Lifting The Fog

By Randy Gaddo

Natatorium—it’s not a word that rolls easily off the tongue. It rarely comes up in daily usage; people generally don’t say, “Hey, let’s go to the natatorium.” They just say, “Let’s go to the pool.”

A natatorium exists in many different configurations. An indoor pool in a facility that also features other associated spaces, such as basketball courts, programming rooms, or gymnastics facilities, can be called a natatorium. There are also indoor, enclosed places that cater primarily to aquatic activities, such as recreational swimming, team swimming, water aerobics, therapeutic pools, spray grounds, water slides, or a combination of any or all of these. A natatorium can also have associated support rooms, such as lavatories, offices, and locker rooms.

Regardless of the form a natatorium takes, there is one specific system that, if it isn’t engineered, designed, installed, used, and maintained properly, can be a major operational and maintenance nightmare—the heating, ventilation, and air-conditioning (HVAC) system.

Strictly speaking, the HVAC system in a natatorium is really a specialized humidity-control system—so notes Paul Stewart, director of sales, marketing, and service at a company that manufactures equipment specifically designed for humidity control. A standard HVAC helps control indoor temperature, and humidity to a limited degree; however, when it comes to the indoor-pool environment, “It’s not a temperature-control issue, it’s a moisture-load issue, and a standard HVAC unit doesn’t do well with moisture-removal requirements,” says Stewart. “I tell people it’s like bringing a knife to a gunfight.”

System Stressors
“The many ways people use buildings with enclosed pools for recreational, competitive, and health purposes continues to evolve,” notes a helpful pamphlet titled, “A Guide To An Integrated HVAC System Design for the 21st Century Natatorium.” The guide is available for download at  The pamphlet, published by Stewart’s company, provides owners, mechanical contractors, and engineers with helpful tips to design or retrofit an indoor-pool facility.

The guide discusses the myriad factors to consider when designing a complex HVAC system. “Enclosing a pool creates a different environment and requires different equipment than just a standard HVAC system,” says Stewart, who works with a wide variety of customers, including parks and rec, commercial and hotel, residential, schools, and more. He has seen the best, and the worst, of issues.

“The worst issues I’ve seen is when either a dehumidifier system is inoperable and no one has taken the time to get it repaired for 6 or 8 months, or when an owner has placed a standard HVAC unit in the pool-equipment room instead of a dehumidifying system,” he says.

Perhaps the most misunderstood issue of the indoor-pool environment, by owners and people in general, is the impact that any change of temperature has on the capacity of the equipment intended to control temperature and humidity. In the parks and rec field, this can happen when a standard pool is “upgraded” to include new water features, like spray guns, dump buckets, or water slides. These features will increase the water surface areas exposed to the air, which will raise the evaporation rate, and that needs to be accounted for, Stewart explains.              

“We see this a lot in the recreation environment where they add these water features, and all of a sudden their dehumidifier doesn’t keep up, and they wonder what happened,” he says. “What happened is you increased the load, and the unit is working as hard as it always had, but you just don’t have the capacity to handle it. It’s like asking a (race) car to go 150, and you have it floored but the speedometer only goes to 120 … the car just doesn’t have the capacity to do what you’re asking.”

Another example of a common problem is changing the water or air temperature to accommodate customer requests or complaints. For example, if a dehumidifying system is designed for 82-degree water temperature and 84-degree air temperature and someone complains that the water temp isn’t warm enough, this may lead a staff member to raise the water temp to appease the customer. Then someone complains that the air is too hot, so staff lowers the air temp. The increased difference between the cooler air and warmer water is what leads to excessive condensation.              

“Now you have a water temperature of 84 and an air temperature of 78, and you’ve more than doubled the evaporation rate of the body of water, and the equipment can’t keep up,” Stewart says. “Is that the equipment’s fault, is it the owner’s fault? It’s really nobody’s fault; it’s an unintended consequence of changing the design parameters, which can have a significant impact in the pool environment.”                    


