I need to disconnect and drain an elevated indirect hot water cylinder as the base on which it is supported is chipboard !! and showing signs of imminent collapse - I plan to replace it with 20mm marine ply. I know how to disconnect hot and cold water supplies and drain cylinder but am unsure how I should isolate heating system water flowing through coil via flow and return pipes - no obvious valves between boiler and cylinder. Will I need to drain the heating system? Any advice greatly appreciated