An air lock is a pocket of trapped air that blocks the normal flow of liquid through a pipe, pump, or fuel line. The term also refers to a sealed chamber used to move people or objects between two environments at different pressures, like a spacecraft and the vacuum of space. These two meanings share a core idea: air where it shouldn’t be, causing problems or requiring careful management.
Air Locks in Plumbing and Heating
In household plumbing, an air lock forms when a bubble of air gets trapped inside a pipe and stops water from flowing freely. Hot water pipes are especially prone to this because they operate at lower pressure than cold water pipes, meaning there isn’t enough force to push the air bubble out on its own. The result is a tap that sputters, produces only a trickle, or gives no water at all.
Central heating systems are another common trouble spot. When air gets trapped inside radiators or the pipes connecting them, you’ll notice some radiators staying cold while others heat up normally. Cold patches across the top of a single radiator are a classic sign. You may also hear gurgling or bubbling noises from pipes and radiators, and your boiler’s pressure gauge may read lower than usual. In more serious cases, the boiler itself can overheat because water isn’t circulating properly.
How to Clear a Plumbing Air Lock
The most common DIY fix uses mains water pressure from a cold tap to force the air bubble out of the hot water line. You’ll need about a metre of garden hose and some duct tape. Connect one end of the hose to the hot tap outlet and the other end to the cold tap outlet at the same sink, taping both ends securely so they won’t pop off. Turn the hot tap on first, then turn the cold tap on for three to five seconds and shut it off. The higher-pressure cold water pushes backward through the hose into the hot water pipe, displacing the trapped air. If the hot water starts flowing normally, the air lock is cleared. You may need to repeat the process a few times.
For radiators, the fix is simpler. A radiator bleed key lets you open a small valve at the top of the unit. You’ll hear air hissing out, and once water starts dripping from the valve, the trapped air is gone and you close it back up.
Air Locks in Diesel Engines
Diesel engines rely on fuel being delivered at precise high pressure to the injectors. When air enters the fuel line, it creates an air lock that prevents diesel from reaching the injectors at all. The most obvious symptom is an engine that cranks but won’t start. In less severe cases, the engine may start but run with noticeably reduced power.
Several things can let air in. Running out of fuel is the most common cause and will almost certainly create an air lock. Leaks in fuel lines allow air to seep in gradually, causing intermittent problems that come and go. Even normal driving introduces tiny air bubbles as fuel sloshes around in the tank. Changing a fuel filter is another frequent culprit, because the replacement filter needs to be filled (primed) with fuel before installation. If it isn’t topped off properly, a large volume of air enters the fuel lines.
Air Locks in Industrial Pumps
Centrifugal pumps, the workhorses of industrial fluid systems, are particularly vulnerable to air locks. These pumps work by spinning an impeller to create suction that pulls liquid through the system. When air gets trapped in the pump casing, it breaks the vacuum seal the impeller needs to generate that suction. The pump keeps spinning, but it moves little or no fluid. It’s essentially running dry.
The standard prevention is priming: filling the pump casing and suction line completely with liquid before starting the pump, displacing all the air. Some pump designs include a dedicated priming chamber that helps remove air automatically. Without proper priming, trapped air reduces output and can damage the pump over time through overheating and cavitation.
Pressure Chambers in Spacecraft and Submarines
The other major meaning of “airlock” is an engineered chamber that acts as a buffer between two environments at different pressures. On the International Space Station, the Quest Joint Airlock lets astronauts move from the pressurized station interior into the vacuum of space without exposing the entire crew compartment.
The process works in stages. During depressurization, most of the air inside the airlock is pumped back into the station to conserve it. Once pressure drops to about 34 millibars, the remaining air is vented into space. The full depressurization takes 30 to 40 minutes. The doors on each side of the chamber use an interlocking mechanism so only one can be open at a time, preventing catastrophic pressure loss.
A proposed airlock design for future Mars missions adds extra safety layers. The concept divides the airlock into multiple compartments of increasing cleanliness, with the innermost rooms closest to the habitat kept at slightly higher pressure. That pressure gradient stops microbes from drifting inward from the Martian surface. The outer compartment would be disinfected with a gaseous cleaning agent after each use. On Mars, the airlock would depressurize to match the planet’s atmospheric pressure of about 6 millibars before the outer hatch opens. The entire process of exiting takes roughly 64 minutes, and re-entering takes about 97 minutes, including both pressure changes and decontamination.
Airlocks in Brewing and Fermentation
In homebrewing and winemaking, an airlock is a small device fitted to the top of a fermentation vessel. It lets carbon dioxide escape (a natural byproduct of yeast activity) while preventing outside air, bacteria, and wild yeast from getting in. It’s a one-way valve, essentially.
Two designs dominate. The three-piece airlock breaks apart into separate components for easy cleaning, which matters during vigorous primary fermentation when foam and residue can clog the device. The S-shaped airlock has two vertical chambers connected by a curved channel, functioning like a drain trap filled with water or sanitizer. Many brewers use the three-piece model during primary fermentation, when clogging risk is highest, and switch to the S-shaped version for secondary fermentation and aging, when gas production is gentler and steady.
Air Embolism: The Medical Parallel
In medicine, the closest concept to an air lock is an air embolism, where air or gas enters the bloodstream and blocks a blood vessel. A venous air embolism occurs when gas enters a vein and travels through the right side of the heart to the lungs. An arterial embolism happens when air enters an artery and lodges wherever the vessel becomes too narrow for the bubble to pass. This can happen as a complication of certain medical procedures involving IV lines or catheters, or from chest trauma. While the plumbing version is an inconvenience, an air embolism is a medical emergency.