sentences:  When the heating system is empty or at the low end of the normal range of working pressure, the diaphragm is pushed against the water inlet; as the water pressure increases, the diaphragm moves, compressing the air on its other side.An older style of expansion tank was larger, oriented horizontally, and had no rubber diaphragm separating the water from the air pocket.This now obsolete style would gradually transfer air from the tank to the highest point in the system, due to air dissolving in the water, and then coming out of solution elsewhere in the system.