How it works
Inside the tank is a rubber membrane. On one side of the membrane is air under pressure and on the other side is water.

In a closed heating system, water cannot be compressed and any increase in water volume due to the increase of its temperature is absorbed by the expansion vessel. As temperature increases, the expanded water volume pushes against the membrane and water enters the tank. With the temperature decrease, the air cushion forces water back into the system. This permits the system to maintain the pressure, helping to reduce energy consumption of the heating system.

The pre-charge pressure can be checked and adjusted using a tyre inflator with a standard (Schrader) valve adapter.