Is the water in your sink not draining away properly? or even not at all? Sounds like there is a blockage somewhere in the system. You may think this is an expensive job to get someone out, but this problem can usually be solved without the help of a plumber, and it will take you no more than 30 minutes. Mostly, slow drainage in your sink would indicate a blockage somewhere; a build-up of dirt and debris that has accumulated over time but another reason for this could be an airlock in the system, so we are going to have a look at how to fix an airlock in a sink drain.

Sink Drain with Plunger
Sink Drain with Plunger

What is an Airlock in a Drainage System?

Airlocks, (also called vapour locks) are common in pipe systems that have been installed without proper ventilation. An airlock is a pocket of air that has no way of escaping therefore creating pressure that prevents the natural flow of liquid through the system.

An airlock or other blockage may manifest itself as the sink draining particularly slowly or the drain making gurgling noises as the water drains away.

How To Fix an Airlock in a Sink Drain

To fix an airlock, you will need to treat it the same as any other blockage and be prepared. So firstly, we recommend getting a few things together so you can sort this problem out quickly and efficiently with no mess.

What You Need

  • Gloves
  • Plunger
  • Towel
  • Bucket
  • Drain Unblocker
  • Drain Rods
  • Old Brush
  • New AAV (Possibly)

1. Check for a Local Blockage

First of all, we need to investigate whether it is actually an airlock in the system or if there is a build-up of dirt and debris clogged up somewhere.

There are a few things you can do to check if there is a local blockage:

  • Run boiling water down the sink
  • Use sink/ drain unblocker
  • Use a plunger
  • Remove and clean the trap (There are different types, but this is the fitting we mostly call the U-bend)
  • Use drain rods
  • Lift the manhole to ensure the blockage is not at your main drain

Once you have potentially ruled out a local blockage by doing the things mentioned above, you can move on to step 2.

2. Locate Your AAV

Once you have ruled out a blockage, you will now want to locate your AAV. An AAV (air admittance valve) is used to allow air back into a system to keep it balanced and the water flowing correctly.

Note: In the UK, AAVs are usually found in the loft although some individual fixtures may have their own; particularly if the fixture was added after the house was built and is not directly plumbed into the original pipework.

3. Inspect the AAV

Once you have located your AAV, inspect the valve. If your AAV is seized, remove it and clean out any debris that may block it.

If unsure, please see the below video on how to maintain your AAV.

4. Test

Once you have inspected and cleaned your AAV (or replaced it if required), you can now test that your drain is working properly.

What to Do if This Doesn’t Work?

If cleaning the AAV has not had the desired effect, this means that there IS a blockage elsewhere that you have not yet located. If this is the case and you want to try and fix the problem yourself, you can try and flush the stack by using a hosepipe and jetting the water down it from the top or using drain rods down the manhole outside. Alternatively, call a plumber out to assist you.


If you’re experiencing a slow sink drain, there’s a good chance that there’s an airlock obstructing the water flow. This guide provides easy instructions on how to locate and remove the blockage using basic tools and supplies that most people have at home. If these methods don’t work, it may be necessary to call in a professional plumber to diagnose and solve the problem.

How to Prevent Airlocks in Your Plumbing Infographic

Plumbing Wizard Tips

“If unsure, call out a plumber to take a look for you!”

“Before checking the AAV, exhaust the possible local blockage options in the drainage system!”

“Make sure that you check for an AAV around the sink that is causing the issue as it may have its own!”

“Make sure that you wear gloves, you may also want a respirator too as you may end up encountering sewer gasses when you remove the AAV!”

Frequently Asked Questions

What is a Durgo valve?

A durgo valve is another name for an AAV (air admittance valve). An air admittance valve is a one-way valve that allows air to enter the drainage system when the water leaves it. This prevents negative pressure from building up in the drainage system, which can cause water backups – they are also designed to prevent noxious sewer gasses from entering the home.

When should you use an air admittance valve?

Air admittance valves should be used in plumbing systems whenever there is a need to vent appliances such as dishwashers, washing machines, or toilets while preventing the backflow of sewer gases into the building. They are also useful for venting traps on sink and lavatory lines where a continuous vent cannot be maintained.

Where should I place my air admittance valve?

The air admittance valve should be placed at the highest point in the drainage system. This will ensure that the valve is open when water accumulates in the drainage system and will allow air to enter, which will help to prevent a build-up of pressure and eventual overflowing of the drainage system.

What is an airlock in a drainage system?

An airlock, or vapour lock, is a pocket of air in the pipe systems that restricts the natural flow of liquid due to trapped air, often resulting in slow drainage or gurgling noises.

How does an airlock differ from other blockages in a sink drain?

While both can slow down the drainage, an airlock is specifically caused by trapped air pockets in the system. In contrast, blockages are typically a build-up of dirt, debris, or other obstructions.

What is an AAV and its purpose in drainage?

An AAV (air admittance valve) allows air back into a system to balance it and maintain the proper flow of water. It also helps in preventing backflow of sewer gases into the building.

Where can I typically find the AAV in my house?

In the UK, AAVs are often located in the loft. However, if a fixture was added after the house was built, it might have its own separate AAV not connected to the original pipework.

How can I determine if the slow draining is due to an airlock or a blockage?

You can attempt various methods, such as running boiling water, using a plunger or drain rods, or inspecting the main drain manhole. If these methods don’t indicate a blockage, it could be an airlock.

What’s the importance of wearing gloves when dealing with the AAV?

Wearing gloves protects your hands from potential contaminants and can prevent exposure to harmful sewer gases that might be released when removing the AAV.

Why do I need a respirator when removing the AAV?

Along with the gloves, a respirator can protect you from inhaling potentially harmful sewer gases that might emanate when dealing with drainage systems.

What if cleaning the AAV doesn’t solve the problem?

If cleaning or replacing the AAV doesn’t work, there may be a blockage elsewhere in the system. You can try flushing the stack or use drain rods. If these methods fail, consulting a plumber is advisable.

How does a Durgo valve relate to an AAV?

A Durgo valve is another name for an AAV. It’s a one-way valve designed to let air into the drainage system when water is flowing out, helping prevent backups and keeping sewer gases from entering the home.

When is the best time to call a plumber for drainage issues?

If you’ve exhausted all DIY methods, including checking for blockages and cleaning or replacing the AAV, and still experience drainage problems, it’s time to consult a professional plumber.


Lee Pearce is not just a master plumber; he’s a veritable Plumbing Wizard. With over 30 years of experience in the trenches of pipes and drains, Lee has become the go-to sage for DIY plumbing, saving homeowners thousands in potential call-out charges. As the founder of Plumbing Wizard, he’s dedicated to demystifying the complexities of home plumbing, offering easy-to-follow advice that stands the test of time and pressure. His online blog is a treasure trove of tips, tricks, and tutorials that empower everyday individuals to take charge of their home’s plumbing health. Lee’s practical wisdom is not just about fixing leaks; it’s about imparting confidence and self-reliance. When he’s not writing or elbow-deep in a plumbing project, Lee is passionate about educating the next generation of DIYers, ensuring that practical skills are passed down and preserved.

More You Might Like