Table of Contents
Designing the HVAC system for an indoor pool (or natatorium) is one of the most challenging tasks in building services engineering. Unlike standard comfort cooling, an indoor pool environment is dominated by a massive, continuous source of moisture evaporation. Failure to properly manage this moisture can lead to severe building degradation, poor indoor air quality, and an uncomfortable environment for occupants.
The numbers tell the story: a 25m pool evaporates 150-250 kg of water per hour during active use (per VDI 2089). Without proper dehumidification, relative humidity climbs above 70%—the threshold where condensation destroys building structures.
Example calculation: A 25m × 12.5m pool at 28°C water temperature with 30°C air at 55% RH:
At rest this pool evaporates 47 kg/hr, increasing to 120+ kg/hr during active swimming. Use our Pool Evaporation Calculator to size your dehumidification system.
The Primary Challenge: Humidity Control
The single most important function of a pool ventilation system is to control humidity. The large, warm body of water in a pool continuously evaporates, releasing vast amounts of moisture into the air. If this moisture is not removed, it will lead to:
- Condensation: Moisture will condense on any surface colder than the dew point of the air, such as windows, walls, and structural steel.
- Corrosion and Mold: Persistent condensation will cause rust, rot, and the growth of mold and mildew, leading to costly structural damage and health concerns.
- Poor Air Quality: High humidity combined with pool chemicals (chloramines) creates a corrosive and unpleasant atmosphere that can irritate the eyes and respiratory systems of swimmers.
- Occupant Discomfort: A hot, humid, and stuffy environment is uncomfortable for swimmers and spectators alike.
Pool Ventilation Design Parameters
| Parameter | Recommended Value | Standard Reference |
|---|---|---|
| Relative Humidity | 50-60% RH | ASHRAE, VDI 2089 |
| Air Temperature | 1-2°C above pool water | VDI 2089 |
| Air Changes | 4-6 ACH minimum | ASHRAE 62.1 |
| Room Pressure | Slightly negative (-5 to -10 Pa) | Best Practice |
| Supply Air Velocity | 0.1-0.2 m/s at occupied zone | Comfort |
| Evaporation Rate | 0.1-0.3 kg/(m²·hr) | Varies by activity |
Key Design Principles for Pool Ventilation
An effective pool ventilation system is built on three core principles:
- Source Capture and Exhaust: Remove the humid, chemical-laden air directly at its source—the surface of the pool.
- Supply of Conditioned Air: Introduce fresh, dehumidified air to replace the exhausted air and maintain a comfortable environment.
- Proper Air Distribution: Ensure that the conditioned air is delivered effectively across all surfaces to prevent condensation and create a well-mixed, comfortable space.
The Ventilation Strategy in Action
The ventilation process follows a continuous cycle: the dehumidification unit removes moisture and conditions the air, which is then distributed through supply air ducts to wash over windows and walls. Return and exhaust grilles pull humid air from the pool surface back to the unit, completing the cycle.
Designing for Effective Air Distribution
How you deliver and remove air is just as important as how much you condition it.
- Supply Air: Supply air should be directed onto the surfaces most prone to condensation, particularly windows and exterior walls. This "washes" these surfaces with dry air, keeping their temperature above the dew point.
- Return/Exhaust Air: Return and exhaust grilles should be located low and close to the pool surface. Since the humid, chloramine-laden air is heavier than the surrounding air, this strategy allows for effective source capture before it can mix with the rest of the room.
Maintaining Negative Pressure
It is crucial to maintain the pool area at a slight negative pressure relative to adjacent spaces. This is typically achieved by exhausting slightly more air than is supplied (e.g., exhausting 110% of the supply air rate).
Why is this important?
- It prevents the humid, corrosive, and odorous pool air from migrating into other parts of the building, such as lobbies, offices, or changing rooms.
- It helps to contain the unique environment of the natatorium.
Calculating the Ventilation Load
The design of a pool ventilation system starts with calculating the evaporation rate from the pool surface. This calculation is complex and depends on several factors:
- Pool Water Temperature
- Room Air Temperature and Humidity - use our Humidity Ratio Calculator to convert between humidity units
- Air Velocity over the Water Surface
- Activity Level in the Pool (e.g., a leisure pool vs. a competitive swimming pool) - see our Air Flow Calculator for airflow conversions
Evaporation Rate Formulas
Per ASHRAE Applications Handbook, the evaporation rate can be calculated using:
Where:
- = evaporation rate (kg/hr)
- = pool surface area (m²)
- = saturation vapor pressure at water temperature (Pa)
- = partial vapor pressure of air (Pa)
- = latent heat of evaporation (≈ 2,450 kJ/kg at pool temperatures)
- = activity factor
| Pool Type | Activity Factor | Typical Evaporation Rate |
|---|---|---|
| Quiet pool (unoccupied) | 0.5 | 0.07-0.10 kg/(m²·hr) |
| Residential pool | 0.5-1.0 | 0.10-0.15 kg/(m²·hr) |
| Public pool | 1.0-1.5 | 0.15-0.22 kg/(m²·hr) |
| Wave pool/water park | 1.5-2.0 | 0.22-0.30 kg/(m²·hr) |
| Competitive swimming | 1.0 | 0.15-0.20 kg/(m²·hr) |
Once the evaporation rate is known, the engineer can select a dehumidification unit capable of removing that amount of moisture. Industry standards, such as those from ASHRAE, provide detailed guidelines for these calculations.
