Indoor Swimming Pools
Indoor swimming pool facilities, or natatoriums, present unique HVAC challenges including extreme humidity loads, corrosive atmospheres, and stringent air quality requirements for swimmer health and building protection.
Design Challenges
Humidity Generation
Pool evaporation generates substantial moisture:
$$W_{evap} = A \times (p_{sat,water} - p_{air}) \times K_{activity}$$
Where:
- A = pool surface area
- p = vapor pressure
- K = activity factor (0.5-1.0)
Evaporation Rates:
| Activity Level | Evaporation (lb/h/ft²) |
|---|---|
| Quiescent pool | 0.04-0.06 |
| Residential use | 0.06-0.08 |
| Public swimming | 0.10-0.12 |
| Competition | 0.12-0.15 |
| Wave pools | 0.30-0.40 |
Chloramine Concerns
Chloramines (combined chlorine) form when chlorine reacts with organic compounds:
Health Effects:
- Eye and respiratory irritation
- Asthma triggers
- Reduced air quality
Control Strategies:
- Adequate ventilation
- Source-level exhaust
- UV treatment of pool water
- Proper water chemistry
Corrosion Environment
Warm, humid, chlorinated air attacks:
- Steel structures
- Electrical equipment
- Ductwork
- Building finishes
Ventilation Requirements
Air Change Rates
ASHRAE recommendations:
| Space | Minimum ACH | Outdoor Air |
|---|---|---|
| Pool area | 4-6 | 0.48 CFM/ft² |
| Spectator areas | 4-8 | 15 CFM/person |
| Locker rooms | 4-6 | 0.5 CFM/ft² |
| Showers | 10-15 | 50 CFM/head |
Outdoor Air
Minimum outdoor air for natatoriums:
- ASHRAE 62.1: 0.48 CFM/ft² pool deck
- Capture chloramines near source
- Prevent recirculation of contaminants
Air Distribution
Supply Air:
- High on walls or ceiling
- Directed across pool surface
- 25-50 fpm at pool surface (minimizes evaporation)
Return/Exhaust:
- Low on walls (near deck level)
- Captures heavy chloramine compounds
- Multiple points around perimeter
Temperature and Humidity
Design Conditions
| Parameter | Pool Area | Spectator |
|---|---|---|
| Dry bulb | 82-86°F | 75-78°F |
| Relative humidity | 50-60% | 50-60% |
| Deck temperature | 84-88°F | N/A |
Water-to-Air Relationship
Maintain air temperature 2-5°F above water temperature:
- Reduces evaporation rate
- Improves swimmer comfort
- Prevents fog formation
$$T_{air} = T_{water} + (2°F\ to\ 5°F)$$
Humidity Control
Target 50-60% RH for balance:
- <50%: Excessive evaporation, swimmer discomfort
60%: Condensation risk, comfort issues
Dehumidification Systems
Refrigerant-Based Dehumidifiers
Mechanical dehumidification with heat recovery:
Operating Cycle:
- Air passes over evaporator (cools below dew point)
- Moisture condenses and drains
- Air reheated over condenser
- Some heat returned to pool water
Efficiency:
- COP: 4-6 for dehumidification
- Heat recovery to pool: 50-80% of removed latent
Outdoor Air Systems
Use outdoor air for dehumidification when favorable:
$$m_{OA} = \frac{m_{moisture}}{W_{return} - W_{OA}}$$
Effective When:
- Outdoor humidity ratio < indoor
- Typically 50°F and below outdoor temperature
Desiccant Systems
Desiccant wheels for specialized applications:
- Very low humidity requirements
- Heat recovery from exhaust
- Combined with refrigerant systems
Hybrid Approaches
Combine methods for optimization:
- Outdoor air when effective
- Mechanical dehumidification when needed
- Heat pump heat recovery
- Energy recovery from exhaust
HVAC System Components
Air Handling Units
Specialized natatorium AHUs:
- Corrosion-resistant construction (stainless, aluminum, coated)
- Large OA/exhaust capacity
- Integrated dehumidification
- High-quality filtration
Ductwork
Material Selection:
| Material | Application | Notes |
|---|---|---|
| Aluminum | Preferred | Corrosion resistant |
| Stainless steel | High corrosion | Higher cost |
| Fiberglass | Supply ducts | No condensation areas |
| PVC | Exhaust | Certain applications |
Avoid: Galvanized steel in pool area atmosphere
Exhaust Systems
Dedicated pool exhaust:
- Corrosion-resistant fans
- Low-level pickup points
- Roof discharge
- Separate from general HVAC
Energy Efficiency
Heat Recovery
Multiple heat recovery opportunities:
| Source | Recovery Method | Application |
|---|---|---|
| Exhaust air | Air-to-air HX | Preheat outdoor air |
| Condenser heat | Desuperheater | Pool water heating |
| Dehumidifier | Heat recovery | Space/pool heating |
Pool Covers
Reduce evaporation during unoccupied periods:
- 50-70% evaporation reduction
- Automatic covers for convenience
- Ventilation reduction possible with cover deployed
Control Strategies
- Setback when unoccupied
- Pool cover integration
- OA economizer when effective
- Demand-controlled ventilation
Special Considerations
Condensation Prevention
Prevent surface condensation:
- Maintain wall temperatures above dew point
- Insulate exterior walls well
- Warm window frames/glazing
- Air movement over surfaces
Structural Protection
Design for durability:
- Vapor barriers on warm side
- Stainless fasteners
- Protective coatings
- Material selection throughout
Acoustics
Swimming pools amplify sound:
- Sound-absorbing materials
- Duct silencers
- Equipment isolation
- Background noise consideration
Indoor swimming pool HVAC systems require specialized design addressing extreme humidity, air quality, and corrosion challenges while providing energy-efficient operation for these demanding facilities.