Anyone that has smelled chlorine odors in a hotel lobby from the 10th-floor indoor pool or has seen competitive swimmers using inhalers at a competition swim meet knows that pool environments aren’t always designed properly.

Designing indoor pools or “natatoriums” properly is one of the more challenging tasks in the HVAC consulting engineering industry. Mastering natatorium design requires a full understanding of why these facilities are so different mechanically than others. Unfortunately, most mechanical engineers get few opportunities to design an indoor pool during their career, so acquiring knowledge beforehand is especially critical.

A successful natatorium environment with good indoor air quality (IAQ) is a synergy of four principles: 1) proper building materials and building envelope design; 2) effective air distribution; 3) the right mechanical HVAC equipment; and 4) pool water chemistry. The latter discipline is actually more of a facility operations and maintenance issue than design concern.

Design

The building envelope is mostly architecture-related; however, the consulting engineer should collaborate with the project architect on the choice of building materials and assure certain installation techniques are applied properly. For example, an excessive use of glass in the design will challenge the engineer’s ventilation design to prevent condensation from forming on it, especially in colder climates. Another architectural precaution is assuring there is a vapor barrier that envelopes the entire building. The contractor must install it without tears or broken seams, which can allow moisture exposure to attack the building’s structural materials. Those are just two of many architecture-related precautions in natatorium building envelope design.

Distribution

Effective air distribution is equally important, because conditioned air must be distributed down to the breathing zone at deck level and pool surface level. Ideally, ductwork must be positioned approximately one foot away from exterior walls and windows to assure proper coverage that will prevent condensation. Chloramines, which is a toxic gas formed by the attachment of human waste and chlorine molecules in the pool water, can’t be totally eliminated in pool ventilation designs, but they can be moved, and their accumulation minimized with proper supply/return air ventilation measures and possibly through the use of source capture devices integrated into the pool gutter.

Equipment

The HVAC equipment, which is typically a DX-style mechanical dehumidifier that can heat, cool and dehumidify the space, needs proper sizing. Knowing the pool’s future activities, such as senior citizen swim periods, or high school swim competitions will help the designer accommodate unique air/water temperatures that determine the equipment sizing. Energy-wise, dehumidifiers can use heat recovery for heating pool water, and energy recovery from energy rich exhaust air to directly pre-heat outdoor air, but those factors must be considered in the design phase and factory built into the equipment.   

Chemistry

Pool water chemistry is typically a facility operations and maintenance issue, rather than a design consideration, but the consulting engineer should always monitor what water sanitization devices are implemented in the design. For example, UV water purification technology or other secondary water sanitation alternatives for reducing chlorine use and subsequently chloramines, could be suggested as part of the pool support equipment.

Historically, uncomfortable and unhealthy pool environments are incorrectly blamed on just the HVAC mechanical equipment. It is rare that there is only one thing contributing to the issues. The true source of the dysfunction is likely a breakdown or design oversight of several of the aforementioned four principles.

Design checklist 

The following is a checklist of design tips and parameters that shouldn’t be overlooked when designing a natatorium environment.

Get operating conditions in writing. Blueprints are part of the design process, but it’s equally important to get the projected pool water temperature, room air temperature, relative humidity (RH), plus the uses for the space on a signed paper from the building owner. The complex psychometrics of a natatorium don’t leave much latitude for equipment that’s not sized for the projected events for the space. For energy conservation, a good rule of thumb is the air temperature should be within 2 F above the water temperature to minimize evaporation. However, occupant comfort is the priority. The natatorium’s environmental control must first satisfy occupants, while minimize operating costs is secondary.

Air changes. Natatorium room volumetric supply air changes should average 4 to 6 per hour. The dehumidifier’s CFM will be determined by the room volume. After determining how much air, then it needs to be distributed down to the breathing zone of occupied space level. Return air grille(s) placement, in relationship to the supply air grilles, should minimize air stratification and short circuiting.   

Outdoor air. Outdoor air measurement as per ASHRAE Standard 62 guidelines has a baseline of 0.48-CFM/square foot for conventional pool water surface areas and a factor for the deck area. Pools with spectators should add 7.5-CFM per spectator during events. Swimmers aren’t considered spectators and they are covered by the baseline calculation. Increasing outdoor air beyond the minimum requirement will significantly raise energy costs in the winter and potentially lower the RH to an uncomfortable level below 50 percent.

Exhaust air. Natatoriums run at a slightly negative pressure of 0.05 to 0.15-w.g., so that humid, chemical-laden air isn’t pushed outside the natatorium into other portions of the building.  Generally, 110 percent outdoor air CFM is recommended. Source capture contaminant devices can figure into the exhaust air as direct exhaust or channeled to the dehumidifier for heat recovery to energy-efficiently condition outdoor air. Source capture contaminant can also take the form of conventional return air grilles above highly active areas, such as a spa.

Load calculation. Calculate the latent load (pools, outdoor air and spectators); sensible cooling load for the space design temperature; and heating load for the space design temperature and outdoor air.

Condensation and vapor migration. Vapor barrier should be specified in the architectural plans for installation on the warm side of the dew point temperature in all walls, ceiling and floors. All exterior windows, doors and skylights should be fully blanketed with supply air of 3 to 5-CFM/square foot.

Sustainability and LEED considerations. Dehumidifiers offer free pool water heating from compressor waste heat. Using exhaust air heat recovery to pre-heat outdoor air will reduce energy substantially. Water conserving condensate reclaim can be returned to the pool, if allowed by local sanitation codes. Using new innovations such as a glycol for heat rejection outdoors instead of refrigerant can reduce the refrigerant charge by as much as 90 percent. Direct drive and ECM fans will also help reduce operating costs compared to belt driven fans.

Swim meet mode. Swim meets are generally infrequent, but they are important for dehumidification sizing and ventilation. This involves calculating additional air changes, such as specifying add-on ventilation ductwork to condition the spectator seating area, or possibly a separate air handling system, even though spectator areas are typically part of the same space containing the pool area.

Service and maintenance. Internet connection is a must. Specification should include today’s state-of-the-art dehumidifier equipment that offers unprecedented internet access, remote monitoring and analyzation through web-based browsers. Factory service professionals can access the equipment for adjustments, recalibration or troubleshooting to keep the unit operating efficiently and maintain the natatorium air comfort via PCs or smartphones.