An apparatus for monitoring air flow in a vent pipe of a radon mitigation system includes a base connectable to the vent pipe. The base houses a circuit that is actuatable to indicate the condition of the radon mitigation system. An air flow switch actuates the circuit, the air flow switch includes a vane that is supported by the base and extends into the interior of the vent pipe through an opening in a wall of the vent pipe. The vane includes a wing having portions that are deflectable to permit installation of the wing through an opening in the vent pipe. The deflectable portions return under their own resilience to their non-deflected conditions once they pass through the opening in the vent pipe.