INTRO (0:00 to 0:30)
Darrel at the furnace, blower running: This system has a complaint behind it, not cooling like it used to. Most techs would walk straight past this furnace, out to the condenser, and put gauges on it. We are not going to touch the gauges. In the next ten minutes I am going to take this system's blood pressure, turn that number into actual airflow using the paperwork that came with this furnace, and show you why the gauges would have lied to us today.
ON-SCREEN: C12 Field Demo: static pressure to CFM, start to finish
MAIN (0:30 to 10:45)
Beat 1: The walkaround before any tool comes out (0:30 to 1:30)
Darrel narrates what he checks before drilling anything: system running in cooling at least 15 minutes so the coil is wet, panels in place, registers open in the house. He opens the filter rack and holds the filter up to the camera without removing it yet: a 1 inch MERV 13 pleat, visibly gray with dust. He says: suspect number one, but we do not guess, we measure. The filter goes back in. We measure the system as we found it, then we fix and measure again. That before and after is the evidence.
ON-SCREEN: Measure as found. Fix. Measure again.
Beat 2: Mapping the cabinet before drilling (1:30 to 3:00)
Close-up sequence. Darrel pulls the blower compartment panel and uses the flashlight and mirror to show where things live: blower below, heat exchanger above it, cased coil sitting on top of the furnace. He narrates the two drilling disasters: a bit through the evaporator coil or through the condensate pan. He marks two drill points with a marker. Return side: the return drop, downstream of the filter, just before the blower cabinet. Supply side: the side of the casing between the furnace outlet and the coil, and he explains why there and not above the coil: this furnace's fan table was written without the coil, so the coil counts as external resistance, and drilling above it would miss the coil's pressure drop entirely. He fits the drill stop on the bit, shows it to camera, and drills both 3/8 inch ports.
ON-SCREEN: Return port: after the filter, before the blower. Supply port: between furnace and coil
ON-SCREEN: Drill stop on. Know what is behind the metal. Never drill into coil or pan
Beat 3: Probes in, signs explained (3:00 to 4:30)
Darrel zeroes the manometer on camera, then inserts the static tip into the return port, tip pointed into the oncoming air, away from the blower. The screen reads minus 0.31. He explains the sign: the blower is sucking on this side, so the duct is below room pressure, negative. Probe moves to the supply port, tip pointed back toward the equipment, into the air arriving from the blower. Screen reads plus 0.51. Positive, because this side is being pushed. He lets each reading settle for a slow ten seconds on camera and explains that a bouncing number means turbulence: move an inch away from a takeoff or a turn and let it settle.
ON-SCREEN: Return: minus 0.31 in WC. Supply: plus 0.51 in WC. Tip always faces the oncoming air
Beat 4: The TESP math and the verdict on the number (4:30 to 5:30)
Tablet in frame. Darrel writes: 0.31 plus 0.51 equals 0.82 inches of water column total external static. He taps the furnace nameplate: this blower is rated to deliver its airflow at 0.5. He gives the medical line: if your blood pressure came back this far over the design number, your doctor would not shrug. Then he reads the split: 0.31 of it is on the return side. That is nearly 40 percent of the total on the suction side of this blower, and the filter lives on that side. The split points the finger before we ever open a panel.
ON-SCREEN: TESP = 0.31 + 0.51 = 0.82 in WC. Rated: 0.5. The return side is carrying 0.31 of it
Beat 5: Finding the fan table and reading it (5:30 to 7:00)
Darrel pulls the installer manual from the literature pocket on the furnace door, flips to the blower performance table, and the second camera gets a clean overhead of the page. He narrates the three steps: one, confirm which speed the cooling airflow is actually set to, and he shows the speed tap or dip switch setting at the control board. Two, find that row. Three, run across to the measured static. He traces with his finger: 1,240 CFM at 0.5. At 0.7, 1,090. At 0.8, 985. At 0.9, 860. Our reading is 0.82, so we are a hair under 985, call it about 975 CFM. He does the division out loud: 975 divided by 3 tons is about 325 CFM per ton. The floor is 350. This coil is starving.
ON-SCREEN: fan table overhead, finger trace. 0.8 in WC = 985 CFM. At 0.82, about 975 CFM
ON-SCREEN: 975 / 3 tons = 325 CFM per ton. Below the 350 floor
Beat 6: What the gauges would have said (7:00 to 8:00)
Darrel, standing, talking straight to camera: here is why this matters. Starve a coil of air and there is not enough heat to boil the refrigerant. Suction pressure drops. A tech who skipped what we just did connects gauges, sees low suction, and adds refrigerant to a system with a perfect charge. Now it is overcharged and still starving, the complaint is still there, and in a week the coil freezes into a block of ice. That tech created the second problem and never found the first. Airflow first. Refrigerant second. Every time.
ON-SCREEN: Low airflow imitates low charge. AIRFLOW FIRST. REFRIGERANT SECOND.
Beat 7: Fix and re-measure (8:00 to 9:45)
Darrel pulls the loaded MERV 13 pleat, holds it next to the correct replacement, and swaps it. Probes back in, same ports. Return now reads minus 0.14. He does the delta on camera: that filter was costing us 0.17 inches all by itself. New TESP: 0.14 plus 0.51 equals 0.65. Back to the fan table: 1,175 at 0.6, 1,090 at 0.7, so 0.65 lands around 1,130 CFM. Divide by 3: about 377 per ton. Inside the window. Then the honest close: this system passes today, but 0.51 on the supply side and a 1 inch rack on a 3 ton system means the permanent recommendation is more filter surface area, a media cabinet, and that goes in the notes with both sets of readings. He caps both test ports with plastic plugs and photographs their locations.
ON-SCREEN: New return: minus 0.14. TESP = 0.65 in WC. Fan table: about 1,130 CFM = 377 per ton. PASS
ON-SCREEN: Dirty filter cost: 0.17 in WC. Cap the ports. Photograph the locations
Beat 8: The cross-check you get for free in heating season (9:45 to 10:45)
Quick whiteboard beat at the furnace. Darrel explains the temperature rise method as the no-drill backup on any gas furnace: the furnace adds a known amount of heat, so airflow falls out of the math. This furnace is 80,000 input at 80 percent, so 64,000 output. If return air is 70 and supply is 120, the rise is 50. CFM equals 64,000 divided by 1.08 times 50, which is 64,000 over 54, about 1,185 CFM. He points at the nameplate rise range: if your measured rise is above the range, the airflow is low, and the furnace is telling you the same story the manometer would. Two methods, one truth. When they disagree badly, something is wrong with one of your measurements, and you find out which before you write anything down.
ON-SCREEN: CFM = output / (1.08 x rise). 64,000 / 54 = about 1,185 CFM
ON-SCREEN: Rise above nameplate range = low airflow
OUTRO (10:45 to 11:00)
Two holes, two readings, one addition, one table. That is the whole measurement, and it changed the entire diagnosis on this system before the gauges ever left the truck. Your practical for this module is doing exactly what I did: drill clean ports, measure TESP, find the fan table, and report the CFM with a verdict. This reading happens on every maintenance and every diagnostic call you will ever run for us. Make it a reflex.
ON-SCREEN: C12 practical: drill, measure, fan table, report CFM with a verdict