Hot Water Tank

[Roly]

If you have an air compressor There is a gizmo advertised that lets you hook up water hose and AC hose with a valve that lets you blow air back through the drain hole. Might loosen stuff enough to drain.  
Ed

Just use a standard air compressor air gun and blow it into the far end of hose connected to drain, see if that starts it draining.  Roly