| Segment | CFM Carried | Round Dia. | Rect. Equiv. | Velocity | Length (ft) | Pressure Drop |
|---|
How reducing trunk ducts work in Canadian forced-air systems
A reducing trunk duct carries the full blower output from the air handler to the first branch, then drops in size as each branch takes off its share of the airflow. By the time the trunk reaches its last branch, it only carries the CFM for that final room. Each segment gets sized to carry exactly its remaining airflow at the design friction rate, so static pressure loss per foot of duct stays constant throughout the trunk. That constant friction rate is what keeps pressure approximately equal at every branch takeoff, which makes field balancing straightforward.
In Canadian two-storey homes, trunk ducts typically run in the basement ceiling along the home's centreline, with branches going up through interior walls or floor cavities. The trunk starts large near the furnace, often 16 to 20 inches wide by 8 inches tall for a 1,400 CFM system, and steps down to 8 by 8 or smaller at the far end. Each step-down requires a reducing fitting, which the sheet metal shop fabricates from the dimensions this calculator provides.
Finding your design friction rate before sizing the trunk
The design friction rate drives every duct size in the system. It equals the available static pressure divided by the total effective length of the longest duct run, multiplied by 100 to express it per 100 feet. You need to know two things first: the furnace or air handler's total external static pressure rating at the operating airflow, and the pressure drop across each piece of equipment in the air stream. Subtract the coil, filter, and any humidifier pressure drops from the rated external static to get available static pressure. Then use the available static pressure calculator to convert that to a friction rate. For most Canadian gas furnaces at their rated CFM, available static pressure runs between 0.08 and 0.15 inches of water column, giving friction rates of 0.07 to 0.12 inches per 100 feet on a system with an 80-foot longest run.
Velocity limits in trunk ducts and why they matter
Standard duct design practice sets maximum trunk duct velocities to control noise. For supply trunk ducts in residential systems, the limit is 700 fpm in occupied-space ceiling locations and 900 fpm in unconditioned basement locations. Exceeding these limits creates a low-frequency rumble that transmits through the duct walls and into the floor structure above. In Canadian homes with hardwood floors, the sound path is direct and homeowners notice immediately. The velocity column in the results table flags any segment that exceeds 700 fpm so you can either increase the duct size for that segment or confirm the location is non-sensitive.
Frequently Asked Questions
The design friction rate equals the available static pressure divided by the total effective length of the longest duct run, expressed in inches of water column per 100 feet. A typical Canadian bungalow with a gas furnace and single-speed blower has available static pressure of 0.10 to 0.15 inches after subtracting equipment pressure drops. With a longest run of 80 feet, that gives a friction rate of about 0.125 inches per 100 feet. Residential systems typically fall between 0.06 and 0.15 inches per 100 feet. Use the available static pressure calculator to find your specific rate before entering it here. Using a friction rate that is too high results in undersized ducts and high velocity noise; too low results in oversized ducts that cost more material without improving performance.
A reducing trunk steps down in size at each branch takeoff, keeping velocity roughly constant and static pressure nearly equal at every branch. It uses less material and fits more easily in tight ceiling joist spaces common in Canadian ranch-style bungalows and split-level homes. A constant-size trunk keeps the same cross-section from air handler to end cap. It is simpler to fabricate and common in older Canadian homes and commercial suspended-ceiling systems. For new residential construction in Canada, the reducing trunk is preferred because it sizes each segment to exactly what it carries, reduces material cost, and gives better balancing results without relying on dampers to correct for uneven static pressure. Use this calculator for reducing trunks. For commercial constant-pressure trunk sizing, pair it with the static pressure calculator.