Hi, Close drain tap and make sure any water heater drain plugs are shut. I then fill water container with a caravan water system sterilizer and open all taps ( make sure shower is in the sink } connect up pump, when water flows steadily turn off cold taps and leave hot open till a steady flow of water from hot taps shows water heater is full then turn off all taps. leave sterilizer to work then flush out system with clean water then all is clean and ready for new season.