Building Pressurization Control for HVAC Engineers

Building pressurization control maintains pressure differentials between spaces to control airflow direction, prevent infiltration/exfiltration, and ensure occupant comfort. Critical applications include laboratories, hospitals, cleanrooms, and stairwell pressurization.

Pressure Relationships

Pressure differential:

$$\Delta P = P_{high} - P_{low}$$

Typical units: Pascals (Pa) or inches water gauge ("w.g.)

Conversion: 1 "w.g. = 249 Pa

Airflow through opening:

$$Q = C \times A \times \sqrt{2 \Delta P / \rho}$$

Where:

$Q$ = airflow (CFM)
$C$ = discharge coefficient (0.6-0.7)
$A$ = opening area (ft²)
$\Delta P$ = pressure difference (lb/ft²)
$\rho$ = air density (lb/ft³)

Simplified for standard air:

$$CFM = 2610 \times A \times \sqrt{\Delta P_{"w.g.}}$$

Worked Example 1: Leakage Through Door

Given:

Door size: 3 ft × 7 ft
Door gap: 0.5 inches all around
Pressure differential: 0.05 "w.g.
Discharge coefficient: 0.65

Find: Leakage airflow

Solution:

Gap area:

$$A = \frac{0.5}{12} \times 2 \times (3 + 7) = 0.833 \text{ ft}^2$$

Airflow:

$$CFM = 2610 \times 0.65 \times 0.833 \times \sqrt{0.05} = 390 \text{ CFM}$$

Answer: 390 CFM leakage through door gaps

Laboratory Pressurization

Negative Pressure Labs

Purpose: Contain hazardous materials (BSL-2, BSL-3, chemical labs)

Typical pressure: -0.01 to -0.05 "w.g. relative to corridor

Control strategy:

graph TD
    A[Measure room pressure] --> B{Pressure < setpoint?}
    B -->|Yes| C[Reduce exhaust airflow]
    B -->|No| D[Increase exhaust airflow]
    C --> E[Modulate exhaust valve/damper]
    D --> E
    E --> F{Fume hood active?}
    F -->|Yes| G[Increase both supply + exhaust]
    F -->|No| H[Maintain base ventilation]

Design:

Exhaust > Supply by 100-300 CFM
Exhaust located low (heavy vapors) and high (light vapors)
Supply diffusers away from exhaust (avoid short-circuiting)

Positive Pressure Labs

Purpose: Protect sensitive materials (cleanrooms, semiconductor, pharmaceutical)

Typical pressure: +0.02 to +0.05 "w.g. relative to corridor

Design:

Supply > Exhaust by 100-500 CFM (depends on room volume)
Pressure cascades: Cleanroom > Gowning > Corridor

Cascade example:

ISO 5 cleanroom: +0.05 "w.g.
ISO 7 gowning: +0.03 "w.g.
Corridor: +0.01 "w.g.
Ambient: 0 "w.g.

Stairwell Pressurization

Purpose: Provide smoke-free egress during fires

Code requirements (IBC/NFPA 92):

Minimum pressure: 0.10 "w.g. (25 Pa)
Maximum pressure: 0.35 "w.g. (87 Pa) with all doors closed

Design airflow:

$$CFM_{stairwell} = CFM_{doors} + CFM_{leakage} + CFM_{weather}$$

Door opening airflow:

$$CFM_{door} = 2610 \times A_{door} \times \sqrt{\Delta P}$$

Typical: 3,000-5,000 CFM per stairwell

Worked Example 2: Stairwell Pressurization

Given:

10-story stairwell
Door size: 3 ft × 7 ft = 21 ft²
Design pressure: 0.30 "w.g. (single door open)
Leakage: 200 CFM

Find: Required supply airflow

Solution:

Airflow for one door:

$$CFM_{door} = 2610 \times 21 \times \sqrt{0.30} = 30,000 \text{ CFM}$$

Total with leakage:

$$CFM_{total} = 30,000 + 200 = 30,200 \text{ CFM}$$

Answer: 30,200 CFM supply fan capacity

(Note: Actual design considers door-opening scenarios per NFPA 92)

Control strategy:

Barometric damper relief (passive)
Or modulating relief damper (active control)
Pressure sensor in stairwell

Makeup Air for Exhaust Systems

Makeup air must replace exhausted air to prevent building depressurization

Calculation:

$$CFM_{makeup} = CFM_{exhaust} - CFM_{infiltration}$$

Sources of exhaust:

Kitchen hoods: 200-500 CFM per linear foot
Laboratory fume hoods: 100-150 CFM per ft² face area
Restroom exhaust: 50-75 CFM per fixture
Process exhaust: varies by application

Consequences of inadequate makeup air:

Difficulty opening doors (high negative pressure)
Backdrafting of combustion appliances
Infiltration of unconditioned air
Comfort complaints

Design guidelines:

Provide 80-100% of exhaust as dedicated makeup air
Remainder from infiltration and general ventilation
Makeup air should be conditioned (heated/cooled)

Relief Air Sizing

Relief air prevents over-pressurization when outdoor air exceeds exhaust

Typical scenario: Economizer mode with 100% outdoor air

Sizing:

$$CFM_{relief} = CFM_{OA,max} - CFM_{exhaust} - CFM_{exfiltration}$$

Relief damper types:

Barometric relief: Gravity-operated (passive)
Power relief: Motorized damper (active control)
Relief fan: Powered exhaust (large systems)

Location: High in building (warm air rises)

Practical Control Strategies

Direct Pressure Control

Sensor: Differential pressure sensor between spaces

Control: Modulates supply/exhaust dampers or fans to maintain setpoint

Advantages: Precise, responds to all influences (doors, wind, stack)

Disadvantages: Sensors drift, requires calibration

Airflow Tracking

Measure supply and exhaust flows, maintain offset

Example: Maintain supply = exhaust + 150 CFM

Advantages: No drift, simple to commission

Disadvantages: Does not account for infiltration/exfiltration

Combined Approach

Use airflow tracking with pressure monitoring as override/verification

Practical Design Considerations

Sensor location: Away from diffusers, doors, HVAC equipment
Door undercuts: 1-inch undercut = ~21 ft² × 1/12 = 1.75 ft² free area
Vestibules: Break pressure differential (two-door airlock)
Wind effects: Can overcome 0.05 "w.g. pressure control
Commissioning: Test with doors open/closed, verify pressure cascades

Related Technical Guides:

References:

ASHRAE Handbook of HVAC Applications, Chapter 16: Laboratories
NFPA 92: Standard for Smoke Control Systems
International Building Code (IBC), Section 909: Smoke Control Systems
ANSI/AIHA Z9.5: Laboratory Ventilation