Indoor pools are among the most punishing environments a building can contain. Large amounts of water evaporate from the pool surface every hour, the air is warm and humid, and it carries chlorine compounds that corrode metals and irritate swimmers' eyes and lungs. In a Canadian winter, that warm wet air sits just inside walls and roofs that are well below freezing on the outside.
Poorly designed or operated pool HVAC leads to failures that are expensive and sometimes dangerous: condensation and mould inside walls, corroded roof structures, rotting window frames and air quality complaints. This guide covers the principles behind a reliable natatorium system.
The evaporation load
The main HVAC load in a natatorium is moisture evaporating from the pool. The evaporation rate depends on:
- Water surface area. Larger pools evaporate more.
- Water temperature and air conditions. Evaporation is driven by the difference between the vapour pressure at the water surface and the vapour pressure in the air. Warmer water and drier air increase evaporation.
- Activity. Splashing, waves, water features, slides and swimmers increase the effective surface area. An active leisure pool can evaporate far more than a quiet lap pool of the same size.
- Air velocity over the water.
The ASHRAE Handbook (Applications volume, chapter on natatoriums) provides the standard evaporation equation and activity factors. Spas and hot tubs have high evaporation rates relative to their size because of their high water temperatures and agitation.
Design temperatures and humidity
Typical design conditions follow ASHRAE guidance:
| Pool type | Typical water temperature | Typical air temperature | Relative humidity |
|---|---|---|---|
| Competition and lap pools | 25 to 28 C (77 to 82 F) | 1 to 2 K above water | 50 to 60% |
| Recreational and leisure pools | 27 to 29 C (80 to 85 F) | 1 to 2 K above water | 50 to 60% |
| Therapy pools | 29 to 35 C (85 to 95 F) | 1 to 2 K above water, within comfort limits | 50 to 60% |
| Whirlpools and spas | 36 to 40 C (97 to 104 F) | As for adjacent pool hall | 50 to 60% |
Keeping the air temperature slightly above the water temperature reduces evaporation and keeps wet swimmers from feeling chilled. Relative humidity of 50 to 60 percent balances evaporation, comfort and condensation risk. Below about 40 percent, evaporation and swimmer discomfort increase. Above about 60 percent, condensation and mould risks rise sharply, especially in cold weather.
In very cold weather, the operating humidity may need to be lowered to protect glazing and the envelope. The real limit is whether any interior surface, including window frames and structural elements, falls below the room dew point. At 29 C (84 F) and 55 percent RH, the dew point is about 19 C (66 F), so any surface colder than that will condense. A grounding in psychrometrics helps make these trade-offs.
Dehumidification approaches
| Approach | How it works | Notes |
|---|---|---|
| Packaged refrigerant dehumidifiers | Cool air below dew point, recover heat to reheat air and pool water | The most common approach in North America; very efficient at reusing latent heat |
| Outdoor air ventilation with heat recovery | Dry outdoor air replaces humid indoor air | Works well in Canadian winters; limited when outdoor air is humid |
| Hybrid units | Combine refrigerant dehumidification with variable outdoor air and heat recovery | Flexible across seasons |
| Desiccant systems | Adsorb moisture on a desiccant wheel | Less common for pools; useful in some large facilities |
Packaged pool dehumidifiers are attractive because the energy removed when water vapour condenses on the cooling coil is recovered as heat, which can reheat the supply air or heat the pool water. This turns the evaporation load into a heat source.
In winter, outdoor air is dry and ventilation alone can often handle the moisture, with an energy recovery device recovering the heat. In summer, outdoor air may be more humid than the target indoor condition, so mechanical dehumidification is needed. See dehumidification strategies for a broader comparison.
Ventilation and chloramines
Outdoor air is needed to dilute chloramines and other disinfection byproducts that form when chlorine reacts with sweat, urine and other organic matter. These compounds are heavier than air and concentrate just above the water surface and around the deck, which is exactly where swimmers breathe.
ASHRAE 62.1 lists an area-based outdoor air rate for pool and deck areas of 2.4 L/s per m2 (0.48 cfm/ft2), applied to both the water surface and deck area. Many designers provide more for leisure pools with heavy bather loads and water features. Provincial public health regulations also govern pool operation and water quality.
Exhaust placement makes a large difference. Low-level exhaust near the pool edge or deck captures chloramines where they form, rather than pulling them up through the breathing zone. Some modern natatorium designs use source capture exhaust along the gutter or deck. Good water chemistry and pre-swim showering reduce chloramine formation at the source.
Air distribution
Air distribution in a natatorium has three jobs:
- Wash cold surfaces such as glazing, skylights and exterior walls with warm dry supply air to keep them above the dew point.
- Avoid drafts over the water and deck, which increase evaporation and chill wet swimmers. Air velocity at the water surface should be kept low.
- Move air from spectator areas toward the pool and the exhaust, so moist air does not stagnate in corners.
Supply ducts are often run along the perimeter and directed at glazing. Return and exhaust grilles should be located to draw air across the pool without creating dead zones.
Pressurization and the envelope
The natatorium should be kept at a slight negative pressure relative to adjacent spaces and to outdoors, so that warm humid, chlorinated air does not leak into the building envelope, change rooms, offices or the rest of the facility. In Canada, the envelope matters as much as the mechanical system. A continuous air barrier and vapour control layer on the warm side, carefully detailed at windows and roof-wall junctions, are essential. Even small air leaks will carry large amounts of moisture into cold assemblies, where it condenses and freezes.
See condensation and mould prevention for more on how air leakage drives moisture damage. Infrared thermography during cold weather is an effective way to locate air leaks and wet insulation in pool enclosures.
Materials and corrosion
The chlorinated, humid atmosphere attacks galvanized steel, standard aluminum and ordinary fasteners. Ducts are commonly specified in aluminum with appropriate coatings, stainless steel, or fabric duct. Dehumidifier coils need protective coatings, and electrical and controls equipment should be rated for the environment or located outside the pool hall.
Operating considerations
- Pool covers used overnight dramatically reduce evaporation and energy use when the pool is closed.
- Night setback should be done carefully. Lowering air temperature increases evaporation and RH, so setpoints should be adjusted together.
- Sensor calibration of humidity sensors in this environment needs regular attention.
- Coil and filter maintenance must account for corrosion and chloramine exposure.
Key takeaways
- Evaporation from the pool surface is the dominant load, and activity can multiply it several times.
- Keep air temperature 1 to 2 K above water temperature and relative humidity around 50 to 60 percent.
- Packaged pool dehumidifiers recover the latent heat of evaporation to heat air or water.
- Low-level exhaust near the water captures chloramines where swimmers breathe.
- Maintain negative pressure and a continuous air and vapour barrier to protect the envelope.
- Specify corrosion-resistant materials throughout the pool hall.
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