As far as I am aware some typs of clean is necessary under the manufacturers instructions. Power flushing is more to do with the litres per min been pumped round the system around about 100 per min along with the chemicals used it dislodges debris and because of the high flow pump it scoures the pipes with the debris and chemical solution. This is much better than a chemical been pushed round by a central heating pump. But first ask yourself a few questions , were the rads hot before were there any cold spots on rads , did system take ages to warm up . If rads were fine then it proberbly dosent need power flush. Another option on combi systems ir to put a drain point onreturn pipe as close to boiler as ppossible then you can shut off valves on boilers flow and return and usingmfilling loop flush the whole system as much as is necessary, first hot then cold , with a cleaner in if you want and repeat untill your happy