INTRO (0:00 to 0:30)
Cold open on a doctor's blood pressure cuff inflating, then a hard cut to a manometer reading 0.90. Voiceover: in C12 you learned to take this reading, the blood pressure of the system, and in D24 you learned that low airflow is one corner of the triangle that fools techs into adding refrigerant to healthy systems. Here is the problem: blood pressure tells the doctor something is wrong. It does not tell them which artery. This video turns your one static number into a map with a finger pointing at the exact component that is strangling the system.
ON-SCREEN: TESP = something is wrong. THE MAP = where it is.
MAIN (0:30 to 4:30)
Beat 1: Four ports, four suspects, one budget (0:30 to 1:30)
Show the D25 static pressure diagnostic map SVG, building port by port. The two C12 ports become four: Port A before the filter, Port B after the filter, Port C before the coil, Port D after the coil. Subtraction hands you each component's bill: filter drop is B minus A, coil drop is C minus D, the return path is A by itself, the supply path is D by itself. Then the budget appears on screen: on a 0.5 system, roughly 0.10 for the return path, 0.10 for the filter, the coil's own published number around 0.20 to 0.25 wet, 0.10 for the supply. The component spending double its budget is the restriction. Run the worked example numbers: 0.36, 0.12, 0.24, 0.18 on a 0.90 TESP. Nobody needs help finding the guilty number in that list.
ON-SCREEN: D25-static-pressure-diagnostic-map.svg, ports A through D lighting in sequence
ON-SCREEN: Return 0.36 vs 0.10 budget. The map convicts the return path
Beat 2: The coil you convict without pulling (1:30 to 2:30)
Show the D25 dirty coil detection SVG. The evaporator hides inside a casing and pulling it is hours of work, so it gets convicted like any suspect: independent evidence that agrees. Four lines on screen, one at a time. One: measured coil drop versus the manufacturer's published wet drop, screen capture of a real data sheet, 0.45 measured against 0.25 published. Two: temperature split above 22, slow air leaving too cold. Three: the D24 recall, low suction with NORMAL subcooling, because low charge drops subcooling and low airflow does not. Four: a flashlight on the air-entering face, the underside, where the dirt mat actually forms. The rule: two lines of evidence to recommend a cleaning, three before a pull-and-clean.
ON-SCREEN: D25-dirty-coil-detection.svg, four evidence paths converging on the verdict
ON-SCREEN: Low suction + normal subcooling = air problem, not charge problem (D24)
Beat 3: The fault that LOWERS static (2:30 to 3:30)
Show the D25 blower fault tree SVG. Everything so far raises static. The blower wheel is the fault that lowers it. Side-by-side photo insert: clean blade cups with crisp edges versus smooth gray crescents packed with dust. Cupped blades cannot grip air, and a loaded wheel gives up 20 to 30 percent of its CFM at the same RPM. The signature on screen: LOW airflow WITH LOW static, the pattern that fools every tech who learned high static is the only enemy. And the trap inside the trap: the fan table assumes a clean wheel, so it promises CFM a bald wheel is not delivering. Cross-check with temperature rise, then put eyes on the wheel. Close the beat with the ECM branch of the tree: a constant airflow ECM hides duct disease by eating watts, so on those systems static and watt draw tell the truth, and a replacement motor installed without mapping static is the next failure with a countdown timer.
ON-SCREEN: D25-blower-fault-tree.svg, the low-low branch highlighted
ON-SCREEN: High TESP + low CFM = restriction. Low TESP + low CFM = blower.
Beat 4: The hunt, and the chain this module breaks (3:30 to 4:30)
Show the D25 duct restriction hunt SVG. The TESP split picks the side: return-heavy means filter, undersized return, blocked grille, crushed return flex. Supply-heavy means coil, crushed supply runs, closed dampers, shut registers. Then the Phoenix reality on screen: attic flex baking at 140 to 160 F, crushed where someone stored boxes on it, kinked at hard turns, and it never springs back. Then the close, on the D25 symptom chain SVG, drawn link by link: restriction, CFM falls, capacity falls, gauges mimic low charge, the wrong tech adds refrigerant, the coil freezes, the compressor floods and dies. Break the chain at link one and it costs a filter. Break it at link seven and it costs a compressor. The map is how you break it early, and the after-reading is how you prove you did.
ON-SCREEN: D25-duct-restriction-hunt.svg, then D25-airflow-symptom-chain.svg building left to right
ON-SCREEN: Fix it, RE-MEASURE it. Before and after CFM or it did not happen
OUTRO (4:30 to 4:45)
Four ideas: the map beats the number, the coil gets convicted by evidence, low static can be the worst news on the call, and every airflow fault is a compressor failure on layaway. In the next video Darrel maps a real strangled system port by port, follows the numbers into the attic, and shows you the before and after CFM that closes the case.
ON-SCREEN: Next: Darrel maps a real system and the numbers find the fault