According to the San Francisco Gate, common causes of air in water lines include valve failures, damaged water tank bladders, accumulated gases in well systems, leaky pipes and damaged water pumps. In some cases, the air pockets are the result of equipment malfunctions and other problems at municipal water treatment plants and in private well systems.
One of the most common causes of air pockets in private well systems is a broken, improperly installed or malfunctioning check valve. In these situations, the valve allows the water in the pipes to fall back into the well reservoir, creating negative pressure in the pipes and sucking air into them the next time a connected faucet is opened. Check valve problems are potentially serious due to the risk of sucking dirt and other contaminants into the pipes along with the air pockets and potable water.
Spluttering faucets are the primary indicator of air pockets in a building's water lines. This is an annoyance but is not dangerous. In some cases, however, the gas trapped in the pipes is methane rather than air. Methane is naturally present in many well systems and is not dangerous unless it accumulates in the pipes. However, large amounts of methane are explosive and flammable. The most effective way to prevent methane build-up is to install a gas release vent on water storage tanks connected to the well.