What do the installation instructions say? In my opinion, the fan will create a significant delay in the operation of the smoke detector in the event of a fire. There are rules that say a smoke detector should not be placed within 3' of an air discharge register. I'm not sure about a rule for fans, but if the fan is in operation it will dilute the smoke and retard its flow to the detector, both of which will increase the response time. I would consult with the local fire marshall.
Don
