Efficiency of Flat Oval Ducts in HVAC Systems
A flat oval duct system is not all flat oval. A lot of it is actually round spiral duct. No one contests that round spiral duct is less expensive and more efficient than rectangular duct. Most flat oval duct systems are only flat oval in the major trunks. Those branch fittings are usually round fittings – efficient round taps manifolded to the flat oval trunks. The “flat oval” in your duct system is mostly straight spiral pipe, elbows, and transitions. This is where you can really start to save money.
Spiral flat oval spiral duct is usually provided in 8’ to 10’ lengths, and can be made up to 20’ long. That compares to about 56” for a standard coil line rectangular duct, so you have a lot less duct joints and duct sealing (we’ll get to that later). Obviously, the less elbows and transitions you have, the straighter duct savings you will pick up with flat oval. Don’t assume the cost of flat oval fittings are exorbitant compared to rectangular fittings, because it’s not the case. Unlike the 1980’s, both flat oval and rectangular fittings are now made on those same plasma torch tables with automated programming. Some of the new seam welding and lock forming equipment make the assembly of flat oval fittings just as easy as banging together the Pittsburgh locks on rectangular fittings. An efficient duct layout will be mostly straight ducts, and flat oval spiral duct will be the less expensive choice.