If the pressure gauge on your boiler is reading below 1 bar, you'll need to repressurize 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, If your boiler uses a lever-type handle, consult the manual. You should hear water flowing into the system. .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 uncomfortable performing this task yourself, contact a Gas Safe registered engineer. Ensure your safety and avoid costly repairs by having it done right.