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Solution of the Month

Taking the Plunge: Sustainable Indoor Pools

February 2011

By David Sokol

In the dead of winter, an indoor dip may seem like an unnecessary luxury—imagine the energy required to heat a pool area to permit stripping to swimsuits, and to dehumidify the natatorium. Yet, rather than eliminate the activity, architects and engineers are applying sustainable techniques to indoor pools. Here are two projects notable for their very different sets of strategies.

Roseville Aquatics Center
Photo courtesy Rainforth Grau Architects
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Into the Blue: Roseville Aquatics Center

To provide a natatorium for their growing population, city officials in Roseville, California, commissioned Rainforth Grau Architects and SEED Inc. to construct its indoor-outdoor proposal—a greenhouse-like natatorium topped by a 21,000-square-foot retractable roof. Completed in late October 2009, the Roseville Aquatics Center’s roof closes when heating is required; for cooling, it is opened to initiate a stack effect, with fresh air entering via side openings. Since the glass-and-polycarbonate building can function like a greenhouse at a minimum temperature of 45 degrees Fahrenheit, the retractable roof alone eliminates cooling and dehumidification loads for the greater part of the year in this dry-weather satellite of Sacramento. 

For those rare times when the center’s heating and dehumidification needs require a mechanical boost, the design team had to accommodate strict California energy codes. To do so it mounted 90 solar panels (comprising 2,700 evacuated tubes) to the building and surrounding property. According to SEED president Stephen Witek, P.E., it is the largest solar thermal water dehumidification system in North America. 

The heat output has three destinations. During winter, while the roof is kept closed, it heats incoming fresh air that dehumidifies the interior. On humid summer days, the heat is diverted to reduce pool boiler loads—since, at these times, the pool is losing heat to both the ground and atmosphere. And regardless of season, there’s hot water for domestic use: Water is drawn through a heat exchanger that transfers heat from the solar heating loop to risers servicing sinks and showers.

This trio of uses also deserves notice, Witek says. “The integration of the solar thermal system into the building’s heating and boiler systems, and its ability to utilize that heat in domestic water and pool water and other areas of the building, is unprecedented. Additionally, only this design can utilize heat for dehumidification via fresh air instead of compressors, because the retractable roof allows us to eliminate system use in summer. The integration of architecture with engineered systems is rare.”

Crossing the Pond: Cook County Residence

For a 6,000-square-foot residential natatorium, St. Charles, Illinois–based consulting engineer J.M. Associates, and Jim Bates, vice president of Batavia, Illinois–based State Automatic Heating & Cooling, decided to turn the decorative into functional. In the upscale property’s 3.8-acre pond, the team submerged four cages of closed-loop heat transfer piping in order to heat and cool the home interior—which includes conditioning and dehumidifying the indoor pool.

The PVC piping is connected to one closed-loop delivery system, which itself links with a mechanical HVAC dehumidifier. Heat recovery from the dehumidifier warms the pool water; under those perfect conditions when the dehumidifier is not required to dry the natatorium air, then a 50,000-Btu pump kicks in the pond-fed heat exchange to make up for the heat recovery.  

Orchestrating systems, explains Harry Topikian, vice president of business development of the dehumidifier’s manufacturer Dectron Internationale, means that the dehumidifier does not require a condensing coil; it is using the geothermal loop for heat rejection. “In addition to pool-water heating, space heating and cooling is relatively free from heat recovery,” he adds. “Heat that would have been handled by those compressors also heats the interior of the natatorium.” The dehumidifier still includes an evaporator coil to condense natatorium air.

A 120,000-Btu, duct-mounted coil does heat the natatorium interior on the coldest winter days; the coil is supplied by the larger house’s gas-fired boiler and operates with water at temperatures above 120 degrees Fahrehenheit, well beyond the upper range of a geothermal system. Air distribution from the dehumidifier is supplied to an 18-inch-diameter fiberglass reinforced plastic perimeter duct trunk line under the pool deck. Extra indoor air quality is built into the air distribution, but it retains the negative pressure required to eliminate pool odor infiltration. And the automatic pool safety cover assists the process, because it hampers the pool’s evaporative rates and heat loss.

Overall this solution maintains 50 percent relative humidity, which ensures comfort while swimmers towel off. It also prevents the mold and potential structural compromise of excessive moisture. An audit estimates that annual heating and cooling costs will be $8,400 versus $15,000 for conventional systems. In terms of capital investment, the pond-based closed loop is cheaper to install than an in-ground geothermal system, and water-to-water heat transfer achieves payback more quickly than less efficient earth-to-water heat transfer.

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