Do you have to fill a hot water heat boiler?

There has to be water in either system for it to work, check with the manufacturer and provided documentation to discern proper water capacity.

A hot water boiler (hydronics) as opposed to steam is a system that is filled with water and has an automatic feeder to make sure the boiler never dry fires

A steam (vapor) system has a limited amount of water so it has space to make steam and can have a manual feeder BUT like a hydronic system much have a low water cut off to prevent dry firing