Sequence of Operation — Forced Air Furnace

The forced-air gas furnace is the most common residential heating system in the US. Its sequence of operation is the most-referenced diagnostic framework in HVAC service — knowing it stage by stage, with measurement expectations at each step, lets you diagnose any no-heat or misoperation call by comparing observed behavior to expected behavior.

This chapter covers the standard integrated-control single-stage gas furnace. Two-stage and modulating units have added complexity at stages 4–6, but the core sequence is identical.

The sequence

Stage 1 — Call for heat

Thermostat closes its internal heat contact. 24 VAC appears at the control board’s W terminal. Board’s microcontroller sees the call and begins the ignition sequence.

Measure: 24 VAC between R and W on the control board. If missing: Thermostat, cable, or 24V supply fault.

Stage 2 — Safety string verification

The board internally checks continuity through the safety string — rollout switch, high limit, flame rollout, auxiliary limit. All must be closed. If any is open, the board either flags an error code or simply doesn’t proceed.

Measure: Continuity across the safety string, or voltage presence at the inducer relay coil input. If missing: One of the safety limits is open. Walk the string with the meter to find which.

Stage 3 — Inducer starts

Board energizes inducer relay. Relay closes, sending 120 VAC to the inducer motor. Motor spins up, pulling a vacuum on the heat exchanger and flue.

Measure: 120 VAC at inducer motor leads during this stage. If missing: Relay failed, wiring fault, or motor failed.

Stage 4 — Pressure switch proves draft

After a brief spin-up delay, the inducer has established draft. The pressure switch (connected to the heat exchanger via a sensing hose) closes when vacuum exceeds the switch’s setpoint. The closed switch signals the board that draft is proven.

Measure: Pressure switch closed (continuity present), or voltage passes through PS. If not closing: Inadequate draft (weak inducer, blocked flue, disconnected hose), pressure switch itself failed, or sensing hose fault.

Stage 5 — HSI warmup

Board energizes the hot-surface igniter (HSI) via a line-voltage output. Typical warmup: 15–45 seconds depending on HSI type. Spark-ignition systems start sparking instead.

Measure: 120 VAC at HSI leads during warmup; HSI visibly glows orange-red. If missing: HSI output relay failed, HSI element failed (open or shorted), or wiring fault.

Stage 6 — Gas valve opens

At the end of HSI warmup, the board energizes the main gas valve coil. Gas begins flowing to the burner. Ignition occurs essentially immediately on contact with the hot HSI (or the active spark on spark-ignition systems).

Measure: 24 VAC at the gas valve’s MV-MVCOM terminals during the ignition window. If missing: Board output failed, or internal module-lockout condition.

Stage 7 — Flame prove

Within approximately 4–10 seconds of gas valve energizing, flame must be detected by the flame sensor rod (rectification signal above the module’s threshold). If detected, the board holds the gas valve open and the ignition trial ends successfully. If not detected, the gas valve is closed and another trial is attempted (or lockout occurs after 3–5 failed trials).

Measure: Flame signal ≥ 0.5–1.0 µA DC (typical minimum) during steady operation. If missing or low: Dirty flame sensor, cracked porcelain, ground fault, reversed polarity, marginal combustion.

Stage 8 — Blower on (heat-on delay)

After flame has been burning for the heat-on delay (typically 30–45 seconds — long enough for the heat exchanger to warm up), the board energizes the blower relay. Blower motor starts on its heating-speed tap. Warm air now circulates through the house.

Measure: 120 VAC at blower motor leads after the heat-on delay; blower running. If missing: Blower relay failed, blower motor issue, ECM communication fault.

Stage 9 — Steady-state operation

System runs at full capacity. Flame is lit, inducer running, blower running. Thermostat remains calling. House temperature rises.

Measure: All voltages as expected; flame signal steady; no error codes.

Stage 10 — Thermostat satisfied

Room temperature reaches setpoint. Thermostat opens its W contact. Board sees call removed. Gas valve is de-energized; flame goes out.

Stage 11 — Post-purge, heat-off delay

Inducer continues running for a post-purge period (typically 15–30 seconds) to clear combustion products from the heat exchanger. Blower continues running for the heat-off delay (typically 90–180 seconds) to scavenge residual heat from the heat exchanger and deliver it to the house.

Measure: Inducer still running briefly; blower continues running.

Stage 12 — Standby

Post-purge and heat-off delays complete. Inducer stops. Blower stops. System returns to standby, waiting for the next call.

SOO as diagnostic framework

Once you know the SOO by heart, every no-heat call becomes a matter of identifying the stage at which the sequence broke down. Watch a complete call attempt:

This mapping is exactly what the interactive flowchart in Chapter 53 is built around. Understanding the SOO lets you use the flowchart effectively — or skip it when the diagnosis is obvious from the symptom.

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Check your understanding

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01A gas furnace's inducer runs, the pressure switch closes, the HSI glows, and the gas valve energizes — you can hear it click open — but no flame appears. What's the most likely cause?

APressure switch failedBHSI failedCGas is not arriving at the burner — low gas pressure, closed valve somewhere upstream, or a gas supply issueDThermostat failed

02During stage 8 of the SOO (blower-on delay), the blower never starts even after the expected delay. The burner continues firing. What is the most productive first measurement?

AMeasure gas manifold pressureBMeasure voltage at the blower motor — if 120V is absent, the blower relay output on the board has failed or the wiring has an open; if 120V is present, the motor has failedCReplace the thermostatDCheck flame signal

03Why does the SOO include a post-purge (inducer continues after burner shuts off) and heat-off delay (blower continues after burner shuts off)?

ACode requirementBPost-purge clears combustion products from the heat exchanger after burner shutoff; heat-off delay scavenges residual heat from the hot heat exchanger into the house, improving efficiency and preventing thermal stressCThey're optional featuresDTo make the house warmer

Before you close the chapter

You should now be able to walk through a complete gas furnace SOO stage by stage, identify what to measure at each step, and map a symptom to the stage where the sequence broke down. The next chapter covers air-conditioning SOO.