Excessive humidity leads to a host of issues, from rain falling inside the structure due to condensation forming on the ceiling to fog forming as water evaporates when air temps are cool and the water is warm. “It’s like you see on a farm pond that first fall day when the air gets cool, but the pond water is still warm. Mist will form just over the pond surface, and the same thing happens inside a pool structure. The equipment wasn’t balanced to put out enough heat to keep the air temperature close enough to the water temperature to prevent excessive evaporation,” Stewart explains.

Compromising Structural Integrity
But the above are minor inconveniences compared to some of the more serious issues caused by the moisture.  Mold and mildew can be caused by excessive moisture. These are serious issues that can impact the health and safety of users and staff members; however, there are even more serious issues that can have a major impact on the structural integrity of the facility itself.              

“Every few years you hear about an indoor-pool facility where the roof falls in, such as the one in Russia a few years ago,” says Stewart. There were actually two such incidents in Russia in recent history: one in February 2004 when 28 people were killed and more than 100 injured after a glass roof snapped due to the sharp contrast between inside and outside temperatures, made worse by a heavy snow load; another occurred in December 2005 when a concrete and metal roof collapsed, killing 14 and injuring about a dozen more.              

Excessive moisture can lead to structural decay in the form of rotting wood and rusting steel that can, over time, become stressed to the point of collapse. “This can be exacerbated by the naturally acidic air in a natatorium, but uncontrolled condensation is the leading factor,” says Stewart.               

He further notes that there are several factors in determining how to control humidity in a pool enclosure. Air distribution is a key one. “You can have the best-sized dehumidifier on the face of the earth, but if the system’s diffusers are pointing inward instead of outward onto the cold surfaces, condensation may form on cold days,” he says.               

The type of facility being controlled also factors into the equation; a hotel pool will have different requirements than, for example, a recreational pool. Exhaust air management also impacts system effectiveness. “In our pamphlet we talk about source capture for controlling (chemical) odor,” says Stewart. This means grabbing the heavy chemical gases, which can be four times heavier than air, meaning they’ll stay low to the floor unless stirred up and eliminated.              

Owners don’t often think of vapor barriers in the same breath as natatoriums. A vapor barrier is a layer of material, such as plastic film or foil, used to protect installed insulation and structural surfaces by retarding the transmission of moisture from the interior or exterior environment.              

In the case of a natatorium, Stewart says, the vapor barrier should be placed directly behind the sheetrock. “This keeps moisture from seeping through sheetrock and settling into structural wood,” he says. If the sub-structure is wood, mold, mildew, and eventually structural damage can occur. If it is a steel sub-structure, even stainless steel, it can corrode and eventually compromise structural integrity.

Anticipatory Planning
However, problems caused by humidity at natatoriums are latent, present but not readily visible, unless there is a conscious effort to find them; and even if owners are doing their best, factors they aren’t aware of may be impacting the operation. “A natatorium can have a properly sized dehumidifier, but if the air isn’t being properly distributed, then it can’t help prevent condensation,” explains Stewart.              

He cites a client at a new high school natatorium that had windows on the outside walls but didn’t have an air-distribution system directed at them.               

“I told the contractor that those windows would condense in the winter, and he told me, well, that’s what the dehumidifier is for,” Stewart recalls. “I told him that if you don’t distribute that warm air onto the cold surfaces, condensation will occur; that’s just physics,” he says, noting that those cold surfaces can be walls, windows, or even skylights. “Architects love skylights because it’s free lighting, but if you don’t direct warm air onto them, you will end up with rain coming down when the conditions are right.”

Part of Stewart’s job is to coach engineers working on natatoriums. Most facility owners hire an engineer for a new or retrofit natatorium project. However, unless an engineering firm specializes in natatoriums, most engineers may not be up-to-speed on them. “I do more natatoriums in a week than most engineers do in a year,” Stewart says--no brag, just fact. As an engineer, he understands the physics and walks the walk, so he can help avoid mistakes.

