Natural Ventilation and Infiltration model Options

Data tab on Model Options dialog.

Natural ventilation

Natural ventilation (and infiltration) can either be 'Scheduled' or 'Calculated'.

• Scheduled - the ventilation rates are predefined using a maximum air change rate modified by operation schedules.
  

• Calculated - the ventilation rates are calculated using wind and buoyancy-driven pressure, opening sizes and operation, crack sizes etc. using the EnergyPlus Airflow Network.

You should generally use the Scheduled natural ventilation model option because it is quicker and easier to enter the related model data and also because the simulations run more quickly.  However if you need to calculate natural ventilation rates which can be achieved with a particular configuration then you should use the Calculated option.

Note: This natural ventilation setting also affects the way that infiltration is treated. It isn't possible to calculate infiltration as a constant (or scheduled) value while using the EnergyPlus Airflow Network for Calculated natural ventilation.

More on Natural Ventilation modelling.

Infiltration units

There are 4 different ways to specify infiltration in calculations not using the AirflowNetwork (Heating and Cooling design and Simulation with Scheduled natural ventilation):

• 1-ac/h - the default option where infiltration is defined in air changes per hour (ac/h), i.e. zone volume per hour under normal operating pressures.
• 2-m3/h-m2 at 50 Pa infiltration is defined as flow rate per unit exposed surface area, when the pressure difference between inside and outside is 50 Pa. Unit commonly used in the UK.
• 3-m3/h-m2 at 4 Pa infiltration is defined as flow rate per unit exposed surface area, when the pressure difference between inside and outside is 4 Pa. Units commonly used in France and Belgium.
• 4-n50 (ac/h at 50 Pa)- infiltration is defined in air changes per hour (ac/h), i.e. zone volume per hour, when the pressure difference between inside and outside is 50 Pa.

The selection made here will affect the units of the infiltration data prompted for on the Constructions tab under Airtightness.

The infiltration defined when using options 2, 3 and 4 is converted to ac/h at standard operating pressure for input to EnergyPlus. The conversion from these options to ac/h uses the methods described in EN 12831.

Airtightness method

When the Calculated natural ventilation method is selected there are 2 ways to define the airtightness of the building:

• 1-Template slider, where 5 predefined airtightness settings are available. This method is most suitable for more early stage analysis.
• 2-Crack template, where custom crack templates can be defined and selected. This method provides more control over the airtightness settings used.