INTRO (0:00 to 0:30)
Voiceover, cold open on M38-duct-system-anatomy.svg: In M37 you calculated the load and picked the equipment. Here is the problem: none of that reaches the customer except through the ducts, and ducts are where good equipment goes to die. The industry's most common installed deficiency is not charge. It is airflow. This video gives you Manual D in four minutes: a budget, a length, a rate, and a chart. Four numbers that decide whether the machine you selected ever performs.
ON-SCREEN: M38-duct-system-anatomy.svg. Title overlay: A budget. A length. A rate. A chart.
MAIN (0:30 to 4:15)
Beat 1: The budget: available static pressure (0:30 to 1:15)
Voiceover over M38-friction-rate-worksheet.svg, top section: Start from the number you have carried since C12: the blower is rated to deliver design airflow at 0.5 inches of water column. Manual D treats that as a bank account, and everything that is not duct withdraws first. Wet coil, 0.21. Media filter, 0.10. Registers, grille, damper, 0.03 each. Watch the subtraction land: 0.10 left. One tenth of an inch for the entire duct system, supply and return combined. And swap that media filter for a 1 inch MERV 13 pleat at 0.30 and the budget goes negative. No duct design exists for a negative budget. The accessories are not details. They are the design.
ON-SCREEN: worksheet subtraction animating line by line, ending at ASP = 0.10 in WC
ON-SCREEN: red overlay: 1 inch MERV 13 pleat = ASP negative 0.10. No design possible.
Beat 2: The length: fittings are the real distance (1:15 to 2:10)
Cut to M38-effective-length-fittings.svg: Air does not count feet, it counts friction, and fittings make friction out of proportion to their size. Manual D prices every fitting in equivalent feet of straight duct. A radius elbow, about 5 feet. A square-throat elbow, about 35. Same turn, 30 feet apart in cost. A panned return path, over 100. Total effective length is the longest supply run plus the longest return run, straight feet plus every fitting's price. Our example: 60 straight feet of supply plus three moderate fittings at 35 each is 165. Return path, 85. Total, 250 effective feet. The tape measure saw 85 feet. The air feels 250.
ON-SCREEN: three fitting tiers with equivalent lengths, then TEL math: 165 + 85 = 250 ft
Beat 3: The rate: one division sizes the system (2:10 to 3:00)
Back to M38-friction-rate-worksheet.svg, bottom section: Friction rate equals available static times 100 divided by total effective length. Here: 0.10 times 100 over 250 is 0.04 inches per hundred feet. The workable window runs roughly 0.06 to 0.18. We are under the floor, and that is the worksheet doing its job: the design failed on paper instead of in the attic. The fix is in the formula. Cut the length: swap the square-throat elbow for a radius, 35 becomes 5, upgrade the takeoff, and the rate climbs into the window. Fittings broke it, fittings fixed it.
ON-SCREEN: FR = ASP x 100 / TEL. 0.10 x 100 / 250 = 0.04. Window: 0.06 to 0.18
ON-SCREEN: fitting swap animation, TEL 250 to 195, FR rising into the window
Beat 4: The chart: lookup, then check velocity (3:00 to 3:40)
Screen recording of ductulator app: With the rate set, sizing is a lookup. Set the friction rate, dial the CFM, read the diameter. Twelve hundred CFM trunk, about 18 inch round at roughly 680 feet per minute. The velocity check matters because noise is the other wall: trunks at or under about 900 FPM, branches near 600, returns at 600 or less, filter grille faces near 300, the same C12 number. And one material warning: flex matches this chart only when it is pulled tight. Compressed flex runs several times chart friction, up to roughly ten times. Size flex for a perfect install, then make the install perfect.
ON-SCREEN: ductulator at FR 0.05, 1,200 CFM, 18 in round, 680 FPM
ON-SCREEN: velocity limits table. Flex warning: compressed = up to 10x chart friction
Beat 5: Existing systems: the ladder (3:40 to 4:15)
Cut to M38-retrofit-priority-ladder.svg: Most of your master-level work is not new design, it is renovation, and the ladder orders it. Rung zero, measure: the C12 static profile and the D25 four-port map, always first. Rung one, returns: the most commonly undersized element, and the cheapest airflow you will ever buy back. Rung two, the worst fittings the survey priced. Rung three, sealing, with the A36 methods, after the breathing is fixed, because sealing raises static. Rung four, resizing, only when the measurements prove nothing cheaper is left. Measure between every rung. Each fix redraws the map.
ON-SCREEN: M38-retrofit-priority-ladder.svg, rungs highlighting in sequence
OUTRO (4:15 to 4:30)
A budget, a length, a rate, a chart, and a ladder for everything already built. In the demo video, Darrel takes this into a real attic: full static map, duct survey, and a ranked retrofit plan, which is exactly what your practical asks you to do. Watch how he prices fittings on sight. That habit is the module.
ON-SCREEN: M38 practical: survey, static profile, three worst restrictions, ranked plan