Total Eff. Length
| Room / Branch | CFM | Calc Dia. | Std Size | Rect. Equiv. | Velocity | TEL | Pressure Drop |
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Branch duct sizing in Canadian residential construction
Branch ducts carry air from the trunk takeoff to a single supply register. In a typical Canadian bungalow, there are six to twelve branch ducts, one per room. Each branch gets sized for the CFM that room requires based on its room-by-room heating and cooling load. The branch diameter comes from the design friction rate, which must match the friction rate used to size the trunk so that pressure loss per foot stays consistent across the whole system.
Getting branches right is where most residential duct systems in Canada fall short. The common mistake is to install every bedroom branch as a 6-inch round because that is what was on the truck. A bedroom needing 80 CFM on a 0.08 friction rate system actually needs a 5-inch round. An oversized branch delivers too much air and steals from other rooms. An undersized branch starves the room and causes comfort complaints the homeowner will blame on the equipment rather than the duct design.
Total effective length is not the same as actual duct length
Total effective length adds the actual measured duct length to the equivalent straight-duct length of every fitting in the branch. A 90-degree radius elbow in a 6-inch round branch adds about 20 feet of equivalent length. A trunk takeoff fitting adds 30 to 50 feet. A supply register boot adds 25 to 50 feet. In a bedroom branch with 12 feet of actual flex duct, one elbow, and a register boot, the total effective length is easily 90 to 100 feet. Sizing the branch only against the 12-foot actual length gives a duct that is two sizes too small and cannot deliver the required CFM. Use the equivalent length calculator to add up each fitting before entering the total effective length here.
Flex duct correction and why it matters in Canada
Flexible duct is more resistant to airflow than rigid galvanised sheet metal at the same diameter because its interior surface is corrugated rather than smooth. A 6-inch flex duct behaves like a 5.5-inch rigid duct in terms of friction loss. Standard duct design practice applies a correction factor that effectively increases the total effective length by 25 percent when flex duct is used. This calculator applies that correction automatically when you select flex duct as the material, so the resulting diameter accounts for the extra resistance. In Canadian construction, flex duct is standard for the last 6 to 8 feet of branch runs into floor or ceiling registers. The trunk and main branches should still be rigid galvanised sheet metal for best performance and longevity.
Frequently Asked Questions
To size a branch duct for a Canadian bedroom you need three values: the room's required CFM from a room-by-room load calculation, the system's design friction rate in inches of water column per 100 feet, and the total effective length of the branch from the trunk takeoff to the supply register including equivalent lengths for all fittings. A typical Ontario bedroom needing 100 CFM on a system with a 0.08 friction rate and a 30-foot total effective length needs approximately a 6-inch round duct. Use the equivalent length calculator to add fitting losses before entering the total effective length here, and verify the room's required CFM with the load estimator.
Total effective length is the actual measured duct length plus the equivalent straight-duct length of every fitting in the branch. A 90-degree round elbow adds roughly 15 to 30 feet depending on diameter. A trunk-to-branch takeoff fitting adds 20 to 50 feet. A supply register boot adds 25 to 75 feet. In a typical Canadian bungalow bedroom branch with one trunk takeoff, one elbow, 15 feet of actual duct, and one register boot, the total effective length could be 80 to 100 feet even though the actual duct run is only 15 feet. This is why branch ducts often size larger than expected. Use the equivalent length calculator to accumulate fitting losses accurately, then enter the total here. After sizing the branches, verify overall system balance with the supply air balancing calculator.