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Dive into Indoor Pool Dehumidification

April 1, 2006
Many of today's hotel/motel natatoriums aren't functioning properly, due to improper construction methods, poorly maintained mechanical equipment, insufficient airflow patterns, or all three combined


The exterior of this indoor pool wall shows signs of a ripped or missing vapor retarder. Moisture ex-filtrating through the bricks has frozen.

This heat recovery dehumidifier from Dectron Internationale keeps relative humidity at 50%, heats the pool water, and cools the space air.

If you want to make a big splash in an untapped service niche, you may want to consider indoor pool system maintenance. Indoor pool maintenance took on more importance with the introduction in the 1970s of commercial dehumidifiers, which protect the pool's building from moisture deterioration, and provides healthy indoor air quality (IAQ).

Unfortunately, a significant number of today's hotel/motel natatoriums aren't functioning properly, due to improper construction methods, poorly maintained mechanical equipment, insufficient airflow patterns, or all three combined. Consequently, many mechanical contractors are getting natatorium service calls with complaints of too hot and/or humid; visible building deteriorations; significant condensation problems on windows and in other areas; and excessive chemical odors.

For the natatorium to work properly, the building envelope and air distribution system, as well as the mechanical equipment, must all work together. A breakdown of one will lead to the improper operation of the other two. Therefore, service contractors addressing this burgeoning market must have fundamental knowledge of proper building construction techniques, airflow, and even pool water chemistry.

Diagnosing Building Envelope Problems
Many natatorium problems are related to improper uses of vapor retarders, windows, ducts, and other construction features.

One of the pool's most critical construction materials is a vapor retarder, which is typically a 4 ml. plastic sheet that envelopes the walls and ceiling of the entire room. At least half of the problematic natatoriums in the industry have vapor retarder deficiencies, such as rips or poor seals.

When vapor retarders leak, moisture will reach the interiors of walls and condense. If the room air is at 84F with 50% relative humidity (RH), the dewpoint of the air is about 63F. If room air exfiltrates into the wall, moisture will condense anywhere the temperature is below 63°F. A ripped, improperly sealed, misplaced, or missing vapor retarder will in most cases lead to a malfunctioning natatorium, regardless of how efficiently the dehumidifier operates. The moisture inside the walls can lead to wall damage and to biological IAQ problems.

To diagnose this problem without tearing open walls and ceilings, one can use interior clues such as discolored or wet drywall. Exterior clues include deteriorating brickwork, ice formation, or mineral salt deposits on outdoor surfaces.

Some natatoriums have no vapor retarders. Removing an electrical faceplate or viewing from above a dropped ceiling will most likely show if there is any plastic film. Foil-backed and extruded plastic materials may be harder to spot. While they are removed, be sure that electrical faceplates have gaskets to reduce air leakage.

An alternative to plastic, foil, or other materials is a continuous double-coat of high quality paint that has been rated as a vapor retarder. This paint is frequently used for block or cast concrete construction. Visible breaks or openings in the coating might indicate impending trouble.

Natatoriums should exhibit negative pressure. As a rule of thumb, exhausting 10% more air than the make-up airflow will produce a suitably negative pressure. For example, if make-up air is 300cfm, exhaust air should be 330cfm.

Diagnosing Equipment Problems
Even though a building envelope or airflow design is usually the cause of a problem, the mechanical dehumidifier typically takes the blame for all complaints of high humidity, condensation odors, and other natatorium IAQ problems. Most medium and high-end dehumidifiers perform other HVAC functions. Besides dehumidifying the space to an adjustable set point, most dehumidifiers also cool the natatorium as needed.

The best dehumidifiers: 1) dehumidify and return the heat to the pool or space as needed; 2) cool the space as needed; 3) supply and/or control additional space heating as needed. This provides a simple and effective system for the natatorium with only one group of set points and no control overlap.

Using conventional air-conditioning equipment instead of a dehumidifier is perhaps the worst application. These pools will rarely have relative humidity (RH) at the desired 50%, since the air conditioner will be unsuccessful in handling the 20- to 50- lbs. of water that evaporate from the average hotel/motel pool every hour. A conventional air conditioner will have an average air supply at 80% to 90% RH.

Routine maintenance is critical:

  • Change filters monthly
  • Check belts for wear and proper tension
  • Oil motors and blower bearings annually, and check belts for proper tension

The refrigeration circuit maintenance is similar to air conditioners, except it's year-round.

