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. Sometimes, the loop is built in; other times, it is external. 3. Open both valves on the filling loop slowly, 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 firmly.5. Turn your boiler back on. If the pressure is still low, repeat the process..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.