Pump expansion valve

A small relief valve in the brew water line that allows water to flow back to the reservoir or drain when heating water in the boiler expands. Prevents over-pressurisation as cold water heats and expands inside a sealed boiler.

Why it matters — Water expands by about 5% from cold to hot. Without an expansion valve, that volume increase pressurises the brew boiler and can cause leaks or component damage. The valve vents the excess pressure safely.

Good to know

• Different from the OPV — the expansion valve handles thermal expansion at idle; the OPV handles over-pressure during brewing.
• Some machines combine both functions in a single valve. A leaking expansion valve usually shows up as a slow drip into the drip tray when the machine is idle.