Natural Ventilation Feature - Overview and Set-up

Natural Ventilation is a great way to save energy for buildings in milder climates and is particularly common in Residential, Educational and some commercial office projects.

Natural Ventilation can be applied as a purely passive strategy, or combined with an HVAC system to operate in milder temperatures with cooling and/or heating and/or ventilation as a back-up. In some instances this can generate substantial savings to the HVAC Design.

This section describes how natural ventilation is set up in Sefaira.

Where to find Natural Ventilation controls

The Natural Ventilation feature is a set of controls that appear in an input tab called "Nat Vent"

Sefaira uses EnergyPlus to make Natural Ventilation calculations. The calculations are based on EnergyPlus’s airflow network calculations, which iterate and determine hour by hour the net effect of temperature differences, wind pressures and space loads on a zone to accurately estimate airflows into zones from outside and between zones.

There are some important things to know about Sefaira when setting up a natural ventilation model:

How to generate openings for Natural Ventilation in the 3D model

Windows drawn in the 3D model and tagged as glazing will be considered openable in Sefaira. However they will only actually become openable if the Natural Ventilation feature is switched on.

Glazing that is added using the Window to Wall Ratio Override is also treated as operable.

Activating Natural Ventilation

Natural Ventilation needs to be “turned on” in the web-application. If it is not turned on, there will be no natural ventilation even if your model has windows.

If it is “off” HVAC is assumed to be used as the only source of heating, cooling and ventilation during occupied hours.

Once natural ventilation is switched “on” it will supplement some or all of the mechanical HVAC system depending on controls selected.

HVAC System constraints and integration

In our implementation of Natural Ventilation, it is always integrated with an HVAC System (even if the HVAC system ends up doing nothing)

Natural Ventilation cannot be operated with an all-air system such as a VAV system or displacement system.

It only works with HVAC systems that have a separate DOAS (Dedicated Outside Air System) for ventilation and zone units for heating and cooling. DOAS systems are labeled on the HVAC Dropdown list (Systems list shown below).

If your project has an unsupported system applied you will not be able to run natural ventilation and you will see this warning:

You can either change the system to a suitable system yourself or use the button in the natural ventilation tab to swap to a 4-pipe Fan Coil Unit system and turn on natural ventilation. This must be done even if all the HVAC components are eventually going to be switched off.

Infiltration constraints and integration

In EnergyPlus, the air-changes and facade area infiltration metrics are not compatible with natural ventilation. To include infiltration when natural ventilation has closed the windows, the infiltration needs to be set to crack flow. If you have not done this, this will be included in the warning message below and can be done automatically.

Note that Crack Flow is not a default in Sefaira. If you have a baseline system you're comparing natural ventilation too, it's a good idea to change that system to also use crack flow as the basis for infiltration.

Other important considerations

  • Assuming some or all aspects of the HVAC system remain, they will be applied per the selected system and all controls set in the airside and waterside tabs for that system. There is no specific “natural ventilation system” – it is assumed to work in tandem with a nominated mechanical HVAC system. For example, if the mechanical system selected is passive chilled beams and natural ventilation is added, the HVAC system will still be passive chilled beams.
  • If the HVAC system is changed while natural ventilation is on, the natural ventilation controls will be remembered but applied to the new HVAC system.
  • Natural ventilation controls do not apply to centre zones. Centre zones will be fully conditioned using the system nominated in the HVAC dropdown.
  • Set-points in the space-use tab are used to control many aspects of natural ventilation. It is therefore important not to set zones to be ignored, or unconditioned or to have extreme setpoints if natural ventilation is to be used. To find out more, read the section “how natural ventilation works with space use control settings”.
  • Exterior shading objects are not obstructing natural ventilation in any way. User can put a shading object that completely overlaps the window area but that will not have an effect on natural ventilation.
  • This feature only supports side ventilation into a sealed zone. It does not currently support cross ventilation or stack flow between zones.
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