By Bob Baker

Many of you readers may get a laugh out of visualizing me sitting in my office on a typical August day in Tampa, Florida and writing about this subject. As they say, a writer must take his inspiration from wherever it shows up. In this case, the weather gives me an opportunity to address a subject that can affect our customers' satisfaction with our work and thus, our success almost every day. Moisture!

Water is a Problem

Over the past several years, we have begun to understand the source of IAQ problems more and more completely. We have also begun to realize that excessive microbial growth plays an important part in many, if not most, air contamination problems. Furthermore, we have learned that these nasty bugs need moisture in order to grow. "Get rid of water!" has almost become a tribal chant among IAQ professionals. They see the damage that water can cause and advise its elimination.

Consultants provide long lists of "problem water examples". These typically include: roof leaks, leaks around windows and other penetrations of the building envelope, water vapor brought through the building shell by negative inside air pressure, water forced up through the slab or foundation, plumbing leaks, water standing in the drain pan, and water droplets carried over by the air stream from the cooling coil.

The "Other" Water

Another less frequently mentioned source of water is probably as important (if not more so) as those mentioned above. That is the water vapor that is one of the normal components of all air but can become a problem when the physical state of air changes. What a day like today reminds us of is that the water vapor content of air can vary a great deal from day to day and location to location. Think about the last time you were in the mountains on a hot muggy day. Can't remember? There are high humidity days in high altitude areas but they are few and far between. High altitude equals lower air pressure and as the pressure of air become lower, its ability to hold water decreases. Cooler air also holds less water.

We all know that when the barometric pressure falls rain is likely to follow. We also learned in school about the cycle of rainfall. Water evaporates from the seas (high pressure) into the air. Air currents carry that moisture upward until it condenses and falls as rain or snow (not much snow here in Tampa). The relative pressure of air on the surface of the ocean compared to high in the clouds has a major impact on the formation of precipitation (rainfall).

Pressure is the Key

We all know how important pressure relationships are in the proper operation of HVAC systems. Early on, we were trained in the various techniques needed to maintain the design "static". As important as pressure relationships are to thermal comfort and ventilation, they are vital to management of water in the system. As air moves through the cooling coil its pressure drops (and it loses a lot of water content). Temperature change is a big part of this physical process but actually heat transfer and pressure change work together to change the water holding capacity of air. If air was near saturation when it entered the coil, it will remain near saturation as it moves through and exits. In addition, air that was relatively dry entering the coil may be near saturation when leaving due to the same temperature and pressure changes. This situation is not limited to hot/humid climates.

Much of the function of a cooling coil is to bring entering air to saturation as it moves through the coil. If, for any reason, pressure drops as the air moves out and through the duct system, additional water can condense out. The one positive part of this whole situation is that air will not lose more heat once it has left the coil. In fact, it may gain heat from poorly insulated ducts, friction, or other factors. That is the reason that duct systems are somewhat forgiving. Heat gain can compensate for pressure drop and maintain air at a point below saturation until it leaves the duct system. Once the air enters the conditioned space, pressure falls dramatically but it quickly mixes with room air and temperature also rises dramatically. This results in an air mixture that is well below saturation.

Dramatic evidence of the pressure drop effect can be seen when an automobile air conditioner is first operated on a humid day. The cold high-pressure air flowing from the vents near saturation meets ambient air that is also near saturation and a stream of water vapor is visible at the point of pressure change. This happens because a supply of drier air is not available to absorb the moisture liberated by the sudden pressure change at the vent exit. Another example is the moisture (and associated microbial growth) often noticed on supply grills. This happens when a system is operating at a critical moisture/pressure balance and the dilution provided by room air is not sufficient to absorb the moisture release triggered by the pressure change at the grill.

Water, Water Everywhere

The capacity of air to hold water is amazing. 100 pounds of air at 59 F contains about a pint of water at saturation. Even a small air handler (1,000 cfm) will circulate air containing up to 5 gallons of water every minute if that air is near saturation. A pressure change of one-inch (wg) would cause that air to give up 0.004% free moisture. The resulting moisture, although only a few drops, would be more than adequate to support formation of a biofilm throughout a duct system.

Moisture control in a building is an important objective. Without proper pressure control, however, extreme efforts to eliminate leaks, seal walls and assure complete removal of standing water will be of little value in eliminating IAQ problems.

Mr. Baker's field of expertise is the control of contamination in air-conditioning and ventilating systems by mold, mildew and bacteria. He writes and speaks frequently about the efficacy, legal risks, and regulatory issues involved in various control strategies. He serves on ASHRAE Technical Committee TC 2.3, TC 2.4, TC 9.8, and Sampling of Airborne Particulate Concentration in Commercial and Residential Buildings GPC 17P. He also serves as a member of ASTM D22.06 (Indoor Air Quality) and is on the Board of Directors of the Indoor Air Quality Association and the Florida Public Health Foundation. Because HVAC applications encompass new uses from an U.S. EPA regulatory standpoint, Mr. Baker works closely with the EPA and industry groups, including serving as the chair of the IAQ committee of the Consumer Specialty Products Association, to help formulate policy in this area.

Bob Baker is Chairman and CEO of BBJ Environmental Solutions, Inc., a company specializing in providing clean air through environmentally responsible products, such as BBJ MicroBiocide , BBJ Micro Coil Clean , "FreshDuct Odor Eliminator ", and BBJ Mold and Mildew Remover™ as well as the revolutionary new Power Coil Clean™. For additional information, Mr. Baker can be reached at (800) 889-2251 or through the company web site at http://www.bbjenviro.com.