By Bob Baker

Until recently, carryover is not a problem that many in the field concerned themselves with. Part of the manufacturer's responsibility is to design cooling equipment that will not allow significant carryover. As a result, this term may not be commonly used by engineers, contractors or field service personnel.

What is carryover?

Carryover is a term used to describe the release of water from the cooling coil into the air stream. This is not a desired result of system operation. As we all know, an important task of the air conditioning system is to control humidity in the conditioned air stream. When ambient humidity is above the desired range of 45-60% RH, this means that a properly operating system must remove significant quantities of water from the over saturated air stream in order to bring the conditions in the occupied space into an acceptable range. The job of the equipment designer is to create an air handler design that will achieve the desired moisture removal (or latent) capacity. Usually this end requires that water that condenses on the heat transfer surfaces flow quickly off those surfaces, into the drain pan or sump and out of the air handler into the sewer or other drain.

Designers and manufacturers of air handlers and cooling coils go to considerable effort to see that this happens. The coil arrangement and fin design must both promote efficient heat transfer and surface water movement. Part of this is providing a smooth metal surface that will promote the sheeting action that helps water drain from the surface rapidly. Manufacturers have found that it is important that the metal surfaces also be as clean as possible or water flow will be changed. Cutting oils, for example, if left of the surfaces can hold water droplets and lead to carryover. Likewise, dust and other contaminates that gen into the air handler during construction can lead to carryover problems. For this reason, manufacturers urge installers to wait until "dirty" parts of construction are complete before installing equipment or seal or otherwise protect air handlers from contamination at the site.

Is carryover a problem?

Carryover is undesirable from several standpoints. If water is carried into the airstream in large quantities, it can cancel out the de-humidification capability of the system and result in comfort complaints. Since system interiors are not constructed to deal with large amounts of moisture, water carried into the plenums or down stream components such as heaters, motors, dampers and lined ducts can cause physical damage that will shorten the life of the system. We are beginning to believe that, as important as the above are, the negative impact of carryover on indoor air quality is even more serious.

How does carryover impact IAQ?

The moisture that is carried over into the air stream does not stay suspended very long. The majority will be deposited on the surfaces of the supply plenum or ducts within ten to twelve feet of the cooling coil. Obstructions in the air path such as blowers, turning vanes, sharp bends and dampers can both increase the amount of moisture released from the air and cause it to fall out more quickly. When this moisture lands on a surface, it will cause any bacterial or fungal spores on that surface to become viable and grow and multiply. If there is insulation debris or dirt on the damp surfaces, growth will be both more rapid and lush.

The resulting colony of organisms will release biological contamination back into the air stream. This may be in the form of live organisms that break off and are carried out into the room or gasses and other waste products that are released by the growing organisms. Most people are sensitive to some extent to live fungi and their spores. The gasses and other wastes tend to be toxic to humans and animals even in small quantities. As we continue to study indoor air quality problems, we find increasing evidence that these biological substances are a major, if not the major, cause of poor indoor air. Certainly, many of the unpleasant odors and occupant allergies can be traced back to microbial contamination that would not occur without the presence of moisture. It is clear that carryover is not desirable and should not be present in a well designed and maintained system.

Water can be carried into the air stream from sources other than the cooling coil. These include humidifiers and air washers. Although these sources of excess moisture are not normally considered when we think of carryover, excess or misplaced moisture from these sources can cause the same problems as carryover from coils. Because of the moisture problems often caused by humidification, manufacturers have developed humidifiers that utilize or generate dry steam. This is because steam adds moisture as water vapor rather than droplets. Even so, manufacturers of these units acknowledge that operation of these units can result in moisture being deposited downstream of the steam release point. Depending on conditions in the plenum or duct, this zone of excess moisture can vary from three to fifteen feet.

The point of all this is that there are many possible sources of excess moisture in the air conditioning system, even when relative humidity is apparently being well controlled. High levels of moisture can result in conditions that reduce the quality of indoor air. Therefore, as indoor air quality providers, we must be conscious of all possible sources of excess moisture and be able to minimize them. In the next issue, we will look in more detail at the various causes of carryover and discuss ways to prevent or minimize carryover.

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.