In the Zeeland West High School natatorium the primary dehumidifier is designed to handle the pool area’s daily tasks of general dehumidification. On event days, a separate dehumidifier is designed to supply spot cooling and eight air changes per hour to the grandstand area.

While complicated indoor waterparks can definitely present humidity challenges (see the cover story "Contractors Ride the Waves at Waterparks" in this issue), natatoriums are not without their HVAC hiccups.

For lack of a better definition, natatoriums are somewhat watered-down versions of indoor waterparks. They are indoor pools - could be large; could be small - minus all of the wave machines, slides, and other water-related entertainment and equipment.

Because natatoriums are not cookie-cutter in construction, it varies as to how a design team provides comfortable temperatures while controlling humidity in each enclosed facility.

For instance, Environmental Systems Design (ESD), based in Chicago, used a different kind of design strategy - implementing both overhead air distribution, as well as hydronic baseboard heating - to supply the correct temperatures inside the natatorium of the $156 million Renaissance Schaumburg (Ill.) Hotel and Convention Center.

In a different twist, a two dehumidification system was put into place to meet the respective temperature needs of both spectators and swimmers inside the $9 million natatorium addition to Zeeland (Mich.) West High School.


James Harrison Jr., a mechanical engineer at GMB Architects+Engineers, located in Holland, Mich., was responsible for designing the HVAC systems for the natatorium addition to the school in Zeeland. In this natatorium, the primary dehumidifier is designed to handle the 15,000-square-foot pool area’s daily tasks of general dehumidification. On event days, which are probably less than 5 percent of the facility’s total usage time, a separate unit is designed to supply spot cooling and eight air changes per hour to the 5,500-square-foot, 500-seat grandstand area.

Admittedly, two dehumidifiers raised dehumidification equipment and installation costs an estimated 35 percent. However, Gord DeRoo, director of facilities and maintenance for Zeeland Public Schools, said air comfort for spectators is remarkably improved over a one dehumidification system.

“One of the main stipulations in the project’s bond issue approval was keeping spectators comfortable because this will be a destination for local and regional swimming competitions,” said DeRoo, who noted that the district’s only other pool - a 1970’s style facility at Countryside Middle School, which utilizes conventional heating and cooling equipment with no dehumidification - is very uncomfortable for both swimmers and spectators.

Students at the $27 million Student Recreation Center’s natatorium at Appalachian State University in Boone, N.C., appreciate the 50-meter pool. Movies are shown on the wall kept blank for movie projection. At this natatorium, the dehumidifiers, custom manufactured by Dectron, not only keep the center at 55 percent relative humidity, but also save energy by providing free pool water heating while simultaneously heating or cooling the space.

Capitalizing on his extensive thermodynamics experience from eight previous natatorium designs, Harrison’s main challenge was providing spectator comfort, but without increasing the room’s overall evaporative rate or using a barrier to segregate the two areas. Consequently, the spectator design set point is the same as the pool: 82°F. However, the smaller dehumidifier - in this case, a Dry-O-Tron DS-202, manufactured by Dectron - is programmed to pre-cool the area down just prior to spectator arrivals.
According to DeRoo, some post-construction experimentation with slightly cooler air after spectator arrival has been successful - without upsetting the enclosure’s dew point temperature.

“We originally considered separating the crowd from the pool with glass,” said DeRoo. “Then there’s no chance of that cooler area raising evaporative rates. However, that idea was abandoned because the design committee didn’t liked their past experiences of sitting in arena or stadium skyboxes that separates spectators from the action.”

While pool and spectator area temperatures are similar, DeRoo pointed out that airflow supply is more proximate to spectators than it is to pool participants, but doesn’t create chilling drafts. Two long runs of linear vents are designed to supply a gentle airflow. Meanwhile, dozens of return air vents near the foot space of the grandstand bench seats are supposed to draw the air downward toward spectators. Thus, as DeRoo explained, the grandstand structure functions as a path for 50 percent of the required return airflow for the unit.

Airflow in the swimming area has return air vents hidden underneath team benches. Meanwhile, the 32-inch-diameter overhead perimeter supply air system is specially positioned to wash all the outside walls and windows with warm, dry air - all designed to eliminate condensation.

Additionally, there are three 40-inch-diameter duct runs above the pool that direct air down toward the water surface. This setup is designed to disperse trapped chloramines away from the swimmers, DeRoo explained.

