A sealed heating system normally has a recommended pressure range shown in the boiler manual. Many systems sit around 1 to 1.5 bar when cool, but the correct value is the one specified for your model.\n\nCheck the pressure display when the system is cool. If it is below the manufacturer’s range, find the exact user instructions for your boiler before doing anything. Filling-loop layouts vary, and using the wrong control can cause damage.\n\nOnly repressurise if the manufacturer’s user guide clearly describes a homeowner procedure and you are confident you have identified the correct controls. Stop at the stated pressure, close every valve used and do not remove the boiler casing.\n\nIf pressure repeatedly falls, rises sharply when heating, water is leaking, a relief pipe is dripping, or you are unsure which controls to use, stop and arrange an engineer visit. Repeated topping-up is not a repair and can hide a fault.\n\nThe linked Vaillant guide illustrates the general process. Always prioritise the instructions for the exact make and model in your home.