Wrong forum Condensor boiler that needs to be topped up

I got an extension a couple of months ago and put in a new boiler. Now the guy who put it in, a plumber, working for the builder, says that it needs to be topped up every so often. He didn't explain this to me at the begining. What does this mean? I know the water pressure in this area is not great at times. Does that have anything to do with by boiler needing to be topped up.

Also, I'm getting an attic conversion in the next couple of weeks and the guy whose doing it recommended changing from an open to a closed system. Is this possible with my current condensor bioler?

Thanks in advance for any comments and help.