A combination includes a selectively driven fan and a bypass flow passage for bypassing the fan. The bypass flow passage communicates with a first check valve to allow air to flow from the bypass passage to a downstream location. A second check valve allows air driven by the fan to pass into a return passage and return to an inlet of the fan in the event that the discharge pressure from the fan overcomes a spring force associated with the second check valve. A heat exchanger pack and an environmental control system for use on an aircraft, and a method are all also disclosed.