Complex sentence: 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.
Split into simpler sentences: When the heating system is empty or at the low end of the normal range of working pressure, the diaphragm will be pushed against the water inlet. As the water pressure increases, the diaphragm moves, compressing the air on its other side.