I recently had an annual boiler inspection (starting to get cold in Minnesota) and the tech didn't see any issues but recommended a deep cleaning of ash from inside the boiler as a "once in ten years" maintenance activity. He used a mirror to show the insides of the boiler, and there looked like pretty significant ash buildup. He quoted me $450, which seems pretty expensive for what essentially sounds like no parts, just time. Is this a recommended activitiy and if so is this a reasonable price? The boiler is about 20 years old.