Our boiler woke us up suddenly with a loud clunking burbling noise. Apparently known as kettling. The Video shows noise at the start and silent boiler at the end after cleaning the system with an additive descale cleaner .The Video also shows how to drain the system add the cleaner and refill the closed pressurised system. Please note I am not a plumber nor am I qualified to show you how to do this job, seek professional advise if in doubt. It is against the law to work on a gas boiler if not qualified.