Installer said radiators do not need power flushing and gunge will just sit at base of the radiators?

Part of the boiler manufacturers installation requirements is that the existing heating system (in this case) MUST be cleaned prior to the install. Their are two options a good heating engineer will employ, if he/she is able to test the heating system before the new boiler is installed he will check in particular the ground floor radiators to see if they are reaching a satisfactory temperature, if they are then a chemical clean is only required. If how ever their are certain ground floor radiators that are not performing well or have cold spots then this means that heavy sludge/debris deposits are present and the only effective course of action is a Power Flush.