Proper dehumidification of indoor pools

Fff 808 AqIndoor pool environments are humid places by design, but, in the opinion of Dectron's James Hogan, far too many suffer from excessive humidity, which manifests itself in the form of foggy windows, premature degradation or outright rotting of building materials, and just plain stinky and steamy air.

Homeowners who've built these indoor pools tend to get a little steamed themselves when their natatoriums (just a fancy word for indoor pools) get this way, and it's usually the builder that bears the brunt of that frustration.

"Pool builders typically aren't the reason for dysfunctional indoor residential pools, but they're usually the ones that get blamed," says Hogan, a product development engineer and CPO in Dectron International's Roswell, Ga., office.

Builders without a lot of familiarity with indoor pools might deserve part of the blame, but there's usually blame enough to share with the architects, mechanical contractors and the homeowners themselves when projects go awry.

"These excessive-humidity problems usually stem from pool construction material choices, building design techniques and/or design/build and installation mistakes," says Hogan. "Therefore it's in the pool builder's best interest to understand the four basic indoor pool principles - architecture, air distribution, mechanical equipment and water chemistry.

"These principles work synergistically in an indoor pool environment. A deficiency in one part affects all the other parts and thus the entire project. The pool builders should keep a watchful eye on each indoor pool project, from the design and construction phase through culmination."

Architecture And Construction

Tim O'Neil is the operations manager and construction engineer for Downes Swimming Pool Co., in the Chicago suburb of Arlington Heights. He and his crew do an average of two indoor pools per year, always residential and always elaborate. And while his projects don't end up like so many others, he does see problems in many hotel pools he's called in to troubleshoot.

"When I get a call from a hotel or something like that, they just call me out of exasperation," he explains. "I come out and I look, and the amount of mis-engineering and installation errors and cutting of corners for these indoor installations just astounds me.

"The pool is usually a toxic, humid, molded, rotted mess. That's because rules weren't followed when it was built and corners were cut because they were worried about putting a $100,000 gold baluster in front of the building instead of spending the extra 10 grand in the pool room."

These problems arise from mechanical engineers and architects who aren't familiar with building indoor pools as often as they do from willful negligence, according to Keith Coursin, president of Desert Aire, a manufacturer of dehumidification systems in Germantown, Wis.

"One of the things that I've seen is that so many of these architects and mechanical engineers design for regular living spaces," he explains. "Everybody thinks about normal environments, and they forget that the aquatic environment is year-round at a nominal 85 degrees and in the 60 degree dew points.

"I always try to paint the picture for most people that an indoor pool is like your worst dog days of August. It's hot and humid. So when you think about it in those terms, that's your inside condition, what are you going to do differently about your choice of materials?"

Vapor barriers are an area where poor choices are often made. Sometimes they're installed incorrectly; sometimes they're not installed at all. The vapor barrier, which can be a plastic sheet, foil-backed or extruded plastic sheet, or a vapor-retardant paint coating, should envelop the entire pool room and is critical in preventing condensation which can rot wood, corrode metal and result in concrete spalling and mold and mildew problems.

"You want a very heavy polyethylene sheeting. You want the heaviest mil you can get in a plastic," says O'Neil. "And the application of that vapor barrier is also very important. If you're just hitting it with a staple gun, you're putting all kinds of little holes in it. What you want to use is something like a hand-applied roofing nail so you're not punching the staples right through the plastic.

"Good, thick polyethylene sheeting isn't inexpensive, but in hindsight, it sure becomes that way, doesn't it?"

As for the material that covers the vapor barrier, O'Neil is partial to cedar, which is resistant to water to begin with, and, when applied over a good vapor barrier, eliminates a lot of issues with degradation caused by humidity. Not all projects are specified this way, however.

"We've seen people using standard drywall," Coursin says. "You've got to use greenboard, which contains an oil-based additive, if you're going to use drywall. It's got to withstand that moisture-laden environment."


Most pool rooms will have some windows, and it's important to choose ones that will hold up to the severe environment of a natatorium, according to Coursin.

"I wish I had a buck for every single-paned window I've seen used on an indoor pool," he says. "Your dew point is going to be reached, and you are going to have condensation. I've been to job sites where the condensation has gotten to the wood frames, which weren't sealed by the way, and they've literally pushed the glass together and shattered it."

Windows should be dual or triple paned to cut down on condensation. Choose a less-expensive window, and you'll need to pay closer attention to air distribution in the room.

Moving Air

The second critical factor in a well-designed and properly built indoor pool environment is air distribution, a job handled by large dehumidification systems. These systems work a lot like super-charged air conditioners, except that they discharge warm, dry air instead of chilled air and are designed to handle much larger humidity loads.

But these systems in themselves aren't going to eliminate poor air quality and damage to the physical environment surrounding the pool if the dehumidified air isn't directed where it should be.

