What it is

The sequence of operation is just the ordered list of things a furnace does between "thermostat closes W" and "burners are running with the blower moving heat into the house." Every modern furnace runs the same basic play in the same order, and the control board is the referee. If you know the sequence cold, you can walk up to a dead furnace, figure out where in the chain it quit, and you're already 80% of the way to the fault.

This covers a typical single-stage furnace with a hot surface igniter and an integrated control board. Two-stage and modulating units add steps, but the spine is the same.

How it works

The board won't skip a step or do them out of order. Each step has to satisfy a safety before the next one is allowed. That's the whole point — the furnace proves the inducer is moving air before it lets gas flow, and it proves there's a flame before it keeps gas flowing. Miss a proof and the board parks the sequence or locks out.

In the field

Here's the play, start to finish:

  1. Call for heat. Thermostat closes the circuit between R and W. 24V hits the board's W terminal. The board wakes up.
  1. Inducer (draft motor) energizes. The board runs the combustion inducer first to clear any leftover combustibles from the heat exchanger (pre-purge) and to establish a draft. Listen for it spinning up.
  1. Pressure switch proves. The running inducer pulls a vacuum on the pressure switch hose. The normally-open pressure switch closes, telling the board "yes, the inducer is actually moving air, not just humming." The board will NOT proceed until it sees this switch closed.
  1. Igniter warm-up. With draft proven, the board sends 120V to the hot surface igniter. It glows for roughly 15–45 seconds depending on the model. This is the warm-up the board uses to be sure there's a hot ignition source before gas shows up.
  1. Gas valve opens. The board energizes the main gas valve. Gas flows to the burners and lights off the glowing igniter (or a spark/pilot on other designs).
  1. Flame proving. The flame sensor (a single rod sitting in the burner flame) passes a tiny microamp current through the flame to ground. The board reads that current and says "I have flame." This has to happen within the trial-for-ignition window — usually about 4–7 seconds. No flame signal in that window and the valve closes.
  1. Igniter de-energizes. Once flame is proven, the hot surface igniter shuts off. Its only job was to light the gas.
  1. Blower (heat) starts on a delay. The board waits a fixed delay — commonly 30–45 seconds — so the heat exchanger warms up before the indoor blower kicks on. That delay keeps the supply air from blowing cold on startup. The blower runs on the HEAT speed tap.
  1. Steady run. Burners and blower run together, dumping heat into the house, until the thermostat is satisfied.
  1. Shutdown. Thermostat opens W. Gas valve closes immediately — flame out. Inducer runs a short post-purge to clear the exchanger, then stops. The blower keeps running on its own off-delay (commonly 60–180 seconds) to pull the remaining heat out of the exchanger before it stops too.

Normal values & targets

  • Inducer-to-pressure-switch proof: switch should close within a couple seconds of the inducer reaching speed.
  • Igniter warm-up: ~15–45 sec (model dependent).
  • Trial for ignition (flame must prove): ~4–7 sec after the valve opens.
  • Blower-on (heat) delay: ~30–45 sec after ignition.
  • Blower-off delay: ~60–180 sec, often dip-switch selectable.
  • Flame-sense current: roughly 1–6 µA DC on a healthy rod; below ~0.5–1 µA many boards drop the flame.

Common faults & what they mean

  • Inducer runs, then nothing. Pressure switch isn't closing. Check the hose (cracked, plugged with condensate), the inducer port, the switch itself, and a blocked or restricted vent. The board is doing exactly what it should — refusing to fire without proven draft.
  • Igniter glows, gas never lights / lights then drops out. If it lights and dies after a few seconds, suspect flame sensing (dirty rod, bad ground, weak microamps). If it never lights, suspect gas supply, the valve, or the igniter position.
  • Furnace fires but blower never comes on. Blower-on delay expired with no fan = check the board's heat-blower output, the blower motor/cap, and the limit. Some boards won't release the blower if the limit is open.
  • Short cycling on the limit. Burners light, then cut out on high temp before the blower starts (or with weak airflow). Dirty filter, closed registers, undersized return, or a slow/failed blower — the exchanger overheats and the high-limit opens. The limit is protecting the exchanger; fix the airflow, don't jumper the limit.
  • Lockout after 3 tries. Board attempted ignition the allowed number of times and gave up. The cause is upstream (no flame proven) — reset and watch which step actually fails.

Tech tips & gotchas

  • Don't troubleshoot by feel — watch the sequence and note the exact step it dies. The step that fails IS the diagnosis. "It quits right after the inducer starts" instantly points you at the pressure switch circuit.
  • The pressure switch is normally OPEN at rest and closes on draft. If you find it closed with the inducer off, that's a stuck switch — the board may flag it and refuse to start as a safety check.
  • A furnace that lights and runs but kills the flame in ~4–10 seconds, over and over, is almost always flame sensing — not the gas valve. Pull and clean the flame rod with light abrasive before you condemn anything.
  • The blower off-delay running after shutdown is normal and good — it's recovering heat. Homeowners think it's "stuck on." It isn't.

Safety / code notes

  • Never bypass or jumper a pressure switch, rollout switch, or high-limit to "make it run." Those proofs exist to prevent the furnace from operating without draft or with a blocked exchanger — defeating them can spill combustion products into the house.
  • Vent sizing and termination follow the appliance listing and the venting categories under the fuel-gas code (IFGC venting provisions). Combustion-air openings are sized per the fuel-gas/mechanical code combustion-air sections.
  • Any time you suspect spillage or a blocked flue, verify draft and check for CO with an analyzer before returning the unit to service.