Feb. 1, 2012
IDECVAV System In the December 2011 Sounding Board (, you published my letter concerning the October 2011 article Evaporative


In the December 2011 Sounding Board (, you published my letter concerning the October 2011 article “Evaporative Pool Dehumidification” ( After reading the author's response, also published in the December 2011 Sounding Board, I remain doubtful that an effective air-distribution system can be provided for a pool space using the IDECVAV concept.

The air-distribution system in a pool must wipe exterior perimeter surfaces to prevent condensation, distribute fresh air throughout the space, wipe the chloramine layer immediately above the pool surface, and prevent stratification and dead spots. 2011 ASHRAE Handbook — HVAC Applications makes a general recommendation of six air changes of total air movement and modifies it if mechanical dehumidification is used. Four air changes is considered a practical minimum for effective air distribution, with eight air changes on the high end. There is a cost tradeoff between fan operating cost and duct cost vs. effective air distribution. The Handbook is not a code, but the recommendation of four to eight air changes for total air movement has proven to be practical. The IDECVAV system often will provide only code-minimum outside air for total supply air (usually, only one air change), and design of effective air distribution to accomplish all requirements seems impossible.

The author implies that −25°F air is the only condition that would result in the system providing code-minimum airflow. After modeling many hundreds of pools, I have seen the code minimum will dehumidify a typical pool space to an outside dew point up to around 40°F (typical range: 35°F to 45°F), not only at winter design temperature. Using TMY3 weather data, this corresponds to 5,075 hr, or 6.95 months, in the Marathon, Wis., area. The code minimum has enough dehumidification capacity to drive the space relative humidity below setpoint when the dew point is below 40°F (often desirable to prevent condensation on exterior surfaces on extremely cold days).

I disagree with the author's statement that, “Directing air at a pool surface is poor design.” The 2007 version of the Handbook recommends in both the ventilation and duct-design sections that some air be directed across a pool surface. The 2011 version adds a third statement in the duct-design section. Recognition by natatorium designers that air needs to be directed across a pool surface to wipe the chloramine layer began in the late 1990s and has increased through the CDC (Centers for Disease Control and Prevention), industry trade magazines, swimming organizations, and ASHRAE. The fear of increasing evaporation rates from air movement over a pool surface justifiably has been superseded by swimmer health concerns.

The author agrees the IDECVAV system cannot control space humidity for all summer conditions, but states that, “The operational periods when humidity cannot be controlled effectively are rare and very intermittent, measured in hours per year, rather than months at a time.” These hours occur when providing 100-percent outdoor air cannot provide enough dehumidification. Using TMY3 data for Marathon, the number of hours will vary from about 500 to 800, depending on total supply-air volume and pool temperature and humidity. This is a theoretical calculation based on an assumed evaporation rate and historical weather data. My original point was that for a valid comparison of the IDECVAV system and a system that could control humidity 100 percent of the time to be made, it should have been stated that only one of the systems could control humidity for an entire year. Five hundred to 800 hr of a loss of humidity control might be acceptable for some owners, depending on their summer pool usage. The same calculation yields 1,300 to 1,600 hr in Indianapolis and 2,200 to 2,500 hr in Nashville, which would make use of the IDECVAV system unlikely, unless there was no summer operation.

Gary Lochner Unison Comfort Technologies Minneapolis, Minn.


The following was submitted to in response to Editorial Director Michael Weil's November 2011 column, “Global Warming: A Lot of Hot Air?” (Weil I'm Thinking of It …,

“I hope that global warming is just a lot of hot air, as Mr. Weil suggests in his column. But when I hear that the scientific consensus is that Earth's climate is warming, humans are causing most of that change, and no scientific body has maintained a dissenting opinion, I don't shrug that off. I believe the opinions of leading scientific bodies and peer-reviewed publications over random articles and Websites.

“And those opinions match my gut-level opinion: With more than 7 billion people now on Earth demanding higher and higher standards of living, how can we not be making a measurable impact on the climate?

“I see climate change as neither a business opportunity nor a bunch of hot air. I see it as an issue that is believable and that we need to address head-on. Our industry can address it by pushing toward more advanced HVAC technologies and, dare I say it, making buildings with no mechanical cooling, heating, or ventilating the norm, rather than the exception.”


Letters on HPAC Engineering editorial content and issues affecting the HVACR industry are welcome. Please address them to Scott Arnold, executive editor, at [email protected].

About the Author

Scott Arnold | Executive Editor

Described by a colleague as "a cyborg ... requir(ing) virtually no sleep, no time off, and bland nourishment that can be consumed while at his desk" who was sent "back from the future not to terminate anyone, but with the prime directive 'to edit dry technical copy' in order to save the world at a later date," Scott Arnold joined the editorial staff of HPAC Engineering in 1999. Prior to that, he worked as an editor for daily newspapers and a specialty-publications company. He has a bachelor's degree in journalism from Kent State University.