"That, to me, is really the key," says Coursin. "Now, the first and most obvious place that needs warm, dry air, which is what comes off the dehumidifier, is the windows. It is, by far, the weakest link of any construction material that you can put in a pool room.

"So you have to design your distribution network to flush the outside windows. Now, if you're talking about a building that's all glass, well, that's a difficult job. If you talk about only one wall, OK, you can design the duct work pretty readily to flush that one wall of windows."

Getting the air where it needs to be is especially important in colder climates, where the exterior temperature can bring windows and exterior walls down below the dew point in the room, usually between 60 and 70 degrees Fahrenheit, unless warm dry air is spread evenly across them. Rooms with high humidity have high dew points, making the moisture in the air condense into water. Keeping surfaces warm helps prevent this from happening, according to Hogan.

"Diffusers should be arranged to direct air at a sharp angle to the surfaces, and you need them close to the surfaces," he explains. "If you shoot the dehumidified air directly against the walls or windows, it just bounces back and doesn't work. If the fluid flow is at a sharp angle, it'll kind of stick to the surface.

"The air distribution should produce 10 to 50 feet per minute of airflow across the pool surface, but should not blow air directly onto the pool surface. That's counter-productive and causes evaporation."

The other side of the airflow equation is the return air, and there are a number of different ways to situate the ductwork.

"For many years, the standard practice said, 'Hot, humid air rises, therefore I want my return air up at the ceiling level,'" Coursin explains. "But there's a new discussion going on within the engineering community that says a portion of the return air should be done down lower by the pool itself to help bring the air over the pool to remove the off-gassed chemicals."

As is the case with other aspects of indoor construction, ductwork is often given too little thought. And sometimes, this thoughtlessness leads to major problems.

O'Neil says he's seen return ducts located near the floor (away from the humid air) and even diffusers located near the ceilings, blowing air directly at the surface of the pool instead of at the exterior walls.

"That's pretty stupid," he says. "So they're taking dehumidified air, which is dry, and blowing it at the surface of the pool. What do you think is going to happen? It evaporates the water really, really well."

The best airflow designs are planned before construction even begins, Hogan says, using computational fluid dynamics (CFD), a computer modeling system that checks for proper air distribution long before groundbreaking.


All the careful work builders, mechanical engineers and architects do on material selection, fluid dynamic evaluation and careful duct placement isn't going to head off potential problems if the right dehumidification system isn't chosen.

According to Hogan, you've got to consider the size of the room, the outdoor temperature, the materials in the room. But that's not enough. You've also got to know how the homeowners plan to use the room. Where will they keep the air temperature? What about the water? These two factors are critical in calculating the size and strength of the dehumidifier that's needed, Hogan says, and once those decisions are made, there's little room for making changes.

"Dehumidifier sizing is very dependent on the homeowners' projected use of the pool," Hogan says. "A common design includes a two-degree temperature differential between the air and water - 84-degree air, 82-degree water. Homeowner preferences that deviate from this standard - 75-degree air and 90-degree water, for example - will significantly affect evaporation rates and dehumidifier sizing calculations. But they're easily accommodated at the design stage.

"But the homeowner should know that air and pool water temperature set point are locked together after the equipment is ordered."

This tight correlation between equipment size, customer use and material selection underscores the need to get manufacturers and local reps involved in the early planning stages.

Chemistry Matters

The final piece of the indoor pool puzzle is pool water chemistry. It's the simplest piece of the puzzle, but like all the other elements, neglect to pay it proper attention and the entire project will suffer.

Hogan stresses the importance of keeping pool chemicals out of the mechanical room (a common sin) or in an unventilated closet to prevent damage to equipment.

"Whether the builder, the owner or others perform aftermarket pool water testing, a DPD or FAS-DPD pool water test kit should be used in order to determine the combined chlorine levels," Hogan says. "The pool builder should set up a maintenance schedule that includes water chemistry as well as a check for the mechanical system's vital signs such as filters, belts and airflow.

"Not everyone is a CPO. It's not hard, but you do need to pay attention to the chemistry. So compliance with national standards and the CPO books is the best way to get long life out of the mechanicals in the building.

"An indoor pool's building envelope, mechanical equipment, water chemistry and air distribution all work together to provide a properly functioning and healthy environment for the user."

The synergistic relationship among these elements requires strict attention to detail, careful planning at the initial stages and the expert advice of a professional dehumidification equipment manufacturer. There are, of course, easier, faster and cheaper ways of putting in an indoor pool, but builders like O'Neil recommend against them.

"Don't be cheap when you're putting an indoor pool in. If you're going to cut corners, don't put it in," he says. "A cheap pool is a bad pool. It really is. And the cost between doing things right and doing them wrong is probably 10 percent.

"It's more important to pay attention to what you're doing than it is to spend an exorbitant amount of cash."

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