Calculated Natural Ventilation Data

HVAC tab in model data

See the Tutorial on Calculated Natural ventilation.

When the Natural ventilation model option is set to Calculated, natural ventilation data is set up using the following procedure:

• Check the On checkbox under the Natural Ventilation header.
• On the Openings tab by setting window, door and vent opening sizes and operation times.

You can override operation of windows, vents, doors to be off as follows:

Note that even when operation is switched off as described above (or because operation schedules on the Openings tab are off) windows, vents and doors still provide a small flow path through the crack between the opening and the surrounding surface if the Model infiltration data is switched on.

Note 1: For Cooling design calculations, natural ventilation is always handled as Scheduled even when the Natural ventilation model option is set to Calculated. In this case you can still enter natural ventilation air change rates and operation schedule data on the HVAC tab but it is used only for design simulations.

The captions for data used for design calculations only is displayed with a lighter grey background in the interface.

Note 2: With both the Scheduled and Calculated options, natural ventilation can be overridden to be off in Heating and Cooling design calculations. In Heating design calculations natural ventilation is switched off by default because it is considered to be a cooling mechanism - windows and vents will normally be closed during the coldest winter periods.

Wind Factor

When carrying out conservative design simulations you may prefer to exclude (or reduce) the effects of wind from Calculated natural ventilation. To exclude wind-driven airflow from the analysis altogether set the Wind factor to 0. For full treatment of wind effects set it to 1 and for intermediate treatment of wind set to a number between 0 and 1.

Control mode

For calculated natural ventilation the control mode defines the way that the openings are opened and closed based on air temperatures or enthalpy. The data can be entered at building, block or zone level.

In the descriptions that follow:

Tout is the outdoor air temperature,

Tzone is the previous time step’s zone air temperature,

Tadjcacent zone is the previous time step’s air temperature for the adjacent zone for interior surfaces,

Tset is the zone natural ventilation setpoint temperature,

Hzone is the specific enthalpy of zone air from the previous time step, and

Hout is the specific enthalpy of outdoor air.

 

The allowed choices for natural ventilation Control mode are:

• 1-NoVent - all of the zone’s openable windows and doors are closed at all times independent of indoor or outdoor conditions. The operation schedule is ignored in this case.
• 2-Temperature - all of the zone’s openable windows and doors are opened if Tzone > Tout and Tzone > Tset and operation schedule allows venting.
• 3-Enthalpy - All of the zone’s openable windows and doors are opened if Hzone > Hout and Tzone > Tset and operation schedule allows venting.
• 4-Constant - Whenever an opening's operation schedule allows venting, all of the zone’s openable windows and doors are open, independent of indoor or outdoor conditions. Note that “Constant” here means that the size of each opening is fixed while venting; the air flow through each opening can, of course, vary from time step to time step. This option allows modelling of a window that is opened for fresh air regardless of inside/outside temperature/enthalpy.
• 5-Adjacent temperature cooling - applies to interior surfaces only. All of the zone’s openable windows and doors are opened if Tzone > Tadjacent zone  and Tzone > Tset and operation schedule allows venting.
• 6-Adjacent temperature heating - applies to interior surfaces only. All of the zone’s openable windows and doors are opened if Tzone < Tadjacent zone  and Tzone > Tset and operation schedule allows venting. This option is a basic attempt to get round the EnergyPlus assumption that natural ventilation is always used for cooling and allow it to heat a zone as well. Note however that the zone setpoint is still used to switch off ventilation if the temperature goes too low (rather than too high as might be expected for a heating control).

Both of the Adjacent temperature control options (5 and 6) revert to 2-Temperature when applied to exterior surfaces. This allows these options to be set at building level.

Modulation of Openings

You can modulate the window/door openings when using Calculated natural ventilation by making appropriate settings to:

• Modulate opening areas - switch on modulation by checking this option.
• Limit value of opening modulation factor - gives the reduction on opening area for low outside air temperatures.
• Lower value of Tin-Tout  - lower value on inside/outside temperature difference for modulating the venting open factor.    
• Upper value of Tin-Tout  - upper value on inside/outside temperature difference for modulating the venting open factor (defines 'low outside air temperatures')
  
  
  
  

This data is used within EnergyPlus to calculate a factor between 0 and 1 that multiplies the opening factor of each window and door in the building. Modulation of the openings can reduce the large temperature swings that can occur if the windows/doors are too far open when they are venting, especially when there is a large inside-outside temperature difference. This can avoid discomfort due to introduction of cold air and/or prevent heating system operation in summer when not really required.

For example if the Lower value of Tin - Tout is 0, the Upper value is 15 and the Limit value of opening modulation factor is 0.05, the windows and doors are only opened 5% of their full opening area when the outside air is 15K colder than inside.When the inside and outside temperatures are the same the windows are open to their maximum opening area and when the temperature difference is 7.5K the windows are opened halfway between these extremes.

Important Notes

1. The purpose of the Modulate option is to prevent discomfort due to very cold air being introduced into the zone. You should keep in mind though that because this control will also prevent ventilation when the temperature in the space is very hot and the outside air is not particularly cold because of the large temperature difference between inside and outside.
   
   
   
   For example if the upper modulation temperature difference is 15K and the limiting modulation factor is 0.05 then the maximum window and vent opening area will be reduced to 5% of their normal size if the temperature in the zone is 35°C and the temperature outside is 20°C. In this case the natural ventilation through the 'almost closed' openings would probably not be adequate to cool the zone back down again. So you should not use this option if you wish natural ventilation to occur when the temperature inside is very high.
   
   
   
   
2. With Modulation active, openings will be completely closed if the difference between inside and outside air temperatures is greater than Upper value of Tin-Tout.
   
   
   
   
3. While the modulation graph above shows the opening factor to be 1 when the inside temperature is greater than outside temperature, by default the natural ventilation openings will all be closed in this case based on the 2-Temperature control mode.

This modulation data can also be accessed from the HVAC tab under the Natural Ventilation > Options headers.

Tip: Although basic DesignBuilder natural ventilation options don't allow controls based on absolute outdoor air temperature, it is possible to apply upper and/or lower outdoor air temperature limits for natural ventilation through use of the mixed mode Temperature control schedule and the Mixed mode minimum and maximum temperatures.