A majority of dehumidifier problems are related to improper installation, and particularly to ductwork that doesn't conform to ANSI/AMCA-201 standards. The standards apply to all air-movement equipment, but are especially important for dehumidifiers. For example, obtainable airflow may be reduced because architects, engineers, and installers sometimes fail to locate an elbow or tee far enough away from the heat exchangers and blowers. Placing a normal duct heater too close to the outlet of a blower will cause the heater to overheat.

The Importance of Return Air
Return air is critical. While larger dehumidifiers may be ordered with properly sized plenum boxes to straighten the airflow, smaller units — such as those commonly used in hotels — may not have boxes. When a plenum box isn't supplied, the return duct should be the same size as the dehumidifier flanges. Any elbow just upstream from the dehumidifier should have airfoil turning vanes and be located no closer to the dehumidifier than 2.5X the effective duct diameter. There must also be at least 3-ft. of straight duct between any elbow/tee and the dehumidifier intake. Deviations from duct installation standards can reduce dehumidifier capacity enough to allow excessive space humidity, or even cause dehumidifier damage.

Circulate Pool Water Through Heating Dehumidifier
Another installation problem is failing to connect the pool water circulation system to a dehumidifier that's designed to heat the pool water. Most natatoriums include a standard gas-fired boiler as a back-up for extremely cold days or pool fill-ups, when the dehumidifier alone might take too long to attain the target water temperature. Not hooking up the pool circulation system to the dehumidifier and using only the back-up boiler is wasting free pool water heating from the dehumidifier.

Skewed temperature set points can cause excessive evaporation that the dehumidifier wasn't sized to handle. Dehumidifier manufacturers require specifics about the pool, structure, and conditions of use in order to select the most economical unit for the application. The lowest cost unit will be designed for room air temperatures that are slightly warmer than the pool water temperature. If the owner changes the set point later to make the room cooler or the water warmer, the increase in evaporation rate may exceed the unit's capacity.

Should the dehumidifier itself malfunction, most modern units have microprocessorbased controllers with diagnostic displays. Since the air distribution can be a problem, a good natatorium service technician should always have a sling psychrometer handy to check any problem areas.

Diagnosing Airflow Problems
The main problem related to airflow, aside from poorly installed ductwork, is poor air distribution to windows, exterior doors, and other possibly cold surfaces. To keep such surfaces dry, it's essential to keep their temperature above the dew point of the air to which they are exposed. Typically, this requires covering them with warm, dry air from the dehumidifier.

Generally, diffusers should be approximately 1-ft. from windows, and deliver 3 to 5 cfm of supply air per sq.ft. of glass. Putting diffusers too far away can cause mixing with the room air before the supply air reaches the windows. For best results, consider linear diffusers. Diffuser angles of 45 degrees or less are preferred to slide the air smoothly over the glass surface versus a 90 degree angle, which can prevent good air coverage. Deep window frames may also block air flow to windows.

When windows stretch from floor to ceiling, smart architects prevent window condensation with under-deck ductwork and linear diffusers running the entire width of the windows, to bathe all window surfaces in warm dry air.

The return grille should usually be mounted from 8- to 15-ft. above the floor. Variances may cause air stratification or insufficient air changes. Return grilles must also be a minimum 15- to 20-ft. from a spa or hot tub to prevent the precipitation of human body oils released by these vessels onto the return grille, ductwork, and the dehumidifier's reheat coil. Precipitated skin oils harden in air into an odorous paste that reduces airflow/heat transfer and can't be washed off.

Many contractors are hesistant to address indoor pool service. Those contractors who have dived into the pool market, however, have found profitability and plenty of work.

The list of companies capable of handling natatorium IAQ problems is not long. The comprehensive natatorium mechanical contractor must be more than the run-of-the mill refrigeration mechanic. Such a contactor can increase profits and differentiate his business from his competitors by becoming better versed in all facets of natatorium construction, pool water chemistry, and mechanical dehumidification.


Poor pool chemical storage practices cause highly corrosive chemicals to pass into the air. Buckets and tanks are known to still leak significant quantities of chemicals into the air even when closed. That leakage can destroy all metal equipment exposed to them, and can irritate eyes, throats, and lungs.

Pool chemicals should not be stored in mechanical rooms. They will corrode the dehumidifier. If chemicals corrode the flue of a boiler, furnace or other combustion device, there is a risk of carbon monoxide poisoning. (See ANSI Z223.1 National Fuel Gas Code, and contact the manufacturer of the combustion device for recommendations.)

James Hogan is product development engineer for Dectron Internationale, an indoor air quality equipment manufacturer based in Roswell, GA. He can be reached at 800/676-2566.