Makeup Air Requirements for Stairwell Pressurization

Overview

Makeup air systems for stairwell pressurization must deliver sufficient airflow to maintain minimum pressure differentials across all stairwell boundaries while compensating for leakage and door-opening events. The design requires careful calculation of supply air quantities, strategic placement of injection points, and proper fan selection to ensure uniform pressure distribution throughout the vertical shaft.

Makeup Air Volume Calculation

The total makeup air requirement combines two fundamental components: continuous leakage compensation and transient door-opening demand.

Base Leakage Airflow

Leakage through stairwell envelope penetrations follows the orifice equation, modified for crack geometry:

$$Q_L = C \cdot A \cdot \sqrt{\Delta P}$$

Where:

• $Q_L$ = leakage airflow (cfm)
• $C$ = flow coefficient (cfm/ft²-√inWC), typically 0.65-0.83 for construction joints
• $A$ = total leakage area (ft²)
• $\Delta P$ = pressure differential (inWC)

For multi-story stairwells, the cumulative leakage area includes:

For an $n$-story stairwell:

$$Q_{L,total} = \sum_{i=1}^{n} C_i \cdot A_i \cdot \sqrt{\Delta P_i}$$

IBC Section 909.6.2 requires maintaining minimum 0.05 inWC across stairwell doors with all doors closed. NFPA 92 Section 4.5.3 recommends 0.10 inWC as a practical design target to account for measurement uncertainty.

Door Opening Demand

When a stairwell door opens, the system must supply makeup air to replace the volume exhausted through the opening while maintaining acceptable pressure:

$$Q_{door} = C_d \cdot A_{door} \cdot \sqrt{2 \cdot \Delta P / \rho}$$

Converting to standard HVAC units:

$$Q_{door} = 2610 \cdot C_d \cdot A_{door} \cdot \sqrt{\Delta P}$$

Where:

• $Q_{door}$ = door opening airflow (cfm)
• $C_d$ = discharge coefficient, 0.65-0.70 for door openings
• $A_{door}$ = effective door area (ft²), typically 50-70% of geometric area
• $\Delta P$ = pressure differential (inWC)
• 2610 = conversion factor for standard air (60°F, sea level)

NFPA 92 requires systems to accommodate simultaneous opening of doors on the fire floor plus two adjacent floors. For a typical 3’-0" × 7’-0" door with 60% effective area:

$$Q_{door} = 2610 \times 0.65 \times (3 \times 7 \times 0.60) \times \sqrt{0.10} = 2,757 \text{ cfm per door}$$

Multiple Injection Points Strategy

Vertical distribution of makeup air prevents excessive pressure stratification in tall stairwells. Stack effect creates a pressure gradient:

$$\Delta P_{stack} = 7.64 \cdot h \cdot \left(\frac{1}{T_{outside}} - \frac{1}{T_{stairwell}}\right)$$

Where:

• $\Delta P_{stack}$ = stack effect pressure (inWC)
• $h$ = height difference (ft)
• $T$ = absolute temperature (°R = °F + 460)

For a 400-ft stairwell with 20°F outdoor and 70°F indoor temperatures:

$$\Delta P_{stack} = 7.64 \times 400 \times \left(\frac{1}{480} - \frac{1}{530}\right) = 0.60 \text{ inWC}$$

This pressure gradient necessitates multiple injection points to maintain uniform pressurization.

Injection Point Placement

graph TD
    A[Makeup Air Fan] -->|Primary Supply| B[Bottom Injection - Floors 1-15]
    A -->|Secondary Supply| C[Mid-Height Injection - Floors 16-30]
    A -->|Tertiary Supply| D[Upper Injection - Floors 31-45]
    B --> E[Pressure Relief Dampers at Top]
    C --> E
    D --> E

    style A fill:#e1f5ff
    style E fill:#ffe1e1

General guidelines for injection point spacing:

Each injection point should serve zones of similar leakage characteristics. Balancing dampers at each injection point enable field adjustment to achieve uniform pressure distribution.

Fan Sizing and Selection

Makeup air fans must overcome system resistance while delivering design flow under worst-case conditions.

Fan Capacity Requirements

Total fan capacity includes safety factors for uncertainties:

$$Q_{fan} = 1.2 \times \left(Q_{L,total} + Q_{door,simultaneous}\right)$$

The 1.2 multiplier accounts for:

• Leakage area estimation uncertainty (±15%)
• Duct leakage (3-5% for sealed metal ducts)
• Control tolerance (±5%)

Static Pressure Calculation

Fan total pressure must overcome:

$$\Delta P_{fan} = \Delta P_{stairwell} + \Delta P_{duct} + \Delta P_{fittings} + \Delta P_{injection}$$

Duct friction loss follows the Darcy-Weisbach equation:

$$\Delta P_{duct} = f \cdot \frac{L}{D} \cdot \frac{\rho V^2}{2 \cdot 144}$$

For rectangular ducts, use hydraulic diameter $D_h = 4A/P$.

