Baxi 761003/810 expansion vessel CP387/10, type 6

Expansion tank CIMM CP387/10 type 6 for Baxi boilers, catalogue article 294.116.006-2 (manufacturer code 294.116.006). This is an expansion vessel, a component that absorbs the increase in water volume as the system heats up. It maintains system pressure within safe limits and works together with the safety valve, which releases pressure if it continues to rise beyond the vessel's capacity. Parts of this type are selected by vessel volume, pre-charge pressure, connection thread size, and membrane material. The membrane must be suitable for the water quality — potable water systems require a different grade than heating systems. When replacing an expansion vessel, check that the pre-charge pressure matches the system's cold fill pressure; otherwise the vessel cannot buffer correctly. Typical failures include loss of pre-charge (often from a leaking Schrader valve or slow seepage) and a ruptured membrane. Either fault causes the system pressure to swing widely and the safety valve to vent repeatedly. A weeping safety valve after a pressure event may indicate debris lodged on the seat, but if the vessel is faulty the valve will continue to lift. Always verify vessel condition before replacing the safety valve alone.