Vent Operation

Openings tab in model data under Vents

The operation of vents can be defined by entering timing data under Operation headers.

Control option (internal vents only)

The control option allows you to define how internal vents behave in natural ventilation systems and whether they operate purely based on a schedule or, when using Calculated natural ventilation, whether they are linked to the main natural ventilation controllers defined on the HVAC tab. This settings is not available for external vents. The options are:

For Calculated natural ventilation:

• 1-Constant - the size of the internal vent airflow opening (when open) is constant and the opening of internal vents is controlled entirely by schedule, i.e. it is not affected by any of the natural ventilation controls on the HVAC tab. In particular the Internal control mode setting on the HVAC tab is not used.
• 2-Use Internal control mode - the internal vents are controlled using natural ventilation Internal control mode settings on the HVAC tab in combination with the operation schedule described below.

For Scheduled natural ventilation:

• 1-Control by schedule - the internal vents are controlled entirely by schedule and are not affected by any of the natural ventilation controls on the HVAC tab.
• 2-Closed - the internal vents are always closed and do not allow any airflow to be exchanged.

The way this data is used depends on the Natural ventilation model option set:

Calculated Natural ventilation

The ventilation rate through the vent is calculated based on the wind and stack pressure calculated at each time step in the simulation.

Operation schedule

The Operation schedule is used to control timing of the vent opening availability. Note that the schedule should have values of 0 and 1 only and intermediate fractional values do not indicate partial opening.

Note: Vents on a roof surface use the operation schedule under the Vents > External > Operation header to define simulated vent opening availability.

Scheduled Natural ventilation

External vent opening is not modelled explicitly with the Scheduled natural ventilation model data setting. Instead, the total natural ventilation flow rate and operation is defined for each zone on the HVAC tab. No operation data is entered on the Openings tab for external vents.

Internal vents are modelled using the EnergyPlus ZoneMixing object which mixes air between the two adjacent zones.  DesignBuilder uses an approximate empirical relationship to calculate the mixing flow:

Flow (m3/s) = Door area (m2) * % area door opens * % time door is open * FAirflowRatePerOpeningArea

where FAirflowRatePerOpeningArea is a factor used to obtain the mixing flow from the opening area. The value of this factor can be defined on the Model options dialog on the Advanced tab under Natural ventilation. The default value is 0.1 m3/s-m2.

A schedule can be applied to control timing of the mixing flow.

Note: The Airflow through internal openings model option must be checked to obtain airflow through internal vents when using Scheduled natural ventilation.

Operation schedule

The Operation schedule is used to control timing of the mixing flow. Note that the schedule should have values of 0 and 1 only and intermediate fractional values do not indicate partial opening.