Rite-Way Plumbing & Heating, of Walker, Mich., needed two boom lifts - one for two installers and one for ductwork hoisting - to complete what Rite-Way project manager Scott Vander Hyde said was “challenging” because of the 50-foot height of the duct installation.

Harrison’s energy-saving specifications, which include a Smart Saver function on the dehumidifiers for exhaust air heat recovery, total a $75,000-per-year and $18,00-per-year for the pool and spectator area savings “versus a standard make-up air unit for the natatorium,” said DeRoo.

The main dehumidifier - a Dry-O-Tron DS-562 - also uses heat recovery to heat the pool water, while also cooling or heating the space on demand. Harrison specified a custom geothermal heat rejection coil in the unit so that in summer mode, 200 to 300 hours of surplus recovered heat is rejected to the school’s central geothermal heat pump loop.

Also specified was a variable-frequency drive, designed to allow the dehumidification system to run at 60 percent capacity during unoccupied hours.

The dehumidification system for the natatorium at the Renaissance Schaumburg (Ill.) Hotel and Convention Center is designed to keep windows free of moisture regardless of outdoor ambient temperatures. It has overhead air distribution, coupled with hydronic baseboard heating, to keep the two exterior walls of window surfaces warm and free of condensation during winter months.


At the Renaissance Hotel and Convention Center, a different dehumidification system was put into place for the 3,000-square-foot natatorium. The main task was to keep the large windows free of moisture, regardless of outdoor ambient temperatures.

Primarily, windows are problematic for enclosed hotel pools, especially in northern climates such as this northwest Chicago suburb, which has a design temperature of -10°. Consequently, condensation and fog have an affinity to cold glazing and can ruin a beautiful view, not to mention the structure itself, with moisture-generating corrosion. ESD solved the problem by turning to overhead air distribution, coupled with hydronic baseboard heating, to keep the two exterior walls of window surfaces warm and free of condensation during winter months.

Typically natatoriums always incorporate overhead duct and sometimes supplement it with the costly method of under-deck air distribution when construction budgets allow. In this case, however, ESD senior vice president Mehdi Jalayerian recommended baseboard heating as a cost-effective solution.

“By introducing convection heat at the base of the glass, it eliminates downdrafts associated with overhead ductwork that typically chill occupants in a swimming environment,” said Jalayerian. “To heat a room of this size, you would normally need 20 to 30 percent more air supply than we have specified, which could be too drafty for occupants when coming from just an overhead source.”

While two boilers supply the baseboard heat, Jalayerian said the majority of the winter space heating temperature of 80° is supplied by the dehumidifier. Not only is heat recovery used for pool water heating, but outdoor supply air is preheated by exhaust heat with the dehumidifier’s Smart Saver option.

According to Jalayerian, additional energy savings is gained in spring and fall months with inherently dry outside air that is brought in with the dehumidifier’s 100 percent outdoor air economizer option. While the entire complex uses an Automated Logic building automation system, the dehumidifier uses Dectron’s Supervisaire®, designed to control and monitor the pool space.

The dehumidifier, a Dry-O-Tron DAS-150 from Dectron, was installed by mechanical engineering contractor Advance Mechanical Systems, from Mt. Prospect, Ill. It is designed to cool the space with its refrigeration cycle. Other hotel areas are cooled via a chilled water loop supplied by two chillers.

While pool odors sometimes travel throughout a hotel, ESD said it eliminated this nuisance with proper airflow techniques and negative pressurization in the natatorium.

One important part of the airflow design is the return air grille, which was strategically placed high on a common wall with the mechanical room to both eliminate air stratification and save the project sheet metal duct labor and installation costs. In this case, the return air grille is located high above the room’s spa. It is designed to handle the significant spa humidity loads and safeguard its circulation throughout the room, explained Jalayerian. Since spas are notorious for depositing body oils on nearby surfaces, Jalayerian noted that a high placement limits the possibility of body oil accumulations from forming on the return air grille and connecting ductwork.

Another design challenge was a small 18-inch high ceiling cavity that was available for aesthetically recessing supply ductwork. ESD’s supply air design uses a center trunk line duct with branches that broadcast air down along the all-important windows with flow-bar diffusers.

Publication Date:08/13/2007