When the system is turned on, the temperature of the water increases and thus it increases also its volume. Water fills the membrane that enlarges by filling gradually the vessel.
The volume of water increases until it reaches maximum working temperature. Then the membrane takes up practically the entire inside of the tank. An airbag stabilizes the pressure and the membrane prevents direct contact between water and the internal surface of the tank.
When the temperature gradually drops and the volume of water is reduced, it is pushed out from the tank until the membrane reaches its initial volume at that point a new cycle begins.