Worked Example: Pool Dehumidification Sizing
Let's size a dehumidification system for a community swimming pool.
Problem Statement
Design the ventilation system for an indoor pool facility:
- Pool dimensions: 25m × 12.5m = 312.5 m²
- Pool water temperature: 28°C
- Design air conditions: 30°C dry-bulb, 55% RH
- Pool type: Public recreational (activity factor = 1.0)
- Outdoor design: 0°C, 80% RH (winter)
Step 1: Calculate Water Vapor Pressures
Saturation pressure at water temperature (28°C):
Partial vapor pressure in air (30°C at 55% RH):
Step 2: Calculate Evaporation Rate
Using the ASHRAE evaporation formula:
Wait—that's unreasonably high. Let's use the more common simplified formula:
With activity factor for public pool:
Step 3: Size Dehumidification Unit
Required dehumidification capacity:
- Design for active use: 70 kg/hr moisture removal
- Add 20% safety margin: 84 kg/hr
Select dehumidification approach:
| Option | Capacity | Energy Use | Best Application |
|---|---|---|---|
| Outdoor air dilution | Unlimited | Very high | Mild climates only |
| Mechanical DX | 50-150 kg/hr | 15-25 kW | Most applications |
| Heat pump pool DH | 50-200 kg/hr | 10-18 kW | Energy-conscious |
Selected: Heat pump pool dehumidifier rated for 100 kg/hr at design conditions.
Step 4: Verify Air Changes
Minimum outdoor air per ASHRAE 62.1:
- Pool area: 2.4 L/s per m² × 312.5 m² = 750 L/s
- Deck/spectator: Additional based on occupancy
Total supply air for 6 ACH:
Summary
| Parameter | Value | Requirement |
|---|---|---|
| Evaporation rate | 70 kg/hr | Design basis |
| DH unit capacity | 100 kg/hr | 43% margin ✓ |
| Supply air | 2,083 L/s | 6 ACH |
| Outdoor air | 750 L/s | ASHRAE 62.1 |
| Room RH target | 55% | 50-60% range ✓ |
Chloramine Control and Air Quality
Beyond humidity, indoor pools face a unique air quality challenge: chloramines. These compounds form when chlorine reacts with organic matter (sweat, urine, skin cells) brought in by swimmers.
Why Chloramines Matter
Chloramines, particularly nitrogen trichloride (), are heavier than air and concentrate at the pool surface—the breathing zone of swimmers. At concentrations above 0.5 mg/m³, they cause:
- Eye and skin irritation
- Respiratory problems (especially in competitive swimmers)
- The characteristic "pool smell" (often mistaken for chlorine)
Ventilation Strategies for Chloramine Control
- Low-level exhaust: Grilles at pool deck level capture chloramine-laden air before it rises
- Increased outdoor air: Fresh air dilutes chloramine concentrations
- Source control: Shower requirements before swimming reduce organic matter introduction
- Air velocity at pool surface: 0.1-0.2 m/s moves chloramines toward exhaust points
Industry Standards and References
This guide follows VDI 2089 (German standard for swimming pool ventilation), ASHRAE Applications Handbook Chapter 5, and BS EN 15288 for pool water treatment. These standards provide the basis for professional natatorium design worldwide.
Energy Efficiency Considerations
Because pool ventilation systems operate 24/7, energy efficiency is a major concern. Modern pool dehumidification units often incorporate energy recovery technologies:
- Heat Recovery: The hot, humid exhaust air is passed through a heat exchanger to pre-heat the incoming fresh air, reducing heating costs.
- Pool Water Heating: Some units can capture the heat rejected during the dehumidification process and use it to help heat the pool water.
Conclusion
Designing a ventilation system for an indoor pool is a specialized task that requires a deep understanding of thermodynamics, fluid dynamics, and building science. By focusing on the core principles of humidity control, proper air distribution, and negative pressure, engineers can create systems that protect the building structure, ensure good indoor air quality, and provide a comfortable environment for occupants. A successful design is one that remains unseen, quietly and efficiently managing the challenging environment of the natatorium for years to come.