Natural ventilation result equal to zero

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I am modeling a project that is totally naturally ventilated, in the results of air changes per hour (ACH), some hours have zero values. Looking at the weather file that I use, in some points the values for wind speed are zero and in those same points I have the result with zero value for air changes per hour (ACH). The results seem to be in compliance, but in the project there is an exhaust system that works passively and causes exhaustion even when the wind speed values are equal to zero. How can I represent this element of exhaustion that makes air changes happen even when the weather file says that the wind speed is equal to zero (without energy consumption)?