Expansion vessel

An expansion vessel is a small tank, split internally by a flexible diaphragm with a cushion of air on one side, fitted to a sealed hot-water or heating system. Its job is simple physics: water expands when it's heated, and that extra volume has to go somewhere — the vessel absorbs it so the system pressure doesn't spike. When the vessel fails (the diaphragm perishes or it loses its air charge), the expanding water has nowhere to go, so the pressure climbs every heating cycle and forces the pressure-relief valve to weep or discharge — the classic symptom of a dripping relief valve or a gauge that keeps rising then dropping. It's a cheap, common part, but diagnosing it (versus a faulty relief valve or an over-pressured supply) and recharging or replacing it is a job for a plumber in Dubai, because getting the pre-charge pressure right is what makes it work.