The vast majority of domestic central heating systems can be cleaned by flushing, there are however exceptions.

Single pipe systems are almost impossible to flush as the water will follow the path of least resistance and never flow into a blocked or partially blocked radiator, in this case the radiators need to be removed and flushed manually whilst the pipework is flushed with the powerflushing machine.

Gravity fed hotwater systems can complicate the flushing procedure for similar reasons, the addition of full-bore gate valves into the pipework allows us to control the flow of water and to effectivly flush the system as two parts.

Despite what you might have heard microbore systems are fully flushable it just takes a bit longer.

Completely blocked systems are unlikely to be cleared by powerflushing, In these cases it is best to physically replace the blocked pipework section and then flush the system to prevent further blockage from occuring elsewhere.