INTRO
0:00-0:30
[ON-SCREEN] D28-sequence-fault-ladder.svg, full frame.
VOICEOVER: Here is the best-kept secret in heating: a gas furnace is the easiest machine in the trade to diagnose, because it diagnoses itself. The board runs the same strict sequence every cycle, every step has to prove itself, and the furnace stops exactly at the step that failed. You learned the sequence in C18: call, inducer, prove, glow, gas, flame, blower. Today that sequence becomes a ladder, and the rung where the furnace stalls names your fault family before you ever open a meter case.
MAIN
[0:30-1:30] Reading the ladder
[ON-SCREEN] D28-sequence-fault-ladder.svg, camera moves down the rungs as each is named.
VOICEOVER: Watch one complete cycle, eyes and ears, and ask one question: where did it die? Nothing at all happens: that is control power, and you walk the 24 volt circuit exactly the way D23 taught you. The inducer runs forever and nothing follows: pressure switch family. The igniter never glows: ignition hardware. It lights, burns a few seconds, and drops out: flame sensing. It runs fine, then quits mid-cycle and comes back later: limit family, which means airflow. And a tripped rollout switch is not a rung at all, it is a red flag over the whole machine, because flame escaped the burner box. One observed cycle, one stall point, one family. Then, and only then, instruments come out.
[1:30-2:20] Ignition and flame sensing numbers
[ON-SCREEN] D28-flame-sensing-circuit.svg. Inset b-roll: oxide-coated rod, then a meter reading microamps.
VOICEOVER: Ignition first. A hot surface igniter gets three tests: eyes for cracks, ohms cold, and volts under command. Silicon carbide elements commonly read forty to ninety ohms cold; open means dead, and line voltage at the plug with no glow condemns the element no matter what it reads. Flame sensing is the microamp game. One paragraph of theory from C18: flame conducts, the rod and the grounded burner are wildly different sizes, so the flame turns AC into a tiny DC current, and DC microamps mean real fire. Healthy is typically one to six microamps. The discipline is clean and retest: measure dirty, polish with fine abrasive, measure again, record both numbers. A rod that goes from point seven to four and a half just got fixed, and you can prove it.
[2:20-3:10] The pressure switch is the messenger
[ON-SCREEN] D28-pressure-switch-diagnostics.svg. Inset b-roll: manometer teed into the switch hose.
VOICEOVER: Now the most expensive habit in furnace work: replacing an open pressure switch. The switch has one job, refusing to close until real draft proves the vent path. So an open switch is a report about the draft, not a confession from the part. Tee a manometer into the hose and read the actual draft against the rating printed on the switch body. Draft beats the rating and the switch stays open: the switch really is bad, and you earned the right to say so. Draft falls short: the switch is innocent, and your fault is the inducer, the vent, the hose, or, on a ninety percent furnace, the condensate path, because a plugged trap backs water into that switch line all winter long.
[3:10-3:50] Limits, rollouts, and the line you never cross
[ON-SCREEN] D28-limit-and-rollout-map.svg, danger zone highlighted.
VOICEOVER: A tripping high limit is the furnace shouting airflow. That is failure pattern three wearing heating clothes: filter, blower, ducts, registers, the summer-dirty A-coil, confirmed with temperature rise against the data plate. Check for overfiring with a manometer too, because high manifold pressure trips limits just the same. Rollout switches are different: a rollout trip means flame physically left the burner box, and it is manual reset on purpose, so a human has to come look. Find the cause before any reset. And hear the rule once more, because it has no exceptions: never bypass a safety to get heat on. Not for ten minutes, not for a cold night, not ever.
[3:50-4:15] Combustion numbers and the evidence bar
[ON-SCREEN] D28-combustion-analysis-targets.svg.
VOICEOVER: The analyzer puts chemistry on the fire: oxygen around six to nine percent on an eighty percent furnace, CO air-free under a hundred ppm when healthy, appliance off above four hundred. And heat exchanger condemnation requires evidence you can show: a scoped crack, flames that disturb when the blower starts, or combustion numbers that shift at blower start. No evidence, no condemnation. That single rule is the difference between a professional and a parts salesman.
OUTRO
4:15-4:30
[ON-SCREEN] D28-sequence-fault-ladder.svg returns, full frame.
VOICEOVER: Watch one cycle. Name the stall. Prove the fault with a number. In the demo video, Darrel runs this ladder on a furnace with staged faults, and in your practical you will do the same. The furnace tells you where it hurts. Learn to listen.