Occasionally a system will become restricted by sludge and even the best boiler will not be able to cope. Sludge is iron oxide (rust) from the radiators whereby the system has been run for a long while without an inhibitor. It is black and powder like and builds up in sharp bends and dams in the pipework as well is in the radiators. Sometimes there will also be cold spots on radiators or your boiler might even overheat. Sludge in the system is also a major reason for component failure such as pumps. A power-flush is a process of coupling a large standalone pump in place of the system pump and together with powerful chemicals, breaking down the deposits of sludge back into powder form and removing them from the system by flushing. Granted, it is not always totally successful such as in the case of plastic central heating pipes, but very seldom does it not improve a dirty system. Only from experience can an engineer determine whether a power-flush is required. A power-flush will often alleviate the need for new radiators or a re-pipe.
I have done many, many flushes over the years, and your system will work more efficiently if it is clean.