This Worcester Bosch charging key assembly is part of the unit's filling loop, used in central heating boilers and systems. It allows the user to manually top-up the amount of water in the system if the pressure drops. External loops are usually removable in keeping with regulations, while internal filling loops normally feature a key that can be removed to interrupt the connection after use. Boiler spare parts must be fitted by a certified Gas Safe engineer. Orders received before 4pm are usually dispatched for next business day delivery.