Q: Heating and cooling contractors in my area can install flexible-duct systems for less than they charge to use metal duct or fiberglass duct board. But is it smart for me to choose that lower-cost option in a high-performance house?
A: Insulated flexible ducts are usually the least costly choice for a heating or cooling duct system. And yes, when they’re properly sized, laid out, and installed, flex-duct systems can perform as well as any system out there. But you have to be careful about quality control. Low-ball HVAC contractors who sell strictly on price usually install flex duct because it’s the cheapest product. But those lowest-bid operators also tend to hire unskilled, low-wage helpers and cut corners on installation—and sloppy or thoughtless installation of any duct material can leave you with a system that doesn’t perform as intended. So if you want quality—but at your best price—flex duct is a good choice; just be careful whom you choose as your HVAC installer.
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.
Size Ducts Correctly
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.
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.”
Seal Connections Tightly
The final key to duct performance is air sealing. This step is easier with flex duct 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.”
