Duct Design for a Retrofitted House

Using the equal friction method, we first determine the total equivalent length (TEL) of the longest run. The longest branch has an equivalent length of 80 ft. The main trunk length is not given, but we can assume a typical length. However, the friction rate is given as 0.1 in. w.c. per 100 ft. The available static pressure is 0.5 in. w.c., but we must also account for the pressure drop of the air handler and other components. The problem states the available static pressure for the duct system is 0.5 in. w.c. Using the equal friction method, we need to select a friction rate that will not exceed the available static pressure. The total equivalent length of the longest run is the sum of the main trunk and the branch. If we assume the main trunk is 40 ft, then TEL = 40 + 80 = 120 ft. The friction rate = available static pressure / (TEL/100) = 0.5 / (120/100) = 0.5 / 1.2 = 0.4167 in. w.c. per 100 ft. That is higher than 0.1, so we could use a lower friction rate. But the problem says to assume a friction rate of 0.1 in. w.c. per 100 ft. So use that. For the main trunk: airflow = 1,200 cfm. Using a duct sizing chart (or duct calculator) for 0.1 in. w.c. per 100 ft, find the duct size that gives a velocity not exceeding 900 fpm. From typical charts, a 16-inch round duct or equivalent rectangular (e.g., 20x12) would give about 1,200 cfm at 0.1 friction and velocity around 900 fpm. Check: for a 16-inch round duct, area = π*(8/12)^2 = 1.396 sq ft. Velocity = 1200/1.396 = 860 fpm, which is below 900 fpm. So a 16-inch round or equivalent rectangular is acceptable. For branch ducts: each 150 cfm. Using 0.1 friction, find size for velocity ≤600 fpm. For 150 cfm, a 7-inch round duct gives area = π*(3.5/12)^2 = 0.267 sq ft, velocity = 150/0.267 = 562 fpm, which is below 600. So 7-inch round branches are acceptable. Alternatively, rectangular equivalents. Thus, main trunk 16-inch round (or 20x12 rectangular), branches 7-inch round. Verify that the total pressure drop does not exceed 0.5 in. w.c. Using 0.1 per 100 ft and TEL 120 ft, pressure drop = 0.1 * 1.2 = 0.12 in. w.c., well within 0.5. So the design is safe.

Flexible duct has higher friction loss, so for the same airflow and friction rate, a larger diameter is needed to keep velocity and pressure drop within limits. The 20% higher friction means that the effective friction rate for flexible duct would be 0.1 * 1.2 = 0.12 in. w.c. per 100 ft for the same size, or we need to upsize to maintain the same friction. Using the duct calculator with 0.1 friction and 150 cfm, a 7-inch flexible duct might have a higher velocity and pressure drop. To maintain velocity ≤600 fpm and friction ≤0.1 per 100 ft, we should increase the branch duct size to 8-inch round. Check: 8-inch round area = π*(4/12)^2 = 0.349 sq ft, velocity = 150/0.349 = 430 fpm, which is fine. The pressure drop for 8-inch flexible at 150 cfm will be lower. Also, the total static pressure will increase due to the higher friction of flexible duct. We need to recalculate the total pressure drop for the longest run using the flexible duct friction. If we keep the same size, the pressure drop would be 0.12 per 100 ft, so for 80 ft branch, drop = 0.12*0.8 = 0.096 in. w.c., plus main trunk drop. But we must ensure total does not exceed 0.5. With upsizing, the friction per 100 ft for 8-inch flexible might be around 0.08, so drop is lower. Recommendation: upsize branch ducts to 8-inch flexible, ensure proper installation with minimal sag and bends, and verify total static pressure does not exceed 0.5 in. w.c. Also, consider using a manual D calculation to account for the actual equivalent lengths of flexible duct fittings.