Quiet Box Fan (QBF) fan design allows for supply and extract ventilation for areas/rooms with high requirements to low noise levels and limited mounting space. Units can be installed above ceilings or in loft spaces. Suitable for round ducts. The centrifugal impeller with back-curved blades is powered by means of 2-pole asynchronous motor with external rotor. The motors are equipped with built-in thermal overheating protection with automatic restart.