Water has dissolved air in it that bubbles out and gets trapped in
radiators, not allowing the hot water to flow fully. Each radiator should
have a little bleeder at the top that operates with a little wrench or
screwdriver. These bleeders might also be at high points of the piping
where the pipe goes up and then down. Repeated bleeding is needed till all
the air is eliminated. Since a boiler uses the same water over and over,
eventually, all the free air gets eliminated.
If your system is typical, then, no it shouldn't use much fresh water.
Maybe the relief valve is leaking, or blowing off for some reason, but when
fresh water is introduced into the system, with it comes more dissolved air.
Then the bleeding must take place again and again.
Again, if your system is typical, the pressure gauge should read about 14
psi. If it's much greater, then the automatic fill valve is faulty. The
temperature is a harder call, since systems vary so much, but if it's more
than 200 degrees, there's probably a problem. You might also simply have a
leak in the piping that's undetected.