If you read the manufacturers installation instructions for any boiler, you’ll read that it’s a requirement for the system to be thoroughly cleaned/flushed prior to the new boiler being installed. This is part of the process of qualifying for your warranty. The other necessary part is having the benchmark commissioning checklist completed at the back of the same manual by the installer.
I, for one, flush every system I fit a new boiler onto. Before the new boiler goes on to it. I also issue a powerflush/system water analysis certificate to prove this has been done.
I take 2 days to fit new boilers and to flush the system and do all the necessary checks required to make sure your system stays healthy for many years to come and so your warranty is validated without any problems. This might seem like a long time compared to how quickly other engineers may want to get your boiler fitted – but in my eyes, I’d rather spend another day on the job making sure everything is spot-on than just trying to get the job done quickly.