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Combination boilers and system boilers both carry pressure gauges and we're often asked about the pressure shown on these gauges. Any sealed central heating system needs an expansion vessel, a pressure gauge and a pressure relief safety valve.
If you heat water it expands. In an open vented central heating system (with a header tank) some of the water moves back up into the header tank. The expansion does not increase the overall volume of water by a huge amount (maybe 3 or 4 litres, depending on the size of the heating system) but it would be more than enough to burst the pipes if it had nowhere to go.
In manufacture the air side is pumped full of air, typically to about 1 bar (about 15 psi). This forces the diaphragm right across to the opposite wall of the chamber. When the central heating system is filled with water and pressurised (to about 1 bar) the water pushes the diaphragm back to about the middle