Builder - September 2008 - (Page 52) HOUSECALL any duct system, says Johnson, “you figure out really quickly what does and does not work.” FIGURE LOADS ACCURATELY So what’s the right way to run flex duct? The job starts with a load calculation using Manual J8, explains Johnson. Compared to the old Version 7 (which dates back to the 1980s), the new manual incorporates a much greater level of detail about building characteristics. Because the calculations are so detailed, says Johnson, contractors rely on computerized versions of the manual. Supplied with all the necessary facts about each room—volume, wall dimensions, insulation levels, window type, and window and wall orientation—the software figures heating and cooling loads on a room-by-room basis and provides a recommended size for the furnace or air conditioner. SIZE DUCTS CORRECTLY STRAIGHT SHOOTING: For best airflow, this duct is stretched to full length and well supported with no kinks or sharp bends. The temporary fiberglass plug will keep dirt out during construction. Once all the room loads are estimated, most Manual J software packages let the user design duct systems with a few more mouse clicks. “If I tell my computer what kind of ductwork I’m going to put in—duct board, sheet metal, or flex duct; perimeter system, spider system, or trunk-and-branch-line system, whatever it’s going to be—it will sit there and calculate all the proper size ducts in a matter of minutes,” says Johnson. In the field, contractors can do duct design with a handheld duct calculator that works like a circular slide rule (the bestknown brand is Trane’s Ductulator). But it’s important to use a calculator that is calibrated for the duct material you’re using. If the contractor uses the answers for a sheetmetal system when he’s really installing flexible duct, he’ll end up with undersized ducts; the corrugations in flex duct create friction in the duct and the diameter has to be larger to compensate. INSTALL SYSTEM CAREFULLY Even if they’re sized right, air ducts won’t perform properly unless they’re installed correctly. “You have to make sure the runs are put in straight, that they’re pulled out to their full length and not allowed to kink, that they don’t have multiple bends in them,” explains Johnson. “You have to hold the installers to a standard of pulling the lines tight and supporting them and not allowing sags. I’ve been in this business for years, and I’m still always amazed at how you can take a duct and straighten out its bends or curves and make it work better and deliver more air.” In one recent case, Johnson was testing a system’s airflow before a house was drywalled. “The total CFM was pretty much right on target, but we had one trunk line from the second-floor attic down to the first floor that was not getting the right amount of air.” The installation crew had introduced multiple 90-degree bends into that run of duct. “They were going diagonally across a floor truss system, and every 16 inches, they would take another bend and go through another truss.” Each 90-degree bend adds the equivalent of 20 or 30 feet of length to a duct’s air resistance, he notes. “So they had an equivalent length of about 200 feet for a 15-foot run.” Rerouting the duct run so it went straight through all the trusses, then made one bend and ran the length of one void between two trusses, made the duct run look about 5 feet longer; but the change reduced air resistance in the duct by at least 80 percent. When inspections catch this kind of mistake, Johnson notes, the installers get a chance to learn better practices. “We made the crew that did it go back out and fi x it,” he says. “The next time they do it, they’ll get it right.” SEAL CONNECTIONS TIGHTLY than it is with metal or duct-board systems, Johnson observes. “In the field, the worst leaking systems are typically sheet-metal duct systems, not flex-duct systems. It’s harder, and it takes longer to seal up a metal system because flex duct comes in 25-foot lengths. You only have a joint every 25 feet instead of every 5 feet.” In Austin, where duct inspections are mandated by code, installers have adapted, according to Austin-based building performance consultant Doug Garrett. “Ninetynine percent of the ducts in Austin have been installed with duct-sealing mastic for the last seven or eight years,” says Garrett. “That’s just the way we do it. They cut the hole in the plenum, and they put the mastic around it, and they put the start collar up into the mastic and embed it. Then they put mastic around the draw band, and they pull the inner liner up, put a zip tie on it, and squish it down into that bed of mastic. Then they pull the outer jacket up, mastic the outer jacket to the plenum and bingo, they’re done—mastic at three locations. The vapor barrier is sealed; the inner liner is sealed; and the start collar is sealed to the plenum. None of it’s going to come off. You’re done for 60 years.”—Ted Cushman HAVE A CONSTRUCTION QUESTION? E-M AIL DENISE DERSIN AT: ddersin@hanley wood.com courtesy stan johnson The final key to duct performance is air sealing. This step is easier with flex duct 52 ■ B U I LD E R sep t e m ber 2008 W W W.BUILDERONLINE.COM http://WWW.BUILDERONLINE.COM
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