Is using cleaner essential after pipework, with old boiler

[John stafford]

I have been replacing my radiators (all too small), and adding some extra ones. Boiler is a baxi back boiler, cast iron.
Is it essential to use a cleaner like x400 to get rid of flux residue etc. Or would fill heat, then drain be sufficient.
Then going to add x100.

 

[king of pipes]

I would just flush the system a couple of times a hot flush then a cold one before refilling and adding x100 , you really dont know what condition your pipework is in especially if buried or under floor boards so keep it simple . kop