How a pressurised expansion vessel takes up expanding hot water and why its air charge matters.
On a sealed or unvented system there is no open vent to swallow the extra volume when water is heated, so an expansion vessel does that job. G3 expects you to understand how it works and why its charge matters.
Inside the vessel a flexible diaphragm separates two sides: a water side connected to the system and an air side pre-charged with air or an inert gas to a set pressure.
As the water heats and expands, it pushes against the diaphragm and compresses the air cushion, taking up the extra volume and keeping the rise in system pressure under control. As the water cools and contracts, the cushion pushes the water back out.
The pre-charge pressure has to suit the system. An incorrect or reduced charge can leave too little usable space for expansion; complete loss of the gas cushion or a failed diaphragm can prevent the vessel working properly. These conditions need different checks and do not all mean that the vessel must be replaced.
A marked pressure rise during heating or relief discharge can point to inadequate expansion provision, but neither symptom identifies a failed vessel on its own. Follow the manufacturer’s fault-finding procedure, checking the vessel pre-charge with its water side depressurised and comparing it with the specified setting. A serviceable vessel with reduced charge may be recharged; damage or diaphragm failure requires the appropriate repair or replacement.
For revision, picture a cushion of air behind a diaphragm doing the work the open vent used to do. Confirm the correct charge pressure and vessel size against the manufacturer data and current guidance.
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