INTRO (0:00 to 0:30)
Darrel at the furnace, system running: This 4 ton system has a complaint behind it. Weak cooling, runs all day, and the last company that looked at it said it might need refrigerant. In C12 I showed you how to take this system's blood pressure. Today we go further, because the static number on this system is ugly, and ugly is not a diagnosis. I am going to split that number into pieces, let the pieces point at the exact restriction, fix it, and prove the fix with an after number. No gauges until the air is proven. That is the law from D24.
ON-SCREEN: D25 Field Demo: from one ugly number to the exact restriction
MAIN (0:30 to 11:15)
Beat 1: The C12 baseline, run fast (0:30 to 2:00)
Darrel runs the baseline he expects every tech to have as a reflex by now: system in cooling 15 minutes, coil wet, the two standard ports drilled (return between filter and blower, supply between furnace and coil, drill stop on, cabinet mapped first). Manometer screens in close-up: return minus 0.48, supply plus 0.42. He does the math on the tablet: TESP 0.90 against a 0.5 rating. Then the fan table overhead shot, finger trace at the verified speed setting: 1,650 CFM at 0.5, 1,320 at 0.8, 1,150 at 0.9. We are moving about 1,150 CFM on a 4 ton system. He divides: 287 CFM per ton against a 350 floor. This coil is starving, and notice what the gauges would show, low suction, looks like low charge. The last company was about to charge a healthy circuit.
ON-SCREEN: Return minus 0.48. Supply plus 0.42. TESP = 0.90 in WC, rated 0.5
ON-SCREEN: Fan table: 1,150 CFM at 0.9. 1,150 / 4 tons = 287 per ton. STARVED
Beat 2: Reading the split before drilling anything else (2:00 to 2:45)
Darrel circles the two numbers on the tablet. Before I drill another hole, the split already talks: 0.48 of this 0.90 is on the return side. More than half the total on the suction side of the blower. So the hunt list is return-side: the filter, the return grille, the return duct. The supply side's 0.42 is high too, but it includes the coil, and we will separate that in a minute. He writes the prediction on the tablet and says it to camera: I am writing down where I think this is going before I look. That is the discipline. The numbers make the prediction, not the gut.
ON-SCREEN: The split: 0.48 return vs 0.42 supply. Return-heavy. Hunt the suction side
Beat 3: Port A, the filter gets separated from the return path (2:45 to 4:15)
Darrel marks and drills Port A in the return drop upstream of the filter rack, narrating the drill rules again briefly: know what is behind the metal, drill stop on. Probe in, tip into the oncoming air. Screen reads minus 0.36. The math on camera: the filter's cost is B minus A, 0.48 minus 0.36, just 0.12. He pulls the filter and shows it: a reasonably clean MERV 8 pleat. Twelve hundredths is a fair price for this filter, the filter is innocent. But look at what is left: the return path alone, the grille and the duct between it and this rack, is eating 0.36 against a budget of about 0.10. Something between the return grille and this cabinet is crushed, blocked, or way too small. That is not a maybe. That is arithmetic.
ON-SCREEN: Port A: minus 0.36. Filter drop = 0.48 minus 0.36 = 0.12. FILTER INNOCENT
ON-SCREEN: Return path alone = 0.36 vs 0.10 budget. THE SMOKING GUN
Beat 4: Port D, clearing the coil while we are here (4:15 to 5:45)
Darrel drills Port D in the supply plenum above the cased coil. Reading: plus 0.18. Coil drop is C minus D, 0.42 minus 0.18, 0.24. Second camera gets the coil manufacturer data sheet: published wet drop 0.22 at rated airflow. Measured 0.24 against published 0.22, this coil is clean, and he stacks the corroborating evidence fast: temperature probes show a 21 F split, inside the 18 to 22 window, and through the blower compartment with the mirror he shows the coil's entering face, no mat. He summarizes the whole map on the tablet: return path 0.36, filter 0.12, coil 0.24, supply duct 0.18. Adds them: 0.90. The map always has to add back to the TESP. One line item broke its budget, and we are going to go find it.