Injection device loss typically ranges 0.25-0.50 inWC for diffusers, 0.15-0.30 inWC for nozzles.

Example for 40-story building:

• Stairwell pressure target: 0.10 inWC
• Duct run friction (250 ft equivalent): 0.85 inWC
• Fittings and transitions (8 elbows, 3 branches): 0.40 inWC
• Injection devices (3 points): 0.35 inWC
• Total fan static pressure: 1.70 inWC

Fan Type Selection

Select fans to operate between 50-75% of maximum flow on the fan curve, ensuring stable operation if system resistance changes.

Duct Routing Considerations

Riser Configuration

Vertical duct risers adjacent to stairwells minimize horizontal runs. Key design elements:

flowchart LR
    A[Outdoor Air Intake] --> B[Air Handling Unit with Heating]
    B --> C[Fire/Smoke Damper]
    C --> D[Primary Vertical Riser]
    D --> E1[Injection Point 1 - Bottom]
    D --> F[Secondary Riser]
    F --> E2[Injection Point 2 - Mid]
    F --> G[Tertiary Riser]
    G --> E3[Injection Point 3 - Top]
    E1 --> H[Stairwell Pressurization]
    E2 --> H
    E3 --> H

    style A fill:#c8e6c9
    style C fill:#ffccbc
    style H fill:#b3e5fc

Velocity limitations:

• Main risers: 2,000-2,500 fpm (balance noise vs. size)
• Branch takeoffs: 1,500-2,000 fpm
• Injection devices: 800-1,200 fpm (minimize noise)

Outdoor Air Intake Location

IBC Section 909.13 and NFPA 92 Section 4.8.4 require outdoor air intakes located to prevent smoke entrainment:

• Minimum 20 ft from potential smoke sources (loading docks, exhaust outlets)
• Above grade flood level per local requirements
• Consider prevailing wind patterns and building aerodynamics
• Install bird/debris screens without excessive pressure drop (<0.10 inWC)

For buildings >75 ft, locate intakes on roof to avoid ground-level smoke migration and maximize pressure recovery from building wake effects.

Seasonal Considerations

Temperature Effects on Airflow

Air density variation impacts volumetric flow relationships:

$$\rho = \frac{P_{abs}}{R \cdot T_{abs}}$$

For standard pressure (29.92 inHg) and varying temperature:

Cold outdoor air increases density, requiring compensation in fan control strategies. Variable frequency drives (VFD) modulate fan speed to maintain constant pressure:

$$N_2 = N_1 \cdot \sqrt{\frac{\rho_1}{\rho_2}}$$

Stack Effect Compensation

Winter conditions intensify stack effect in tall buildings. Systems must overcome additional pressure differential:

Worst-case winter scenario (for 400-ft building):

• Outdoor: -10°F
• Indoor: 70°F
• Stack effect: 0.72 inWC (from previous equation)
• Required stairwell pressure: 0.10 inWC
• Total fan discharge pressure required: 0.82 inWC + duct losses

Summer reverse stack effect (outdoor >indoor temperature) aids pressurization at lower floors but opposes it at upper floors, necessitating careful injection point balancing.

Heating Requirements

NFPA 92 Section 4.8.5 permits heating makeup air to prevent discomfort during door openings. Heating capacity:

$$Q_{heating} = 1.08 \cdot CFM \cdot \Delta T$$

For 10,000 cfm system with 60°F temperature rise (-10°F to 50°F discharge):

$$Q_{heating} = 1.08 \times 10,000 \times 60 = 648,000 \text{ Btu/hr}$$

Heating must activate with pressurization fans to prevent cold air dumping during emergency egress.

System Redundancy and Emergency Power

IBC Section 909.11 requires emergency power for pressurization systems. Design considerations:

• Dual makeup air fans (100% standby capacity)
• Emergency generator connection via automatic transfer switch (<10 second transfer)
• Battery backup for controls and damper actuators (minimum 90-minute capacity)
• Monthly automatic testing per NFPA 110

Redundant systems may share common ductwork with isolation dampers, or provide completely independent air paths for enhanced reliability in critical applications.

Testing and Commissioning

Field verification confirms design assumptions:

1. Leakage area measurement: Pressurize stairwell with all openings sealed, measure flow at multiple pressures to determine actual $C \cdot A$ product
2. Pressure distribution: Verify ±0.02 inWC uniformity between injection zones
3. Door opening performance: Simulate simultaneous door openings, confirm pressure maintenance
4. Seasonal adjustment: Program VFD control algorithms based on outdoor temperature sensors

Acceptance criteria per NFPA 92 Section 4.6:

• Minimum 0.05 inWC, maximum 0.35 inWC across closed doors
• Maximum 30 lbf door opening force (0.12-0.15 inWC depending on door geometry)
• Pressure recovery within 30 seconds after door closure

This content provides engineering-level guidance for makeup air system design. Specific projects require calculations based on actual building geometry, construction details, and local code amendments.