If the pressure gauge on your boiler shows low pressure, 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. It may have a valve or tap.3. Open both valves on the filling loop carefully, usually counterclockwise. . You should hear water flowing into the system. .4. Keep an eye on the pressure gauge. When the pressure reaches the recommended level (usually between 1 and 1.5 bar), close both valves on the filling loop firmly.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. Incorrectly repressurizing the system can cause damage.