Exhaust Air Strategies for Indoor Pool Ventilation

Exhaust Air Strategies for Natatoriums

Effective exhaust air strategies are critical for maintaining indoor air quality, controlling humidity, and removing chloramines in natatorium environments. The strategic placement and volumetric balance of exhaust points directly impacts occupant comfort, air quality, and the longevity of building materials.

Fundamental Exhaust Requirements

The primary objectives of natatorium exhaust systems include:

• Chloramine extraction from the breathing zone and water surface
• Moisture control to prevent condensation on building surfaces
• Contaminant dilution to maintain acceptable indoor air quality
• Thermal stratification management to prevent hot, humid air accumulation at the ceiling

ASHRAE Applications Handbook Chapter 6 (Natatoriums) specifies minimum outdoor air ventilation rates and emphasizes the importance of exhaust location relative to contaminant sources.

Exhaust Air Volume Calculations

Total exhaust airflow must balance supply air while maintaining appropriate building pressure. For natatoriums, the exhaust rate is typically calculated based on ventilation requirements:

$$ Q_{\text{exhaust}} = Q_{\text{supply}} - Q_{\text{infiltration}} + Q_{\text{exfiltration}} $$

Where natatoriums typically maintain negative pressure of 0.05 to 0.10 inches water column relative to adjacent spaces to prevent moisture migration:

$$ Q_{\text{exhaust}} = Q_{\text{supply}} \times (1 + P_{\text{factor}}) $$

Where $P_{\text{factor}}$ ranges from 0.05 to 0.15 depending on building tightness and desired pressure differential.

Minimum outdoor air requirements per ASHRAE 62.1 for pool deck areas:

$$ Q_{\text{OA}} = 0.48 , \text{cfm/ft}^2 \times A_{\text{deck}} + 0.06 , \text{cfm/ft}^2 \times A_{\text{water}} $$

Exhaust Location Strategies

The effectiveness of contaminant removal depends significantly on exhaust grille placement relative to contaminant generation zones.

graph TB
    subgraph "Natatorium Exhaust Strategy"
        A[Water Surface] -->|Evaporation + Chloramines| B[Breathing Zone 4-6 ft]
        B -->|Buoyancy| C[Upper Zone 8-15 ft]
        C -->|Stratification| D[Ceiling Zone >15 ft]

        E[Deck-Level Exhaust<br/>15-25% of total] -.->|Captures| B
        F[Mid-Level Exhaust<br/>20-30% of total] -.->|Captures| C
        G[High-Level Exhaust<br/>45-60% of total] -.->|Captures| D

        H[Pool Gutter Exhaust<br/>Optional] -.->|Captures| A
    end

    style A fill:#e3f2fd
    style B fill:#fff3e0
    style C fill:#fce4ec
    style D fill:#f3e5f5
    style E fill:#c8e6c9
    style F fill:#c8e6c9
    style G fill:#c8e6c9
    style H fill:#c8e6c9

Deck-Level Exhaust

Low-level exhaust grilles positioned 4 to 6 feet above the pool deck capture chloramines and moisture in the breathing zone before buoyancy transports contaminants upward. This strategy is particularly effective for:

• Swimmer and spectator breathing zones
• High-occupancy periods with elevated chloramine generation
• Facilities with competitive swimming where athletes breathe heavily near the water surface

Deck-level exhaust should constitute 15 to 25 percent of total exhaust volume. Grille placement should be:

• 3 to 8 feet from pool edge
• 4 to 6 feet above finished floor
• Positioned to avoid short-circuiting with low-level supply air

High-Level Ceiling Exhaust

Upper exhaust points capture buoyant warm, humid air that naturally stratifies at the ceiling. This exhaust location:

• Prevents condensation on ceiling surfaces and roof structure
• Removes the bulk of moisture load through natural convection
• Captures chloramines that have risen through thermal plumes

High-level exhaust typically represents 45 to 60 percent of total exhaust and should be distributed across the ceiling to prevent dead zones.

Gutter and Water Surface Exhaust

Some designs incorporate exhaust integrated into pool gutter systems or positioned immediately above the water surface. This approach:

• Captures evaporation and chloramines at the source
• Reduces contaminant concentration before breathing zone exposure
• Requires specialized grille design to prevent water entrainment

This strategy is optional and typically adds 5 to 15 percent to total exhaust capacity.

Exhaust Approach Comparison

Contaminant Removal Effectiveness

The air change effectiveness for exhaust systems depends on placement relative to contaminant sources. The ventilation effectiveness can be quantified:

$$ \epsilon_v = \frac{C_{\text{exhaust}} - C_{\text{supply}}}{C_{\text{breathing zone}} - C_{\text{supply}}} $$

For well-designed multi-level exhaust systems in natatoriums, $\epsilon_v$ ranges from 1.2 to 1.8, indicating superior performance compared to mixing ventilation ($\epsilon_v = 1.0$).

The chloramine removal rate depends on exhaust proximity to generation sources:

$$ \eta_{\text{removal}} = 1 - e^{-\frac{Q_{\text{local}} \times t}{V_{\text{zone}}}} $$

Where:

• $Q_{\text{local}}$ = local exhaust flow rate (cfm)
• $t$ = residence time (minutes)
• $V_{\text{zone}}$ = zone volume (ft³)

Stratification Control

Thermal stratification naturally occurs in natatoriums due to:

• Warm water surface temperatures (78-82°F)
• Evaporative heat release
• High ceilings (typically 15-25 feet)

Temperature differentials between deck and ceiling can reach 8 to 15°F without proper air distribution. Exhaust strategies must work in coordination with supply air patterns to:

1. Extract the warmest, most humid air at the ceiling
2. Prevent excessive temperature gradients that reduce comfort
3. Maintain adequate air movement through the breathing zone

The stratification factor can be estimated:

$$ S_f = \frac{T_{\text{ceiling}} - T_{\text{deck}}}{T_{\text{supply}} - T_{\text{deck}}} $$

Well-designed systems maintain $S_f < 0.5$.

Design Implementation Guidelines

Exhaust Grille Sizing and Selection:

• Maximum face velocity: 500-700 fpm to minimize noise
• Grille free area: 50-60% of nominal size
• Corrosion-resistant construction (Type 316 stainless steel, powder-coated aluminum)

Duct Placement:

• Insulate exhaust ducts in conditioned spaces to prevent condensation
• Maintain duct velocity: 1200-1800 fpm to prevent moisture accumulation
• Slope horizontal runs toward drain points

Pressure Balancing:

• Install exhaust dampers for zone balancing
• Provide volume control at main exhaust branches
• Monitor building pressure relative to adjacent spaces

Chloramine Concentration Targets:

• Trichloramine (NCl₃): < 0.3 mg/m³ per WHO guidelines
• Breathing zone sampling at 3-5 feet above deck
• Continuous monitoring recommended for high-use facilities

Operational Considerations

Exhaust strategies must adapt to varying occupancy and pool use:

• High occupancy periods: Increase deck-level exhaust proportion
• Unoccupied periods: Reduce total exhaust while maintaining building pressure
• Water treatment events: Temporarily increase exhaust during shock chlorination

The exhaust system should integrate with dehumidification equipment controls to optimize energy recovery while maintaining air quality and humidity setpoints.

References

• ASHRAE Handbook—HVAC Applications, Chapter 6: Natatoriums
• ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality
• WHO Guidelines for Indoor Swimming Pools and Similar Environments