My fill line is inside the house at the water heater. I have a T on the boiler loop with a check valve and I use a washer style hose to fill from my well's pressure tank into the boiler loop. The check valve keeps boiler water from getting into domestic water.
Exactly, no need to run a separate line clear out to the boiler just to fill it. T off in the basement into a line coming from or going to the boiler.
My well kicks in at 50 PSI, I’ve never seen my boiler system carry more than a few PSI, so no need really for a check valve although it doesn’t hurt.
When I switched to all copper in the basement last winter I installed a double check and a 120V solenoid valve and ran more than three wires to the G400, I can fill it from outside at a push of a button.