The Helios RR 250 A is a high-performance centrifugal fan engineered for in-line installation in circular duct systems. Designed to handle medium to low airflow volumes under high resistance, this model features a robust casing made from galvanized sheet steel with duct-compatible spigots. Its backward-curved impeller, also constructed from galvanized steel, is directly mounted to the motor and dynamically balanced for smooth, quiet operation and excellent efficiency. The fan is powered by a totally enclosed, single-phase external rotor motor that is maintenance-free, thermally protected, and equipped with sealed-for-life ball bearings. Speed can be adjusted using an electronic controller, a 5‑step transformer, or operated in dual-speed mode via the DS 2/2 accessory (Ref. no. 1267), making it an adaptable solution for demanding ventilation setups.