Abstract: An air charging device is connected to a supply tank, a pump and a use facility, such as a faucet. When the faucet is opened, water flow from the tank through the device to the faucet charges a chamber in the device with air; while when the pump is actuated, water flow from the pump through the device to the tank causes the air in the chamber to be compressed and escape into the tank. The flow of water in either of two directions causes a freely movable valve member to engage and shift a plunger. Movement of the plunger in one direction causes intake of air into the chamber, and movement thereof in the other direction causes compression and discharge of air into the supply tank.