Stack Effect Ventilation

Overview

Stack effect ventilation harnesses buoyancy forces generated by temperature differences between indoor and outdoor air to create natural airflow through buildings. This passive ventilation mechanism becomes the dominant driving force in tall buildings and vertical shafts where wind effects are minimal.

The fundamental principle relies on the density difference between warm interior air and cooler exterior air, creating a vertical pressure gradient that drives airflow without mechanical assistance.

Stack Pressure Physics

Fundamental Relationship

Stack pressure develops from the hydrostatic pressure difference between air columns of different temperatures. The driving pressure at any height is:

$$\Delta P_s = \rho_o g h \left(1 - \frac{T_o}{T_i}\right)$$

Where:

$\Delta P_s$ = stack pressure (Pa)
$\rho_o$ = outdoor air density (kg/m³)
$g$ = gravitational acceleration (9.81 m/s²)
$h$ = vertical height between openings (m)
$T_o$ = outdoor absolute temperature (K)
$T_i$ = indoor absolute temperature (K)

Simplified Calculation

For practical applications with small temperature differences, the relationship simplifies to:

$$\Delta P_s = C_s h \Delta T$$

Where:

$C_s$ = stack coefficient ≈ 0.0342 Pa/(m·K) at standard conditions
$\Delta T$ = temperature difference $T_i - T_o$ (K)

This approximation holds for temperature differences up to approximately 30 K.

Airflow Calculation

Volumetric Flow Rate

The airflow through stack-driven openings follows orifice flow principles:

$$Q = C_d A \sqrt{\frac{2 \Delta P_s}{\rho}}$$

Where:

$Q$ = volumetric flow rate (m³/s)
$C_d$ = discharge coefficient (typically 0.6-0.65 for sharp-edged openings)
$A$ = effective opening area (m²)
$\rho$ = air density (kg/m³)

Combined Openings

For buildings with inlet and outlet openings, the effective area becomes:

$$A_{eff} = \frac{1}{\sqrt{\frac{1}{A_{inlet}^2} + \frac{1}{A_{outlet}^2}}}$$

This relationship shows that the smaller opening dominates flow resistance and limits total airflow.

Neutral Pressure Plane

Definition and Location

The neutral pressure plane (NPP) represents the horizontal level where indoor and outdoor pressures equalize. Above this plane, indoor pressure exceeds outdoor pressure (exfiltration), while below it, outdoor pressure exceeds indoor pressure (infiltration).

For a single enclosed space with openings at top and bottom, the NPP location is:

$$h_n = \frac{h}{1 + \sqrt{A_{bottom}/A_{top}}}$$

Where:

$h_n$ = height of NPP above bottom opening (m)
$h$ = total height between openings (m)
$A_{bottom}$ = area of lower opening (m²)
$A_{top}$ = area of upper opening (m²)

Design Implications

Equal Opening Areas: NPP locates at mid-height, creating symmetric pressure distribution.

Unequal Opening Areas: NPP shifts toward the larger opening, affecting pressure gradients and infiltration patterns.

Multiple Floors: Each floor experiences different pressure zones, requiring careful analysis for each level.

Thermal Chimney Design

Operating Principle

Thermal chimneys are vertical shafts designed to maximize stack effect by:

Solar Gain: Absorbing solar radiation to increase air temperature
Thermal Mass: Storing heat to extend operation beyond daylight hours
Height Optimization: Maximizing vertical distance for pressure development
Area Sizing: Balancing flow resistance against discharge requirements

Design Parameters

Chimney Height: Taller chimneys generate greater pressure differentials. A minimum height of 3-4 meters is typically required for effective operation, with optimal performance at 6-10 meters for residential applications.

Cross-Sectional Area: Sizing follows momentum and continuity principles. For a target flow rate:

$$A_{chimney} = \frac{Q}{v_{design}}$$

Typical design velocities range from 0.5-1.5 m/s to maintain laminar flow and minimize noise.

