Every gas furnace runs the same play, every cycle, in the same order. Techs learn the sequence of operation early because it’s the backbone of diagnosis: find the step where the sequence stopped, and you’ve found the neighborhood of the fault.
What’s less discussed is that most of the sequence is observable from outside the cabinet — each step has a mechanical signature. That’s worth walking through, because it’s how a dispatcher can know where a cycle died before anyone opens a panel.
The sequence, step by step
1. The heat call. The thermostat closes the circuit between R and W. Nothing moves yet — but this is the moment the furnace is asked to do something, and everything after is the furnace answering. If you’re watching the W wire, you have a timestamp.
2. Inducer prepurge. The control board starts the draft inducer to clear the heat exchanger and establish combustion airflow. From outside: a small motor spins up and runs alone for a short purge. This is usually the first mechanical evidence that the furnace heard the call.
3. Pressure switch proves the draft. The inducer’s draft pulls in a pressure switch, telling the board it’s safe to ignite. Nothing audible changes — but if the switch never proves (blocked vent, failed switch, weak inducer), the sequence stalls right here: an inducer running with nothing following it. That stall pattern is itself a diagnosis.
4. Ignition. Depending on the furnace: a hot-surface igniter warms up, or a spark igniter fires. Then the gas valve opens.
5. Flame proving. The flame sensor must confirm flame within a short proving window, or the board closes the gas valve and retries. This is the step behind one of the most common no-heat patterns in the field: the flame lights — combustion is briefly there — then drops out seconds later because a dirty or failed sensor never proved it. The board retries a few times, then locks out. From outside, that reads as an unmistakable rhythm: flame on, flame gone, again and again, then silence.
6. Blower on. After a heat-exchanger warm-up delay, the circulating blower starts — the biggest mechanical event of the cycle, and the moment the house actually starts getting heat.
7. Steady run, then shutdown. The call satisfies, the gas valve closes, the inducer post-purges, and the blower runs off the remaining heat.
Reading a failure as a missing step
Flip the sequence around and it becomes a diagnostic checklist:
- Heat call with nothing after it — the furnace never answered. Board, power, or safety chain.
- Inducer runs, nothing follows — the draft never proved. Pressure switch, venting, or a rollout/limit holding the sequence.
- Inducer runs, no flame ever appears — ignition failed. Most often the igniter.
- Flame appears, drops within seconds, retries, lockout — flame proving failed. Usually the flame sensor.
- Cycle completes but repeats constantly — short cycling. Airflow, limit, or oversizing territory.
Notice what makes this list work: you don’t need to see inside the control board. The order and timing of observable events — heat call, inducer, flame, blower — carries the diagnosis.
That’s the idea FurnaceWatch is built on. A sensor on the cabinet follows exactly this sequence on every cycle, and when a cycle stops at one of these steps, your office gets the failure by name — before the customer calls. If you run a service company and want to see what that looks like on a real fleet, book a demo.