DENVER - Watch out, contractors. Engineers in Denver for the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Annual Meeting were asking themselves, "What often-ignored factors affect the performance of residential forced-air systems?"

Not surprisingly, some of the first items cited during a program on residential applications were load calculations, duct sizing, and duct integrity. Proper usage of duct tape was even called into question. According to a unitary manufacturer, "Most systems are oversized and most ducts are undersized."

A code official pointed out that in her jurisdiction, the state code requires Manual J compliance for new systems. The regulation will be monitored for design, she said, but testing will not be required. Optimum results would be achieved from design and testing after installation, other participants pointed out.

Duct Insulation

According to an engineer, installers need to be aware of the resistance of various types of duct insulation to moisture and wetting, and subsequent degradation. "In some cases, once you get it wet, you've lost your R value."

A utility representative added, "There's a bubble, foil-type barrier promoted for duct insulation that's promoted as being an R-6 insulative value, when it's actually more like R-2." He and other forum participants described it as "aluminized bubble wrap."

Duct placement is still a problem, participants said. Even in new "green" construction, ductwork often is installed in unconditioned spaces.

Fans And Humidity

There has been some disagreement in the engineering community about whether continuous fan operation is a bad thing as far as airflow goes. Some engineers are stating that continuous fan operation is necessary to meet airflow requirements. However, engineers attending this forum conceded that consumers need greater awareness (make that any awareness) that fan operation can lead to increased indoor humidity.

"It can lead to 15 percent to 20 percent re-evaporation of moisture into the airstream," pointed out a participant. "It's technically well understood, but not communicated."

Also, where should registers be located? Old standards say under windows, but better insulated homes and better windows might mean this is not strictly necessary.

"We may consider keeping duct runs near the core, resulting in shorter duct runs," said an employee of an energy agency. "No one is coming out and saying it yet, but it might be OK."

Filtration And Maintenance Effects

The location of the return air filter might also be revisited, said another member of the energy community. "They are more commonly located at return grilles. Return ductwork has 30 percent to 40 percent greater return-side pressure," he pointed out. Therefore, return duct often leaks at a greater rate. "When dirty air travels to the coil, it becomes nutria for microbial growth."

"There are efficiency standards for filters, but no rating for pressure drop," complained an engineer. "The consumer doesn't understand the difference."

A unitary manufacturer pointed out that "For some reason, we have a homeowner who feels they don't have to do anything to their system. We have commonly seen 1-inch static pressure drop across the system that it wasn't designed to handle," due to a high-efficiency filter, filter loading, and duct leakage. "You just can't push that much air across the system."

"Cabinet leakage is an area I'd like to see addressed," said a member of academia.

"With air handlers in attics and garages" - unconditioned spaces - "leakage around the filter and system access points can add to the load."

California's Title 24 addresses duct leakage on a limited basis for some climate zones, pointed out a participant. Programs such as Energy Star and Building America discuss duct leakage, "but they're not codified."

"What we need are good installation techniques and fastening methods," an engineer stated.

More applied research may cover how to actually fix the system. "How do you actually pull it off?"

Publication date: 08/15/2005