Surface Absorptivity: Dark-colored surfaces (α > 0.8) maximize solar heat gain. Glazed chimneys enhance greenhouse effect, achieving temperature increases of 15-25 K above ambient.

Solar Chimney Configuration

┌─────────────────┐
│   Outlet Vent   │ ← Hot air discharge
├─────────────────┤
│                 │
│    Absorber     │ ← Dark surface, solar heated
│    Surface      │   Temperature: To + 15-25 K
│                 │
│   Air Channel   │ ← Rising warm air
│                 │   Velocity: 0.5-1.5 m/s
│                 │
│   Glazing       │ ← Transparent cover
│                 │   Greenhouse effect
│                 │
├─────────────────┤
│   Inlet Vent    │ ← Cool air intake
└─────────────────┘

Atrium Ventilation Strategies

Atrium as Thermal Engine

Building atriums function as large-scale thermal chimneys when properly designed. The high volume allows significant thermal stratification, with temperature differentials reaching 10-15 K between floor and roof levels.

Design Configurations

Single-Sided Atrium: Openings at ground level and roof create direct vertical flow path. Suitable for moderate climates with consistent temperature differentials.

Cross-Ventilated Atrium: Combines stack effect with wind-driven ventilation through strategic opening placement on opposite facades.

Segmented Atrium: Multiple floors with individual ventilation zones connected to central shaft, allowing independent control while sharing exhaust capacity.

Opening Strategy

     Roof Vents (Exhaust)
            ↑
    ┌───────┴───────┐
    │               │
    │    Stratified │  ← Hot layer (Ti + 10-15 K)
    │    Hot Zone   │
    ├───────────────┤
    │               │
    │    Mixed      │
    │    Atrium     │  ← Intermediate temperatures
    │    Volume     │
    │               │
    ├───────────────┤
    │               │
    │    Cool Zone  │  ← Near ambient temperature
    └───────┬───────┘
            ↓
    Ground Level Inlets

Upper Exhaust Openings: Located within the top 10-15% of atrium height to capture stratified warm air. Automated control responds to temperature sensors.

Lower Intake Openings: Positioned to draw cooler outdoor air or conditioned air from adjacent spaces. Shading and strategic orientation minimize solar heat gain at intake.

Intermediate Openings: Floor-level openings at multiple heights allow occupied spaces to access the pressure gradient without direct connection to extreme temperature zones.

Building Height Effect

Stack pressure increases linearly with height, making tall buildings particularly effective for stack ventilation:

Low-rise (h < 10 m): Stack effect minimal, wind dominates
Mid-rise (10-30 m): Stack effect significant, combined with wind effects
High-rise (h > 30 m): Stack effect dominant, requires pressure relief strategies

For buildings exceeding 50 meters, stack pressures can reach 50-100 Pa, necessitating vestibules, revolving doors, or mechanical pressure relief to prevent excessive infiltration and door operation difficulties.

Practical Design Considerations

Temperature Differential Maintenance: Natural ventilation requires consistent indoor-outdoor temperature differences. Effectiveness diminishes as outdoor temperatures approach indoor setpoints.

Seasonal Performance: Stack effect reverses in summer when outdoor temperatures exceed indoor temperatures, requiring alternative ventilation strategies or hybrid systems.

Opening Control: Automated dampers or manually adjustable vents allow optimization for varying conditions. Temperature-actuated mechanisms provide passive control.

Integration with Mechanical Systems: Hybrid approaches combine natural stack ventilation during favorable conditions with mechanical backup, optimizing energy consumption while ensuring year-round performance.

Performance Metrics

Effective stack ventilation design achieves:

Air change rates: 4-12 ACH depending on building geometry and temperature differential
Pressure differentials: 2-20 Pa for residential, up to 50+ Pa for tall commercial buildings
Energy savings: 30-60% reduction in HVAC fan energy compared to fully mechanical systems

These metrics require careful integration of opening design, building geometry, and climate-responsive control strategies to harness buoyancy forces effectively.