For modelling cooling effects of natural ventilation in an atrium, I use the airflow network in energyplus to simulate the natural ventilation. To control the conditions and calculate the change of cooling loads, I use the HVAC template Zone: Ideal Load Air system, and fix indoor temperature in 21 degree.

To control the natural ventilation, I use temperature as Ventilation control mode, and in the Ventilation Control Zone temperature setpoint schedule, I use 20.999 degree. Then, in the indoor and outdoor temperature difference lower limit for maximum venting open factor and upper limit, I set 9 and 9.001 to ensure the openings to be opened in 12-21 degree and opening factors in only 0 and 1.

However, I found the opening factors still have some values less than 1.0, like 0.5, why?

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Thanks