What’s Inside a Boiler? Series - By Terry Matthews Part 1: Expansion Vessel If your boiler pressure keeps dropping, swings up when the heating is on, or you’ve noticed water dripping from the copper pipe outside, the expansion vessel is one of the first things to suspect. It is a really common cause of pressure issues in combi and system boilers, and if it is left, it can trigger the PRV and put strain on other components. This guide explains what the expansion vessel is, wha