Baxi 910806/9 expansion vessel RP/7,5, type 3

This expansion tank, listed as CIMM RP 7.5 type 3 and intended for Baxi boilers, is a component of the heating system’s closed water circuit. Parts of this type are fitted on the return or system side of the circulation pump. Their function is to absorb the volumetric expansion of water as it heats, thereby keeping system pressure stable and within the set limits of the safety valve. When the water cools and contracts, the vessel returns the stored water back to the circuit. Without a correctly sized and pre-charged vessel, the system pressure would rise sharply on heat demand, causing repeated venting or safety valve discharge, and could lead to water hammer or component stress. Such vessels are selected primarily by their total volume, pre-charge pressure (typically set to match the system’s cold fill pressure), connection thread size, and membrane material. For potable water circuits, a vessel with a suitable membrane and lining is required. When replacing a vessel, always confirm the pre-charge pressure with a tyre gauge (the valve is under the cap) and adjust it after isolating the system and draining the water side. A common failure is loss of pre-charge – often due to a leaking Schrader valve or a ruptured membrane – which manifests as rapid pressure swings and frequent relief valve weeping. The membrane can also fail from age or contaminants; if water comes out of the air valve, the membrane has split. This article is a spare part for service or replacement in Baxi boilers; its specific volume and connection details correspond to the type designation given.