You didn't specify what kind of boiler you have. Some low pressure steam boilers have a manual fill valve as well as an automatic float switch valve. Hot water boilers have a pressure reducing valve that constantly monitor the boiler pressure and maintain 12 to 18 PSI for the safety of the boiler. If the boiler runs out of water there is a good chance it will rupture and become unrepairable. If you are saying your boiler won't fill, and its a hot water boiler, the pressure reducing valve is probably blocked with sediment and needs replacement. The valve is in the water line that feeds directly into the boiler return line from city water. Hope this helps.
The previous answer was good I would just add that a quick temporary bypass when the pressure reducing valve is blocked is to run a hose from the drain valve on your hot water heater to the drain on the boiler and open them both until the pressure in the boiler is at 15 lbs. turn them both off and bleed the radiators until you have water coming out of the bleeders if you cannot get water coming out of the bleeders on the top floor repeat the fill until you do if the system is full it will work fine. get pressure frequently to see if you have a leak in the system if it stays in the normal operating range change the pressure reducing valve at the end of the season or when you have time to have the heat off while it is being done. If you have a steam system there should be a bypass for the automatic feeder that would let you add water the above method will work if the bypass was not installed just bring the level up to theproperlevel in the class guage.
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