Open fill valve that is attached to the water line. Fill to proper level. Level will depend upon altitude of the highest pipe in system. Due to expansion you may have to bleed a bit when the water heats up.
Now if you want to get rid of this problem install a automatic fill valve. Once set to proper pressure it will automatically fill boiler when low.
The above instructions are for hot water if yopu have STEAM do not use these directions and consult a professional.
Linear Mode