Retrofitting an older facility into an enclosed natatorium can present a whole host of challenges. It is much easier to properly engineer and build a humidity-control system from scratch. For starters, getting old equipment out and fitting new and often larger equipment into the existing space can be a major issue.

Another challenge, Stewart says, is that sometimes shortcomings from a previous design simply cannot be addressed. For example, if the duct work in the wall is sized for 20,000-cubic-feet per minute (CFM) and through evaluation during the engineering phase of a project it is determined that 25,000 or 26,000 CFM are required, the existing duct work may not be able to handle the flow. “So now you’re faced with the decision—do we work with the cards we were dealt, use existing duct work and hope for the best, or do we figure out how we are going to increase the duct work?” says Stewart.              

Purging For Air Quality
The citizens of the Fraser Valley Metropolitan Recreation District in Colorado dealt some good cards to the parks department about a decade ago. That’s when residents passed a bond that mandated the department to build four community-enhancement projects. One of them was the Natatorium at Grand Park Community Recreation Center in Winter Park.              

Scott Ladean, facility manager for the natatorium, has been with the park district since the mid-1990s. “So I was lucky enough to be here for the development of the specifications and preparation for the bond issue and for the passing of the bond,” he says. “It’s coming up on eight years that we’ve been in operation, and things have been pretty good.”              

To control humidity in the 12,000-square-foot natatorium, there is a 50-ton, custom-built, commercial rooftop unit.  To assist human control of the complex system, an additional air-handling system uses outside air to help dehumidify the indoor space. This system naturally dehumidifies indoor spaces with 20- to 50-percent outdoor air, greatly reduces mechanical dehumidification loads, and improves air quality in the space by exhausting greater amounts of chemical odors.              

“Our dry Colorado climate makes using outside air very effective when controlling humidity and improving air quality, along with minimizing corrosion by removing greater amounts of chloramines from the space,” says Ladean. Chloramines are colorless, unstable chemicals with a pungent odor, often present in the high-chlorine environment of a natatorium.              

Ladean says the natatorium schedules four or five air purges during each 24-hour cycle to ensure that all noxious fumes are being eliminated. Additionally, if management knows there will be a greater bather load due to special events, additional purges can be programmed as needed.              

Associated support spaces, such as restrooms, show rooms, offices, activity spaces, and the entrance foyer, are heated and cooled by a separate rooftop HVAC system.              

The natatorium’s humidity-control system also features an automated building-management component. “The system is designed to monitor and control our building for optimum indoor air quality and comfort,” says Ladean. “It allows us to visually monitor and control building status from our computers, and if we are experiencing humidity- or chemical-related issues, we can use the system to purge the air at the push of a button.” The system can also be controlled from the operator display on the central building-control unit.              

Ladean explains that management was fortunate to have a facilities maintenance manager who was very familiar with the brand-named HVAC systems, and was able to stay on top of preventive maintenance. There is also a service contract with a local HVAC company to do twice-yearly, preventive-maintenance servicing.  In addition, the equipment manufacturer provides support and training as needed.              

The total system in the Grand Park natatorium was chosen during the design phase of the project to get the bond approved.  The design was agreed upon after input from the architect, the mechanical engineer, the pool builder, the project-management firm, and the building-maintenance manager. Having all of these technical shareholders at the table during that phase ensured that the right system was selected.              

The care in choosing the system and the depth of maintenance support paid off recently when the natatorium’s experienced facilities manager left for a new job. As of this writing (August), Ladean was in the process of sending out announcements for the position; in the meantime, the facility’s humidity-control equipment—instead of being “left up in the air”—is in good hands and being safely maintained by staff and consultants as needed.                              

Randy Gaddo, a retired Marine who also served for 15 years in municipal parks and recreation, is now a full-time photojournalist who lives in Bay Minette, Ala.; he can be reached at (678) 350-8642 or email