Depends where the fan is located for temperature reasons. Otherwise it doesn't really matter in terms of amount of airflow which way it is pointing, as it results in the same flow. You just feel the results more when it's blowing to the inside.
If the fan is upstairs, exhausting air will be more effective (remove the heat). If the fan is downstairs, blowing air in is more effective. Doing both at the same time is best.
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