I think new boiler warranty demands a power flush/chemical clean before installation, from (horror) posts I've read on here, it has to be properly carried out with multiple flushings and water testing so a good idea to do it before removing the old boiler, but will the new boiler warranty be valid if you then run the system with the old boiler for a few days/weeks before new boiler installation?. I presume that the normal method is to do the whole operation in one, ie, remove old boiler, clean the system, then install new boiler. Chemical cleaning is probably "worse".