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 slowly, usually counterclockwise. . 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 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 uncomfortable performing this task yourself, contact a Gas Safe registered engineer. Incorrectly repressurizing the system can cause damage.