Hi,when firing up,the boiler will run for a few minutes then cut out(you can hear a humming, but then manually switch out and it goes quiet).We have had a new hot water cylinder fitted a couple of weeks ago, and had a few problems with air etc etc, but the boiler was on constant,however as now milder turned to timer, and problems occuring, any ideas.Somebody suggested sludge build up??Wanted second and third opinion before draining system again and adding inhibitor.So is it this simple or do we call in an "expert"