AirflowNetwork in EP3
AirflowNetwork is EnergyPlus’s sophisticated module for modeling multizone airflow, including natural ventilation, infiltration, and mechanical ventilation systems. It solves mass conservation equations at each node (zone) in the network to calculate airflow rates driven by wind pressure, buoyancy (stack effect), and HVAC system operation.
Note: EP3 currently supports AirflowNetwork modeling for building envelope components (windows, doors, cracks, and other envelope openings) but does not yet support HVAC system components within the AirflowNetwork. This documentation focuses on envelope-related natural ventilation and infiltration modeling.
Core Principles
AirflowNetwork treats your building as a network of nodes connected by airflow components (cracks, openings, ducts, fans). At each timestep, it solves a system of nonlinear equations based on:
- Wind-driven pressure: External wind creates positive pressure on windward surfaces and negative pressure on leeward surfaces
- Buoyancy (stack effect): Temperature differences between indoor and outdoor air create density-driven flows
- HVAC system pressures: Mechanical systems can interact with natural ventilation (not currently supported in EP3)
For natural ventilation specifically, the module calculates airflow through openable windows, doors, and other large openings based on wind and buoyancy pressure differences.
Wind Pressure Coefficients (Cp): The Two Approaches
Wind pressure coefficients (Cp values) are critical because they determine how wind velocity translates into surface pressures. The pressure at any point on a building surface is:
P = Pref + 0.5 × ρ × V² × Cp
P – Total surface pressure (Pa)
- The absolute pressure at a specific point on the building surface
- This is what the AirflowNetwork uses to calculate airflow through openings
Pref – Reference pressure (Pa)
- The static atmospheric pressure at a reference height (typically at the weather station height or building reference level)
- Accounts for the baseline atmospheric pressure before wind effects
- In EnergyPlus, this is typically the outdoor air barometric pressure
0.5 – Constant coefficient
- This comes from Bernoulli’s equation for dynamic pressure
- Dimensionless constant
ρ (rho) – Air density (kg/m³)
- Density of the outdoor air
- Varies with temperature, humidity, and elevation
- Typical value at sea level: ~1.2 kg/m³ at 20°C
- EnergyPlus calculates this based on outdoor conditions using psychrometric relationships
V – Wind velocity (m/s)
- The approach wind speed at the building reference height (typically roof height or 10m meteorological height)
- Obtained from weather file data
- May be adjusted for terrain and height using the power law wind profile
V² – Wind velocity squared (m²/s²)
- The dynamic pressure term is proportional to velocity squared
- This is why higher wind speeds have disproportionately large effects on pressure
Cp – Wind pressure coefficient (dimensionless)
- Relates the dynamic pressure to actual surface pressure
- Varies with wind direction, surface orientation, and building geometry
- Typically ranges from about -0.8 to +0.8 for building surfaces
- Positive values = pressure (windward surfaces)
- Negative values = suction (leeward and side surfaces)
- This is the parameter you either input manually or let EnergyPlus calculate with SurfaceAverageCalculation
- Cp depends on wind direction, building geometry, and surrounding terrain
AirflowNetwork:SimulationControl
You must create an AirflowNetwork:SimulationControl object in order to have access to the other AirflowNetwork objects within EP3. The most critical input field is Wind Pressure Coefficient Type. This determines how you will specify Cp’s for your building surfaces. EnergyPlus offers two choices:
Option 1: Input (User-Defined Cp Values)
With the Input option, you manually specify Cp values for each external surface at different wind directions (typically 36 directions at 10° intervals).
Advantages:
- Complete control over values
- Can incorporate wind tunnel test data or CFD simulation results for complex geometries
- Useful for unusual building shapes or heavily obstructed sites
- More accurate for buildings where simplified methods don’t apply
Disadvantages:
- Labor-intensive to generate Cp values
- Requires expertise or access to wind tunnel/CFD data
- Easy to introduce errors if values are inconsistent
When to use: Complex building shapes, buildings with significant local shielding, or when you have measured/simulated Cp data available.
Option 2: SurfaceAverageCalculation (Automatic Cp Calculation)
With SurfaceAverageCalculation, EnergyPlus automatically calculates Cp values based on building geometry using empirical correlations from the literature (primarily based on rectangular building research).
The algorithm:
- Analyzes your building geometry to determine surface orientations
- Calculates surface-averaged Cp values based on wind angle of incidence
- Uses correlations that account for building aspect ratio and height
- Updates values for each wind direction during simulation
Advantages:
- Minimal user input required
- Consistent and reproducible
- Adequate accuracy for typical rectangular or simple building forms
- Significantly faster to set up
Disadvantages:
- Less accurate for complex geometries (L-shapes, courtyards, unusual forms)
- Cannot account for site-specific shielding from adjacent buildings or terrain
- Based on idealized conditions (uniform approach flow)
When to use: Rectangular or simple building geometries, preliminary design studies, buildings in relatively open terrain.
Key AirflowNetwork Objects for Natural Ventilation (Supported in EP3)
AirflowNetwork:SimulationControl – Enables the module and sets solver parameters. This object, must be created before any of the other objects will be visible in EP3.
AirflowNetwork:MultiZone:Zone – Defines each thermal zone as a network node.
Recommended EP3 Implementation: Assigned under ZoneTemplate
Allows AirflowNetwork properties to quickly be assigned to multiple Zones
Alternate EP3 Implementation: AirflowNetwork:MultiZone:Zone
AirflowNetwork:MultiZone:Surface – Defines which surfaces have openings and their leakage characteristics
In EP3, the AirflowNetwork:MultiZone:Surface can be applied at the Construction level. Surface properties will then be applied to all surfaces with the given construction:
AirflowNetwork:MultiZone:Component:DetailedOpening – Models large openings like windows with detailed two-way flow calculations (handles both inflow and outflow through the same opening)
AirflowNetwork:MultiZone:Component:SimpleOpening – Simpler model for openings using orifice equations
AirflowNetwork:MultiZone:ExternalNode – Represents outdoor conditions at different heights/locations – required only when Input is selected as your Wind Pressure Coefficient Method
AirflowNetwork:MultiZone:Surface:Crack – Models air leakage through cracks in the building envelope.
AirflowNetwork:MultiZone:Surface:EffectiveLeakageArea – Alternative method for specifying envelope leakage
AirflowNetwork:MultiZone:Surface:EffectiveLeakagePercentArea – This is an EP3 object type that allows specification of an EffectiveLeakageArea based on the percentage of the overall surface area, allowing much faster modeling for surfaces of varying sizes.
Modeling Strategy for Natural Ventilation in EP3
A typical workflow involves:
- Identify ventilation surfaces (operable windows, doors) and assign opening components
- Choose your Cp approach based on building complexity and available data
- Define opening schedules to control when windows are open (based on indoor temperature, occupancy, etc.)
- Include envelope leakage through cracks for realistic infiltration
The module then calculates airflow rates through each opening and updates zone temperatures and humidity accordingly, fully coupled with the thermal simulation.
Practical Considerations
Convergence: The nonlinear solver can struggle with very large openings or extreme pressure differences. Start with moderate opening sizes and check convergence warnings.
Timestep sensitivity: Natural ventilation involves rapid airflow changes. Use shorter timesteps (6-10 per hour minimum) for accuracy.
Control strategies: You may want to explore different control strategies to control when openings operate based on indoor/outdoor conditions, simulating realistic occupant behavior or automated control systems.
Free Cooling Example
Below is an example setup of an AirflowNetwork:MultiZone:Surface applied to a window construction, and used for free cooling through operable windows:
