Zone ventilation mass flow rate reports zero

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Hi

I am modeling a simple house and I have chosen to have a "ZoneVentilation:DesignFlowRate" as intake But when I run the simulation, the hourly output variable "LIVING_1:Zone Ventilation Mass Flow Rate kg/s" reports zero always. To check to see if I am doing something wrong or not, I selected "ZoneInfiltration:DesignFlowRate" and this one reports a correct hourly values under "LIVING_1:Zone Infiltration Mass Flow Rate kg/s".

Is there something in terms of solver or airflow networks that I need to do to make sure that the ventilation object has been used properly?

Regards