Natural ventilation design features
BEM can be used during conceptual design to evaluate the potential impact of natural ventilation design features such as operable windows. A BEM analysis can estimate potential HVAC energy savings when natural ventilation is used in a mixed-mode strategy, or the analysis can estimate indoor air temperatures when natural ventilation is the sole source of comfort cooling. These analyses can be used to explore the feasibility of natural ventilation as well as to evaluate how other design decisions, such as building form and massing and fenestration design, affect natural ventilation performance.
Impact of Natural Ventilation
Natural ventilation serves two primary roles: ventilation for indoor air quality and comfort cooling. The following are some of the potential impacts.
- Energy savings due to reduced fan and cooling system operation.
- Potential undesired increase in heating energy, depending on controls.
- Elimination of the need for a cooling and ventilation system in some cases.
- Expanded comfort range for occupants with access to operable windows (adaptive comfort).
- Improved resilience and passive survivability, potentially allowing occupancy during loss of power.
The actual impacts will depend on how natural ventilation openings are controlled. Savings will depend on occupant behavior unless the openings are controlled automatically. Some designs use a system that notifies occupants when conditions are favorable to open windows.
Natural Ventilation Alternatives
A BEM analysis during conceptual design could take a number of approaches to comparing alternatives, such as the following:
- A single design concept simulated with and without natural ventilation, to estimate the potential benefits.
- A single building form simulated with variations in natural ventilation opening size and location, to determine appropriate opening sizes.
- Variations in both building form and openings, to compare the impact of basic building design decisions on natural ventilation performance.
There are several design strategies for natural ventilation in buildings, and each takes advantage of one or both of two driving forces -- wind and air temperature differences -- which create the pressure differences that force air through openings such as windows. There are three basic approaches to natural ventilation:
- Cross ventilation - primarily wind driven
- Stack ventilation - primarily driven by air temperature differences between warmer, lower-density indoor air and cooler, higher-density outdoor air
- Single-sided ventilation - driven by a combination of window and air temperature differences
The primary design factors that affect natural ventilation airflow are the size and location of openings that serve as inlets and outlets. The following are some typical natural ventilation design strategies.
- Single-sided ventilation for individual rooms, ideally with high and low openings
- Cross ventilation for larger open rooms
- Narrow building footprint to maximize floor area with natural ventilation
- Atrium for stack ventilation
In all cases, ceiling fans can be used to provide air movement to expand the indoor air temperature comfort range and increase the hours that natural ventilation can provide comfort.
Some BEM tools can simulate different types of natural ventilation controls. For example, it is usually possible to model full automatic control, which would provide best performance. It may also be possible to model manual control, with probabilities of operation to approximate occupant behavior. Consider creating models that represent varying levels of control to understand the range of impact that natural ventilation might provide. The range of controls that are used for natural ventilation include the following.
- Manual control
- Manual control with indicators to encourage occupants to open or close windows
- Window interlock switches that shut off HVAC supply to a space when a window is open
- Full automatic control of openings by the building management system, which might implement schemes such as passive night cooling.
When selecting natural ventilation strategies for BEM analysis, consider how to integrate strategies for building form and massing, fenestration and daylighting design, and constructions and thermal mass. Success of natural ventilation is strongly affected by building form. Design strategies that are good for natural ventilation are also often good for daylighting. Thermal mass can affect that number of hours that natural ventilation can provide comfort. BEM analysis can be used to understand the integrated impact of these design decisions.
Guidance on Modeling Approach
BEM is one of a few natural ventilation analysis methods that may be appropriate during conceptual design. A climate analysis can provide an understanding of the number of hours per year when natural ventilation might be useful. Manual airflow calculations can be used for rough sizing of inlets and outlets based on desired airflow. Then BEM can be used to estimate annual impact of natural ventilation, such as energy savings or comfort hours, for different design strategies. For detailed studies of airflow within a space or around a building, then computational fluid dynamics calculations may be necessary.
A simple box model approach is typically appropriate during conceptual design. Natural ventilation simulation capabilities vary among BEM tools, so a first step in a natural ventilation study is to become familiar with the tool’s modeling approach and required inputs. Some BEM tools include airflow network models that can estimate both airflow from outdoors to indoors and airflow between interior zones. Other BEM tools limit calculations to airflow between outdoors and individual zones. Consider running several simulations varying uncertain inputs in order to gain an understanding of the sensitivity of results.
Two fundamental approaches may be appropriate depending on the goals of your BEM analysis
- Natural ventilation alone. Create a model without a cooling system and evaluate indoor air temperature based on natural ventilation alone.
- Mixed-mode. Create a model with both natural ventilation and mechanical cooling and associated controls to estimate the number of hours per year that natural ventilation can provide comfort and estimate the energy consumption for cooling during other hours.
In all cases, it is a good idea to closely examine the simulation results to see that natural ventilation controls are operating as expected. Hourly outputs are especially helpful.
Guidance on Presenting Results
BEM results that are typically found useful are described in this page: Analyzing Model Outputs. Other important considerations:
Example presentations of natural ventilation simulation results.
ASHRAE. Handbook 2021 Fundamentals, Chapter 16
Center for the Built Environment. Ceiling Fan Design Guide
Center for the Built Environment. Mixed Mode Case Studies and Project Database
Menchaca, et al. Modeling Natural Ventilation in Early and Late Design Stages
Price Industries. Engineering Guide Natural Ventilation
Whole Building Design Guide. Natural Ventilation