Natural Ventilation Options

Advanced tab on Model Options dialog.

Model holes (including holes in virtual partitions)

You can optionally switch off all holes in the model including those holes generated by defining partitions as virtual partitions. In this case the building is modelled as if no holes were defined.


Tip: Unchecking this data can be a quick way to model virtual partitions as standard partitions.

Calculated Natural Ventilation

Wind Factor

See Natural ventilation in HVAC model data.

Discharge coefficient for open windows and holes

Enter the discharge coefficient for all open windows and holes. The default of 0.65 is a good general purpose setting.



Suggested values for discharge coefficient

The literature suggests a large range of different values for discharge coefficient (symbol Cd often used in orifice equations). For example IEA Annexe 20 Air flow Through Large Openings in Buildings indicates that discharge coefficients reported in the literature vary between 0.3 and 0.8 and that it is not understood what causes the differences.


Other sources recommend a value close to 0.6. For example the CONTAMW 2.0 User Manual states that the discharge coefficient, is related to the dynamic effects and is typically close to 0.6 for an orifice and slightly higher for other openings in buildings.


ASHRAE propose the following correlation based on interzone temperature differences:


Cd = 0.4 + 0.0045 δT


for the range δT 0.5 to 40.


Given other uncertainties in natural ventilation calculations (wind pressure coefficients, effective areas of real-world openings and crack flows etc), using a discharge coefficient between 0.60 and 0.65 should provide sufficient accuracy.

Modulation of Openings

See Natural ventilation in HVAC model data.

Scheduled Natural Ventilation

Airflow through internal openings

When using Scheduled natural ventilation, windows, vents, doors and holes in internal partitions can be modelled by mixing a predefined amount of air from one zone with the adjacent zone by checking the Airflow through internal openings checkbox and entering the Airflow rate per opening area. This inter-zone airflow is transferred equally in both directions and is modelled using the EnergyPlus ZoneMixingoption.