Pump

When the boiler is down, it can stem from several causes: a broken safety valve, a blocked pump, a burnt control board, ...
The gas boiler expansion vessel or oil allows you to absorb the overpressure present in the hot water circuit. Sometimes, the gas between the membrane of the expansion vessel and the metal housing runs out, and it becomes necessary to add some. An expansion vessel pump works like a bicycle pump: you need to unscrew the valve of the expansion vessel and then insert the hand pump or foot pump for the expansion vessel. Push to add air to the expansion vessel to re-inflate it. A professional (heating engineer or plumber) can insert nitrogen into the expansion vessel, achieving the same effect as air.
If the boiler spare part is in poor condition, then the expansion vessel itself must be replaced with a new one. It must have the same amount and shape to ensure its function.