Indoor Swimming Pools (Natatoriums)

Indoor swimming pools (natatoriums) present unique HVAC design challenges requiring specialized knowledge of moisture transport, chemical air quality management, and corrosion control. The interaction between warm water surfaces, chlorinated water chemistry, human occupancy, and building envelope performance creates a complex psychrometric and chemical environment that demands careful engineering analysis.

Design Challenges

Natatorium HVAC systems must simultaneously address multiple competing requirements:

Moisture Management: Pool water evaporation rates typically range from 0.5 to 2.0 lb/h·ft² of water surface area depending on activity level, water temperature, and air conditions. This massive moisture load—often 10 to 50 times greater than a comparable building without a pool—drives equipment sizing and energy consumption. A 1,500 ft² competitive pool can evaporate 1,000 to 3,000 pounds of water per hour during heavy use.

Chemical Air Quality: Chlorine-based disinfection systems produce airborne chloramines (primarily trichloramine, NCl₃) when chlorine reacts with nitrogenous compounds from swimmers. These irritating compounds accumulate in indoor pool air and must be diluted through ventilation or destroyed through chemical filtration. Trichloramine concentrations should be maintained below 0.5 mg/m³ (0.3 ppm) to prevent eye and respiratory irritation.

Corrosion Control: The combination of high humidity (50-60% RH recommended), elevated chloramine concentrations, and warm temperatures creates an extremely corrosive environment. All HVAC components, ductwork, fasteners, and controls exposed to pool air require special corrosion-resistant materials and protective coatings.

Energy Efficiency: Maintaining proper indoor conditions year-round while processing large ventilation and dehumidification loads results in substantial energy consumption. Energy recovery becomes essential, with heat recovery from exhaust air and dehumidification condensate commonly recovering 50-70% of energy that would otherwise be wasted.

Psychrometric Considerations

The natatorium psychrometric process differs fundamentally from conventional building HVAC. The pool water surface acts as a constant moisture source, typically maintained at 82-84°F for recreational pools and 78-80°F for competitive facilities. Indoor air is conventionally maintained 2-4°F warmer than water temperature to minimize evaporation and prevent swimmer discomfort.

Recommended design conditions for competitive pools:

• Pool water temperature: 78-80°F
• Air temperature: 80-82°F
• Relative humidity: 50-60%
• Air velocity at water surface: <30 fpm (minimize evaporation)
• Deck air velocity: 50-100 fpm (adequate for comfort)

For recreational and therapy pools with higher water temperatures (84-92°F), air temperatures must increase proportionally. The dewpoint temperature of indoor air must always remain below all indoor surface temperatures to prevent condensation on windows, walls, ceilings, and building structural elements.

Load Calculation Fundamentals

Natatorium load calculations require detailed analysis of:

1. Latent load from evaporation (dominant load, often 60-80% of total cooling)
2. Sensible heat gain from solar radiation through glazing
3. Pool water heat loss to air and through evaporation
4. Ventilation loads for outdoor air introduction
5. Occupant loads during peak use periods
6. Lighting and equipment loads

The evaporation rate depends on water surface area, air and water temperatures, relative humidity, air velocity across the pool surface, and activity level. Empirical correlations from Carrier, Shah, and ASHRAE provide calculation methods with varying complexity and accuracy.

System Types

Natatorium dehumidification can be accomplished through several approaches:

Mechanical Refrigeration Dehumidification: Direct-expansion or chilled water cooling coils condense moisture from pool air. Provides excellent humidity control but requires substantial energy and typically needs reheat to avoid overcooling.

Desiccant Dehumidification: Solid or liquid desiccant systems absorb moisture chemically. Effective at lower temperatures but require significant regeneration energy. Often combined with mechanical cooling.

Heat Recovery Dehumidification: Integrated systems use refrigeration cycle to simultaneously cool/dehumidify pool air while recovering condensing heat for pool water heating, domestic hot water, or air reheat. Most energy-efficient approach for year-round operation.

Ventilation-Only: Introducing outdoor air for dilution and exhausting humid pool air. Only viable in dry climates or during specific seasonal conditions. Generally insufficient as standalone solution.

Design Approach Overview

Successful natatorium design requires integrated analysis of multiple systems:

• Pool water evaporation rate calculation and verification
• Dehumidification equipment selection and sizing
• Chloramine control through ventilation and/or UV treatment
• Building envelope analysis and condensation prevention
• Energy recovery integration for economic operation
• Pool deck heating for comfort and condensation control
• Spectator area isolation and conditioning
• Corrosion-resistant materials and protective coatings for all wetted components

Each of these elements must be carefully analyzed and coordinated to create a healthy, comfortable, durable, and energy-efficient indoor pool environment.

Code and Industry Standards

Natatorium design should reference:

• ASHRAE Handbook - HVAC Applications, Chapter on Swimming Pools
• ASHRAE Standard 62.1: Ventilation rates for indoor pool areas (0.06 cfm/ft² pool surface + 0.48 cfm/ft² deck area minimum)
• CDC Model Aquatic Health Code: Water and air quality requirements
• NFPA Standards: Fire protection and electrical requirements for wet environments
• Local health codes: Specific ventilation and water quality requirements

Indoor pool facilities require specialized engineering expertise integrating mechanical, structural, architectural, and aquatic chemistry considerations. The following sections provide detailed technical guidance for each critical design element.

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