If the pressure gauge on your boiler shows low pressure, 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. 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. 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 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.