Which would you guys recommend for the job, got an isar that's got a partially blocked main heat exchanger as you can hear it whilstling when you run ch or dhw. My plan was to try a cleaner in it and leave it for a couple of days as the customer is home for a day on Tuesday and then back on Thursday so can go back and drain it. Is x800 to aggressive to leave for that long as don't want to kill the heat aluminium heat ex