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. It may have a valve or tap.3. Open both valves on the filling loop carefully, 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 not sure how to repressurize your boiler, it's best to consult a professional. Incorrectly repressurizing the system can cause damage.