Dehumidifier sizing for indoor/outdoor pools
Besides architectural products that open the enclosure, dehumidifier sizing is especially critical for a pool destined for outdoor use. Luckily, the homeowner’s unconventional plan of using the space in an occasional 25.5-C (78-F) temperature already mandated a significantly larger dehumidifier than what more traditional indoor pool activities would require. Therefore, set-point temperature and humidity recovery is quick after closing the retractable windows on a high humidity or cooler day, simply because the dehumidifier has more capacity.
“The dehumidifier, which was sized to provide eight volumetric air changes per hour, can bring the enclosure’s relative humidity (RH) down to its targeted 50 per cent RH air comfort level within a half-hour of sealing the enclosure after outdoor operation,” said Edward G. Sneed III, a manufacturer’s representative and HVAC manager with Stillwell-Hansen in Edison, N.J., who consulted with engineers and assisted on the project.
Another reason for specifying a considerably larger dehumidifier than what is typically required for residential indoor pools was the homeowner’s unconventional proposed water and space temperatures (32.2 C [90 F] water and 27.7 C [82 F] space). This differential produces higher evaporative rates than the two-degree differential (such as 26.6 C [80 F] water, 27.7 C [82 F] space) recommended in the natatorium chapter of the American Society of Heating, Refrigerating and Air-Conditioning Engineers’ (ASHRAE) Handbook.
The project’s HVAC design is anchored by a 16-ton dehumidifier, which is three to four times larger than a typical residential pool unit. While the manufacturer can produce mechanical dehumidifiers ranging up to 140 tons for large facilities with tremendous humidity loads, such as indoor waterparks, the 16-ton unit was one of the largest dehumidifiers manufactured for a residential application. The 226.5-m3/min (8000-CFM) unit can dehumidify to 50 per cent relative humidity (RH) and heat/cool the space.
The dehumidifier, which typically runs 24-7 to protect the enclosure from the effects of stratified humid air—whether in an occupied or unoccupied partial load mode—has built-in sustainability for reducing energy costs. One way the system does this is by recovering waste heat from compressor operation to provide free pool water heating at a significant energy savings. Additional efficiency comes from its electronically commutated (EC) direct-drive fan motors, which can cut ventilation operating costs by up to 15 per cent versus conventional equipment. The dehumidifier also features a condenser with owl-wing fan blades (one of the lowest decibel-operating fans in the HVAC industry) to maintain quietness near the home’s bedroom windows.
(10 ft) high and nearly encircle the entire pool perimeter.
A typical residential indoor pool requires a smaller 4- to 8-ton dehumidifier and only a single-phase electric service. In this case, however, the volumetric size of the two-storey building and its substantial amount of glass, combined with the significant evaporative rate of a large 6.1- x 12.2-m (20- x 40-ft) pool, and 2.4-m2 (8-sf) spa, mandated a 16-ton dehumidifier, which far surpasses the dehumidifier size of most hotel and health club indoor pools.
Instead of three 6-ton units running on single-phase electric, the local utility made the exception of running three-phase electrical service to the household to handle the HVAC requirements. One large unit saved the homeowner thousands of dollars in installation and future maintenance costs versus three units.
The towering windows afford swimmers open, uncropped views of the 0.6-ha (1.7-acre) property’s esthetic backyard landscaping, which treks upward towards one of the two most prominent ridges of the Watchung Mountain. That said, the substantial amount of windows proved challenging for the project’s engineers because the design goal guarantees condensation-free windows to maintain the views even during sub-zero winter days.
“Architects and building owners love a lot of windows in indoor pools, but engineers don’t because they’re difficult to keep free of condensation during wintertime dew points,” said Sneed, who has assisted engineers on hundreds of residential and commercial indoor pool projects. “However, with the right building materials, HVAC equipment sizing, air distribution, and other proper engineering practices, condensation can be eliminated regardless of the amount of glass.”
The difficulty in keeping glass free of condensation was one reason the homeowner abandoned his original design, which included a 4.5- x 9.1-m (15- x 30-ft) skylight. This is discouraged for indoor pool enclosures because it produces a glare on the water surface.
“We didn’t like the skylight concept, because they’re difficult to keep the glass conditioned and free of condensation,” said Vladimir Ayzenberg, P.E., director of engineering, Jarmel-Kizel, who led the project’s mechanical design team.
