"Bob Lang" wrote in message

I understand that there are manual and automatic filling systems.
In either case, must the boiler be on for it to fill? (It is
not on now.) All the water supply valves leading to the boiler
are already on. However, inline with the supply pipes, there is
also what looks like a pressure regulator with a little lever on
top.

With the supply valve turned on, it should fill by itself. Yes, the lever
on top will open the valve manually. The problem, of course, is the air in
the system that must be purged. Thee should be a valve to drain the system.
While filling, if you open that valve it should allow the water to flow
through and purge the air. Once you do that, turn the heater on, let it
come to temperature, then turn up a thermostat so it calls for heat. That
will start the circulator and you will again be able to purge residual air.
Ed