Solder for hot water heating system
My understanding is that you only need to use lead-free solder for potable water supply piping. For every other kind of piping, you can use lead solder, and that would include the copper pipe in hot water heating systems.
Since the heating system is a closed system, when soldering, make sure you allow some way for the heated air inside the pipe to escape. Otherwise you build up air pressure inside the pipe that works against capillary pressure trying to suck the molten solder into the joint. The result can be joints that won't take solder.