Vents and Holes

Openings tab in model data

External Vents

External vents are typically used for modelling airflow through external vent openings for the purposes of providing fresh air and/or zone cooling. They are only used when the Calculated natural ventilation model option is selected. When using Scheduled natural ventilation external vents should, in general, not be added to the model since, in this case, outdoor airflow is defined explicitly at zone level and the vent settings cannot be accessed to define the conductance and texture.

Vents can be added to the model by navigating to the appropriate surface and either:

1. Setting the Auto generate option and settings the Preferred width and height options. In this case a vent will be positioned under each of the windows (if the vent spacing is the same as the window spacing), or,

2. Alternatively, for more control over the placement position of the vents, you can draw the vents using the Draw vent command.  Note that on surfaces where custom openings have been defined, any changes to vent layout model data on the Opening tab is ignored.

Note: if you have drawn custom vents at the surface level then these will be used even if the Auto generate checkbox is not checked. The Auto generate checkbox only controls the automatic generation of vents, not their use in the calculations.

Vent type

The properties of the vent are defined by the selection of the vent type:

Select the vent component that holds additional property data on the vent type such as the construction, texture and discharge coefficient to be applied.

The additional vent model data that can be added is described under:

• Vent Placement - defines the size and position of vents when using the Auto generate option.
• Vent Operation - defines the times when vents can operate.

Important Note: The external vent operation settings are only made available for editing when the Natural Ventilation > On checkbox is checked on the HVAC tab.

Roof Vents

Roof vents are defined as any vents placed on an external pitched or flat roof surface. The vent type for roofs is defined separately under the Roof header.

Although roof vents have a separate vent type selection you should use the same operation schedule as for External vents to define simulated vent opening availability. This data can be accessed under the Vents > External > Operation header.

Internal Vents

Regardless of the Natural ventilation model option selected, data on internal vents can always be accessed. In this case the properties of the selected vent component(s) define the resistance to heat conduction and airflow (see details below under Vent Modelling).

Note: Internal vent data is not available at zone level to avoid ambiguity when different settings are made in adjacent zones. Surface level internal vent settings are generally inherited from block level, however settings for inter-block vents are inherited from the building level. For details on these rules see: Zone Level Data for Internal Surfaces.

Otherwise the same comments made above for External vents also apply to Internal vents.

Vent Modelling

 

Calculated Natural Ventilation (Simulations only)

 

Each vent is modelled using:

 

• An EnergyPlus Door. The heat conduction through the vent Door component is combined with the Wall and Partition results.
• An EnergyPlus AirflowNetwork:MultiZone:Component:DetailedOpening and other associated AirflowNetwork components, providing a controllable flow path. You can define Vent operation.

 

Scheduled Natural Ventilation (and Heating and Cooling Design Calculations)

 

Each vent is modelled using:

 

• An EnergyPlus Door. The heat conduction through the vent Door component is combined with the Wall and Partition results.
• Outdoor air ventilation airflows are defined explicitly for zones as a whole and external vents do not contribute to this.
• Internal vents also have an EnergyPlus ZoneMixing object using parameters set in the Model options dialog. External vents do not exchange air explicitly with Scheduled natural ventilation.

Holes

Holes are modelled in a similar way to vents with some exceptions:

• Holes must be drawn at surface level.
• Airflow through holes is uncontrolled, i.e. the airflow path is active all the time.
• Drawing holes can be used as a way to merge two zones when the Merge zones connected by holes Model option is set.
• Holes are used internally with DesignBuilder to model the heat transfer and airflow through Virtual partitions.
• The discharge coefficient to be used for all holes (and doors) in the model is defined on the Advanced tab of the Model options dialog under the Natural Ventilation header. The default discharge coefficient is 0.65 is used. For vents the vent component defines the discharge coefficient.

Hole Modelling

 

Calculated Natural Ventilation (Simulations only)

 

Each hole is modelled using an EnergyPlus sub-surface and an airflow connection:

 

• Sub-surface:
• When using the 1-Minimum shadowing or 2-Full exterior solar distribution Model options external holes are modelled using 'perfectly clear' windows which transmit all solar radiation and visible light, while internal holes are modelled using EnergyPlus Doors and an Infrared Transparent construction.
• When using 3-Full interior and exterior solar distribution, all holes are represented using 'perfectly clear' windows which transmit all solar radiation and visible light. In this case heat conduction through window components are displayed in results as Internal Natural vent and External Natural vent as appropriate.

 

• EnergyPlus AirflowNetwork:MultiZone:Component:DetailedOpening and other associated AirflowNetwork components, providing a non-controllable flow path.

 

Scheduled Natural Ventilation (and Heating and Cooling Design Calculations)

 

Each hole is modelled using an EnergyPlus sub-surface as follows:

 

• External holes are represented using 'perfectly clear' windows which transmit all solar radiation and visible light. In this case the heat conduction through windows is displayed in results as External Natural vent. External holes do not exchange air explicitly with Scheduled natural ventilation.
• Internal holes are modelled using an internal "door" between the 2 zones and, if the Airflow through internal openings Model option is selected, an EnergyPlus ZoneMixing object is also included using parameters set in the Model options dialog.