ON-SCREEN: Port D: plus 0.18. Coil drop = 0.42 minus 0.18 = 0.24 vs 0.22 published. COIL CLEAN
ON-SCREEN: THE MAP: return 0.36 | filter 0.12 | coil 0.24 | supply 0.18 = 0.90
Beat 5: While the panel is off, the wheel check (5:45 to 6:30)
Blower compartment open. Darrel rakes the flashlight across the blower wheel blades and pulls the camera in tight. These cups are clean metal with crisp edges, this wheel grips air. He holds up a photo of a dust-loaded wheel for contrast: smooth gray crescents, and a wheel like that gives up 20 to 30 percent of its airflow at the same RPM. Here is the trap I want you to remember: a bald wheel LOWERS static. If this system had read 0.45 with weak airflow, the ducts would have looked fine and the wheel would have been the story, and the fan table would have lied to me because it assumes a clean wheel. High and low are both findings. Only normal static with normal airflow is good news.
ON-SCREEN: Clean cups = grip. Dust-cupped blades = up to 20 to 30 percent CFM gone
ON-SCREEN: Low TESP + low airflow = blower problem, and the fan table lies
Beat 6: Into the attic, following the number (6:30 to 8:15)
Attic sequence, headlamp and kneeboards. Darrel narrates the geography: the return grille is in the hallway ceiling, the return flex runs maybe twenty feet across the attic to the furnace platform. He walks the run with the flashlight. Halfway along, the find: the return flex passes beside the storage decking by the hatch, and stacked plastic bins have slumped against it, crushing it to less than half its diameter. He puts his hand in the dent: this flex has baked up here through a lot of Phoenix summers, it is stiff, and it did not bounce back, it just stayed crushed. He moves the bins, supports and re-rounds the run, shows that the liner recovers most of its shape, and states the honest call: this section is heat-aged. Today it re-rounds and we will prove the airflow came back. If it will not hold shape, the right repair is replacing this section, and that gets scoped honestly, not taped.
ON-SCREEN: Crushed return flex at the storage decking. The 0.36 in WC, in the flesh
ON-SCREEN: Phoenix flex bakes at 140 to 160 F. Crushed flex stays crushed
Beat 7: The after numbers, the only proof that counts (8:15 to 10:00)
Back at the furnace. Probes back in the same ports. Return now reads minus 0.22, Port A reads minus 0.08. The return path went from 0.36 to 0.08. Supply reads plus 0.46, and Darrel flags it before anyone asks: notice the supply went UP from 0.42, that is not a problem, that is physics. We are moving more air now, and more air costs more pressure everywhere. Which is exactly why you never judge a repair by TESP alone. New total: 0.22 plus 0.46, 0.68. Fan table trace: 1,560 at 0.6, 1,450 at 0.7, so 0.68 lands around 1,470 CFM. Divide by 4: about 368 per ton. From 287 to 368, starved to inside the window, and he writes both number sets side by side on the tablet for the close-out photo. THAT is a closed case. Not "I moved some boxes." Before: 0.90 and 287 per ton. After: 0.68 and 368 per ton.
ON-SCREEN: After: return minus 0.22, Port A minus 0.08, supply plus 0.46. TESP = 0.68
ON-SCREEN: Fan table: about 1,470 CFM = 368 per ton. BEFORE 287 / AFTER 368
ON-SCREEN: Supply rose 0.42 to 0.46 because MORE AIR IS MOVING. Expected
Beat 8: What the gauges would have done to this call (10:00 to 11:15)
Darrel closes panels and talks straight to camera. Rewind this call and give it to a tech who skips the air. Low suction on the gauges. Subcooling would have read normal, but only if they checked it and knew what it meant from D24. They add refrigerant, the suction comes up a little, the house still cannot catch up, and now the system is overcharged with a crushed return still in the attic. Next month the coil freezes. Next year, after enough flooded starts, the compressor dies, and somebody calls it old age. That whole chain hangs off one crushed piece of flex and one static reading nobody took. He caps all four ports with plugs, photographs the locations and the repaired run, and states the close-out: as-found map, the find, the repair, the after map, all of it into the job record with the photos.
ON-SCREEN: The chain: restriction, low CFM, fake low charge, freeze, floodback, dead compressor
ON-SCREEN: Cap all four ports. Photograph everything. Both number sets in the record
OUTRO (11:15 to 11:30)
One ugly number, four ports, one map, one trip to the attic, and a before and after that closes the case. Your practical for this module is exactly this hunt: I will plant one airflow fault, and your job is to find it with numbers, fix it, and prove the fix with the after reading. The map does the finding. You just have to draw it.
ON-SCREEN: D25 practical: map it, find it, fix it, prove it with the after number