RE: The old age question, to leave open or closed a boiler automatic fill valve?
I would close the fill valve. When the system is installed properly there will be no loss of pressure and no need to add water. The only time to add water will be when the water pressure drops due to a leak or air being removed from the air elimination device that came out of solution. Always ensure there is a low water cut off devi e installed and the boiler will be protected.