It’s not as old as the chicken and egg debate, but the question of round duct versus rectangular is one in which there are valid points on either side. When making the decision on which one to use, there are a couple key points to keep in mind: space and pressure.

On large scale commercial jobs, the engineer is the one making the decision on which duct system to use. On smaller scale jobs and residential, the contractor is the one making the decision. Both have to take into consideration the space they are given to work with by the architect. That space may have to be shared by electrical, plumbing or other items in close confines so the choice of which duct to run makes a difference.

“Round spiral duct allows for more efficient airflow with less friction loss, especially on long runs,” says Bob Murray, our expert on the subject at Lake Mechanical. “For example, round duct can be run in lengths up to 30 feet with minimal joints, connections and sealing requirements. Rectangular duct is shop fabricated in lengths up to 10 feet that require sealing both longitudinally and at the connecting assemblies,” Murray says.

While round spiral duct is easier and faster to install, one disadvantage is it takes up more room. It may not be as wide, but it takes up more height. For instance, a 24x12 in. duct with insulation is only 14 in. high, but a 16-in. round duct with insulation takes up 18 in. of space. Another factor to consider when deciding between round and rectangular is the system pressure.

“Round spiral duct is more economical to install – that is on medium or high pressure duct system,” Murray says. “Low pressure snap lock duct is not as rigid and requires more sealing of longitudinal seams and joints. In that instance, it’s easier to install a rectangular duct more efficiently versus a round duct on a low pressure system.”

If you have an engineer working the project, you can depend on them to make the correct decision, but if you are the one making the call, take into account space constraints and the pressure the system will be under. All things being equal, some people just have favorites.

“If they're engineered and installed properly, they're about the same in terms of airflow,” Murray says, “but I would take a round duct system if I had the choice.” 

John Smith, the 2009 Associated Builders & Contractors (ABC) Contractor of the Year, is Chairman/CEO of Lake Mechanical Contractors in Eustis, FL.