Question 1

What is better — round or rectangular ducts?

Accepted Answer

Round ducts are more efficient because they have less surface area per unit of airflow, which reduces friction losses by up to 30% compared to rectangular ducts of equal capacity, according to ACCA Manual D guidelines. Rectangular ducts are best for tight spaces like between floor joists. Regardless of shape, proper sizing ensures the correct air velocity — typically 600–900 FPM for supply ducts — for good distribution.

Question 2

What is the difference between metal and flex duct?

Accepted Answer

Metal is smoother and creates less air resistance. Flex duct is easier to install but produces more friction. For the same CFM, a flexible duct usually needs to be one or two sizes larger than a metal one to perform equally.

Question 3

What happens if HVAC ducts are too small?

Accepted Answer

Undersized ducts lead to whistling noises, higher energy bills, and hot spots in far rooms. The U.S. Department of Energy estimates that duct-related issues account for 25–40% of heating and cooling energy loss in a typical home. Matching the duct diameter to the system’s actual CFM output prevents these issues and keeps static pressure within the recommended range of 0.5″ WC or less.

Question 4

How do I size a return air duct?

Accepted Answer

Return ducts often need to be slightly larger than supply ducts to reduce noise and static pressure. Input the total system CFM to get the required intake dimensions to keep the blower healthy.

Question 5

Can I use this calculator on a job site?

Accepted Answer

Yes. It is optimized for mobile, so you can pull it up on your phone to verify dimensions before you start cutting or ordering materials.