If the pressure gauge on your boiler shows low pressure, you'll need to add water to the system. Repressurizing is a straightforward process: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. 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. If the pressure is still low, repeat the process..If you're uncomfortable performing this task yourself, contact a Gas Safe registered engineer. Incorrectly repressurizing the system can cause damage.