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What is an Air Lock?

Mary McMahon
Updated May 17, 2024
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An air lock is a bubble of air in a liquid piping system that creates an obstacle, slowing the rate of flow or stopping it altogether. Air locks can occur in a wide variety of systems, and there are several techniques operators can use to address them. This term should not be confused with the airlock, a closed chamber separating two or more chambers from each other or allowing for movement between spaces of differing pressures.

When an air lock occurs, the bubble slowly forces its way to the top of the plumbing, because it is lighter than the surrounding liquid. Along the way, it can collect and incorporate smaller air bubbles, creating an even larger air pocket. When the pocket hits the top of the plumbing, it will prevent liquid from moving through the system, because it cannot force its way past the air bubble or push the air bubble forward in the piping. Air lock commonly occurs in fuel lines, boilers and radiators.

This situation can be accompanied by knocking and other strange noises caused by the pressure inside the symptom. One option for addressing an air lock is to change the pressure in the lines, either lowering pressure to allow the bubble to escape, or increasing pressure to allow the fluid to punch past the bubble. Another technique is to open a valve attached to a high point at the piping. Many systems include such valves for bleeding the lines to release air and allow the fluid to flow normally again.

Frequent recurrence of air lock in a system can indicate a problem with the pressure or other characteristics. A technician will need to evaluate the system to determine how and where air is getting in, and what can be done to prevent blockages in the future.

A special kind of air lock can arise in the context of fermentation vats. As beer and wine ferment, gases are generated and start to rise to the surface of the vessel, causing a rise in pressure. Without relief, the vessels would explode, generating a catastrophic mess. Leaving the vessel open to permit ventilation is not an option as the mixture can become infected or oxidized. The solution is the installation of a valve to allow carbon dioxide out, while keeping oxygen away. These systems rely on an air lock where the gas rises to the top while liquid remains at the bottom to avoid inadvertent release of beer or wine.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a About Mechanics researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments

By MaPa — On Oct 17, 2011

My sister in law is turning into quite the homebrewer, and you have to be careful to avoid a fermentation air lock when you put up your beer after brewing. The large glass containers that the beer is stored in have thick, rubber plugs in the top. If the volume of beer is not properly accounted for the pressure can build up and shoot the stopper an impressive distance, usually bouncing it off the ceiling.

By emtbasic — On Oct 16, 2011

@Charred - For household plumbing, they have something called an accumulator that can act to relieve pressure/air problems and minimize pipe hammering. They are usually copper and stick sideways off of the water pipe. You may have seen them if you've been in a newer construction house lately.

For cars, I have no idea. Was it a cooling system that has been modified in some way, or was it stock? I've never had anything like that in my cars, but I tend to drive pretty boring compacts.

By KLR650 — On Oct 15, 2011

A lot air locks come about as a result of bad designs. Most systems go for years and years without this kind of thing happening. I know that it's cheaper to cut corners when you're installing new plumbing or upgrading an old system, but a professional design by someone who knows what they're doing can save you years of headache.

By Charred — On Oct 15, 2011

@Mammmood - I'm glad that you fixed the problem, but someone needs to come up with a magic bullet for this kind of problem. It haunts not only automobiles but sometimes plumbing lines in houses too.

I understand the concept of opening an air lock valve, but I have some other ideas. I think someone should come up with some kind of treatment to pour into the plumbing that will just puncture the air bubble.

I am not a scientist so I don’t know how to make such a thing, but given that we have other ways of treating water – and even fuel – I think you would be able to create something that would pop the bubble and free the line once again.

By Mammmood — On Oct 14, 2011

Air locks are a pain and can cause you a lot of problems, especially if they get into one of the pipes in your car’s fluid systems.

I had an air lock in my cooling system for quite some time, and the only way that I could get rid of it was to do a complete coolant flush.

Believe me, it was worth it. There’s no point in pouring coolant into the car if the coolant won’t reach its destination because of a bubble in the line.

For the longest time I thought maybe I had a coolant leak, but it turned out to be just the opposite: it was a coolant stoppage.

Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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