If the pressure gauge on your boiler is reading below 1 bar, you'll need to add water to the system. Consult your boiler's manual for specific instructions, or follow these general steps::1. Turn off your boiler and allow it to cool.2. Locate the filling loop. This is usually a flexible braided hose connected to your mains water supply and boiler, under the unit. Sometimes, the loop is built in; other times, it is external. 3. Open both valves on the filling loop carefully, If your boiler uses a lever-type handle, consult the manual. Listen for the sound of filling.4. Keep an eye on the pressure gauge. When the pressure reaches the appropriate pressure, close both valves on the filling loop turning clockwise.5. Turn your boiler back on. Confirm that pressure is at an appropriate level. If problems persist, contact a qualified HVAC technician. If you're not sure how to repressurize your boiler, it's best to consult a professional. Ensure your safety and avoid costly repairs by having it done right.