I first flush with mains pressure cold water, then backflush. After this I run some boiling sodium percarbonate/napisan through it (its more effective at high temperatures), I leave some in for it to soak awhile (or overnight). My brew kettle normally gets the same treatment except the water is only about 60*C no way I am boiling a full 50L. Flush the chiller with tap water, and I am done. I have in the past followed this up with an idophor solution but have since become a tad lazy and skip this process now. On brew day I sanitise by allowing a a few litres to run though with boiling wort before I fill the cold side of the chiller.