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Pools: Swimming in the Details

Natatoriums can be a source of great enjoyment for swimmers, but also a source of frustration and high levels of maintenance for Owners, if not designed and constructed properly. Many factors must be considered when designing a comfortable pool environment such as humidity control, air flow distribution, and material selection.

Air with a high level of entrained moisture will negatively affect comfort and, in extreme circumstances, the health of the occupants. Additionally, the moisture in the air will condense on cooler surfaces such as walls, exterior doors, and windows when surface temperatures drop below the dew point. This condensation can increase maintenance costs and accelerate the deterioration of building materials. A dehumidification system controls the moisture within the natatorium space with the highest level of success when compared to other means of ventilation without active cooling. Most modern dehumidification systems maintain comfort by monitoring the temperature and humidity of the pool environment, the outside air conditions, and also the pool water temperature. To ensure comfort, the pool ambient air temperature should be maintained at two degrees Fahrenheit above the pool water temperature.

“The design of the air distribution system is critical to maintain proper comfort levels in the pool environment.”

The design of the air distribution system is critical to maintain proper comfort levels in the pool environment. Good design practice dictates that 4 to 8 air changes per hour should be implemented depending on the type of pool and number of spectators predicted. Above grade supply and return ductwork is the most common and economical means of air distribution. The supply air diffusers should be positioned in such a manner that an even blanket of air is forced over cold exterior surfaces, such as windows, sky lights, and exterior doors. The majority of the total supply air, approximately 80%, should be distributed at the exterior walls with the remaining 20% supplied near the ceiling to prevent stratification and stagnation of the air. Supply air should never be directed over the surface of the water as this causes drafts felt by the swimmers. Airflow over the pool also increases the evaporation rate of the water limiting the effectiveness of the dehumidification system. The natatorium should be maintained at a slightly negative pressure to prevent moisture and chloramine odors from migrating to other areas of the building. Due to the corrosive nature of the pool environment all ductwork should be constructed of aluminum or stainless steel. Galvanized coated steel can be used, but must be painted, which introduces a point of maintenance.

Care should also be taken during design to eliminate the possibility of short cycling the supply air. This phenomenon is caused by having the return grilles and supply diffusers placed too close together so the dry supply air recycles prematurely. The return air inlet should be placed far from the supply to ensure that all of the warm, moist air is returned to the unit without leaving areas of stagnation within the natatorium. Multiple points of return are effective, though good air circulation can be achieved with a single well-placed return grille. Due to the natural tendency of warm, moist air to be buoyant and rise, the return grille should be located within 10-15 feet from the floor level.

With the proper design of mechanical dehumidification units and ductwork distribution systems, natatoriums will be a source of enjoyment for swimmers, Owners, and Maintenance Staff alike.

– Mark J. Harrington, PE