing constant drip noises. Is this normal?
Seth on January 07, 2020
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