On the whole, “The greatest number of airborne fungi was dispersed from metal duct, followed by fiberglass duct liner and rigid fibrous glass duct…. Decreasing airborne concentrations in the room were measured during the second and third cycles, but no significant differences in the concentrations of airborne fungi were observed for the three duct materials during these cycles.”

The spores that are not dispersed, however, have the potential to breed within the ductwork. “If the surface provides nutrients, almost any duct materials could provide significant fungal contamination.”

The “oil can effect,” in which metal ducts pop due to pressure changes, can also release a big burst of spores. This falls into the category of vibration, which plays a role in dispersal.

ADDING COILS

When it comes to IAQ problems, “We find we’re looking at new buildings,” built after 1970, said Boone. The results presented here were from a field study.

Boone was outspoken against fiberglass liner. “Fiberglass liner collects debris — and you don’t need liquid water to grow mold,” she said. Moreover, the liner can’t be cleaned. This could be part of the reason why “The difference [in fungal levels] between occupant space and hvac is dramatic; fungal populations are much higher in hvac.”

The paper states that “It has been found that air conveyance systems, which have internal surfaces in contact with the airstream, which readily collect debris, are more likely to become sources of fungal organisms found in the occupant space.”

Boone pointed out that heavy fungal growth would occur within 10 ft of the cooling coil. From the field study: “On March 25, 1999, the cooling coils were cleaned using detergent and water. On March 31 all the tests were repeated. The repeat tests were taken within 25 cm of the March 24 locations, but were not taken at the same locations.” (See Table 1.)

Numbers skyrocketed after the coils were cleaned with detergent and water.

“These studies show that fungal levels in fibrous glass insulation in contact with the airstream surface in an air conveyance system are hundreds of times higher than the fungal levels found in closed-cell foam or on galvanized metal surfaces. Cleaning air conveyance cooling coils with detergent mixed with water and then rinsing increased the fungal levels in the fibrous glass insulation but not in the closed cell foam insulation or the galvanized metal located within 3 m downstream of the cooling coils,” the report states.

“The fungal levels in the fibrous glass liner increased greatly and indicated the active growth of fungi in this material. Active growth of fungi in fibrous glass liners in low-moisture areas is less likely to occur because adequate moisture for growth is less likely to exist in these areas. Fibrous glass liners located in low-moisture areas can collect fungal organisms from the airstream and release these organisms to the occupant space.”

Coil-cleaning methods include forward-flush, back-flush, high-pressure, or low-pressure. The type used needs to take into consideration the type of ductwork downstream of the coil as well as the coil type. Overspray should be avoided as much as possible.

FOULED COILS

The impact of biological fouling on cooling coils is fairly well known, said the speaker, Jeffrey A. Siegel (graduate student, Energy Performance of Buildings Group, Lawrence Berkeley National Laboratory). “Particulate fouling causes increased energy use” as well as IAQ problems, he said.

Coils in this research project were cleaned according to an Australian maintenance standard. They still had bacteria.

Cooling coils are a viable location for bacterial and fungal growth, he pointed out, because they are moist. Therefore, they can aid in the spread of bacteria and fungi. These substances can also shed off the coil and enter the airstream. Dust provides the nutrients essential for growth.

There is a cleaning standard, he said, but it entails the use of extreme chemicals, and because of the hard-to-reach locations of many coils, it is hard to do correctly. Moreover, the cleaning has little effect on established biofilm.

Filtration is a better solution, he continued.

“We need to research different strategies for control,” he said. Drain pan issues and air contamination also need to be taken into consideration.

In short, “When you have a wet coil, you have a filter,” Siegel said. “You don’t have a coil.”

Publication date: 12/17/2001