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I tested with an ExampleFile VentilationSimpleTest.idf (V24-1-0). I tested with an ExampleFile VentilationSimpleTest.idf (V24-1-0). I changed the Coefficients as follows so that it is not affected by outdoor air temperature and velocity. The ventilation rate should be constant.

ZoneVentilation:DesignFlowRate,
    ZONE 1 Ventl 1,          !- Name
    ZONE 1,                  !- Zone or ZoneList or Space or SpaceList Name
    Constant,                !- Schedule Name
    flow/zone,               !- Design Flow Rate Calculation Method
    6.131944,                !- Design Flow Rate {m3/s}
    ,                        !- Flow Rate per Floor Area {m3/s-m2}
    ,                        !- Flow Rate per Person {m3/s-person}
    ,                        !- Air Changes per Hour {1/hr}
    NATURAL,                 !- Ventilation Type
    0,                       !- Fan Pressure Rise {Pa}
    1,                       !- Fan Total Efficiency
    1,                       !- Constant Term Coefficient
    0,                       !- Temperature Term Coefficient
    0,                       !- Velocity Term Coefficient
    0;                       !- Velocity Squared Term Coefficient

I output Zone Ventilation Standard Density Volume Flow Rate, Zone Ventilation Current Density Volume Flow Rate and Zone Ventilation Mass Flow Rate, but all fluctuate. Why? What kind of ventilation rate is supposed to be constant other than current density volume, standard density volume and mass? How can I keep the ventilation rate constant?

I tested with an ExampleFile VentilationSimpleTest.idf (V24-1-0). I tested with an ExampleFile VentilationSimpleTest.idf (V24-1-0). I changed the Coefficients as follows so that it the ventilation rate is not affected by outdoor air temperature and velocity. The I expected the ventilation rate should to be constant.

ZoneVentilation:DesignFlowRate,
    ZONE 1 Ventl 1,          !- Name
    ZONE 1,                  !- Zone or ZoneList or Space or SpaceList Name
    Constant,                !- Schedule Name
    flow/zone,               !- Design Flow Rate Calculation Method
    6.131944,                !- Design Flow Rate {m3/s}
    ,                        !- Flow Rate per Floor Area {m3/s-m2}
    ,                        !- Flow Rate per Person {m3/s-person}
    ,                        !- Air Changes per Hour {1/hr}
    NATURAL,                 !- Ventilation Type
    0,                       !- Fan Pressure Rise {Pa}
    1,                       !- Fan Total Efficiency
    1,                       !- Constant Term Coefficient
    0,                       !- Temperature Term Coefficient
    0,                       !- Velocity Term Coefficient
    0;                       !- Velocity Squared Term Coefficient

I output Zone Ventilation Standard Density Volume Flow Rate, Zone Ventilation Current Density Volume Flow Rate and Zone Ventilation Mass Flow Rate, but all fluctuate. Why? What kind of ventilation rate is supposed to be constant other than current density volume, standard density volume and mass? How can I keep the ventilation rate constant?