Why is Boiler is banging when starting?

A boiler may bang when starting due to a phenomenon called "kettling," which occurs when limescale builds up on the heat exchanger, causing water to overheat and produce steam bubbles. When these steam bubbles collapse, they create a banging noise. Other potential reasons include air trapped in the system, loose components, or issues with the burner or ignition system. It's advisable to have a qualified technician inspect the boiler to diagnose and resolve the issue.