L 5 is 5 l/per minute through the heater or this will over heat the boiler and switch off prematurely.Increase your flow or ajust you start up valve at the bottom next to your drain. ( a brass long splined rod.)at the bottom but you need to speak to Richard Clue as he has the same hater. I dont have a L5, but they are all very much alike. Be careful they are not toys.