anything if you’re trying to fix it & don’t know how,,, most boilers now have auto-fill but there’re many still working fine that don’t,,, 1 valve for fill & another for drain.
btw, boilers are never filled to capacity,,, suggest you call your hvac guy & have him stop by for a look-see – BE THERE when he is,,, you could be jeopardizing your family’s safety poking where you’re not familiar.
knowing what you are doing you could end up blowing up yourself and the entire household! Boilers are not for the inexperienced to mess with. Call a professional and have them teach you how to use your heating system before you create a tragedy.
to offer any help. An old boiler could include anything from the years 1600 to 2000 installed in any of a 1000 different ways.
And on a second point: Internet etiquette considers posting in all capital letters to be shouting. There’s no reason to shout here. It’s considered rude.
The more information you can give us, the more help we can offer. With no information there isn’t much we can do. From your post the only things we KNOW are that you don’t know what you are doing and that you have ‘an old boiler’. That is a dangerous combination.