Gas Furnace Sequence of Operations: Step-by-Step Guide, Safety, Timing, and Troubleshooting

Knowing the gas furnace sequence of operations helps pinpoint problems quickly and safely. This guide walks through each step—from thermostat call to post‑purge—plus safety controls, timing, airflow, and staging differences. It also covers troubleshooting tips for no-heat, short cycling, ignition lockouts, and airflow faults. Whether dealing with hot surface ignition, direct spark, or intermittent pilot, understanding the sequence of operations gas furnace logic makes diagnostics faster and repairs more accurate.

What The Gas Furnace Sequence Of Operations Looks Like

The internal furnace control board manages a repeatable ignition sequence. While details vary by brand, the steps below reflect most modern, 80% and 90+% AFUE furnaces.

1. Call For Heat (W Signal): The thermostat closes W to R, sending 24 VAC to the control board. The board confirms safeties are normal before proceeding.
2. Inducer Motor Starts: The inducer clears residual gases from the heat exchanger and establishes draft. Expect a brief pre‑purge.
3. Pressure Switch Proves Draft: Negative pressure closes the switch. If it does not close within a set time, the board stops and retries or locks out.
4. Ignition Warm‑Up Or Spark: For hot surface ignition (HSI), the silicon nitride or carbide ignitor heats to a glowing orange. For direct spark ignition (DSI), the ignitor sparks at the burner.
5. Gas Valve Opens: After proof‑of‑draft and ignitor readiness, the board energizes the gas valve. Gas flows to the burners.
6. Flame Establishes And Is Proven: Flame rectification through a flame sensor sends a microamp DC signal to the board. If not proven promptly, the board shuts gas and retries.
7. Blower On Delay: After flame is stable, the indoor blower starts following a programmed time or temperature rise. This prevents blowing cold air.
8. Heating Run: The furnace maintains flame until the thermostat is satisfied. Two‑stage or modulating units may vary firing rate during this period.
9. Call Satisfied—Gas Off: The thermostat opens W. The board closes the gas valve and stops ignition.
10. Blower Off Delay: The blower continues briefly to extract remaining heat and protect the heat exchanger.
11. Inducer Post‑Purge (Some Models): The inducer may run briefly to clear the exchanger and vent.

Key idea: Each stage “proves” the preceding condition via a switch or sensor. Any failed proof pauses or aborts the sequence for safety.

Safety Controls That Govern The Sequence

Furnaces use redundant safeties to prevent unsafe combustion and overheating. Understanding these helps explain lockouts and fault codes.

• Pressure Switch: Confirms proper draft through the heat exchanger and vent. Stuck open or stuck closed conditions halt ignition.
• Flame Sensor: Uses flame rectification to confirm flame after the gas valve opens. Typical signal is 2–6 μA DC. Poor signal triggers flame failure.
• High Limit Switch: Opens if supply air temperature exceeds design, usually from low airflow. Repeated trips indicate filter, blower, or duct issues.
• Rollout Switch(es): Manual‑reset devices near burners that trip if flame “rolls out” of the firebox. This indicates a serious venting or exchanger issue.
• Auxiliary Limits: Some units include additional temperature or blocked‑vent switches for enhanced protection.
• Blocked Condensate Switch (90+%): Opens when the condensate drain is restricted. Prevents water backup into the heat exchanger.
• Gas Valve Redundancy: Many valves contain dual internal solenoids for fail‑safe operation.
• Control Board Lockout: After a preset number of failed ignition attempts, the board enters soft or hard lockout and flashes a diagnostic code.

Do not bypass safeties. Any diagnostic jumpers must be temporary and used only by trained personnel during controlled testing.

Variations By Ignition Type And Staging

Hot Surface Ignition (HSI)

HSI is common on modern furnaces. The control board heats the silicon nitride or carbide element for several seconds before opening the gas valve. The flame must establish quickly to prevent ignitor damage. Do not handle ignitors with bare hands; oils reduce lifespan.

Direct Spark Ignition (DSI)

DSI produces a spark directly at the main burner and opens the gas valve simultaneously or in tight sequence. It avoids a fragile hot surface element but requires proper grounding and electrode alignment.

Intermittent Pilot (IP) And Older Standing Pilot

Intermittent pilot lights a small pilot flame with spark, then proves pilot flame before opening the main valve. Older standing pilot systems use a thermocouple for flame safety. Many have been upgraded or replaced for efficiency and safety.

Single‑Stage, Two‑Stage, And Modulating Sequences

• Single‑Stage: Runs at full fire whenever heating is demanded. Simpler sequence and controls.
• Two‑Stage: Starts in low fire (W1), then steps to high fire (W2) if demand persists. Staging may be controlled by thermostat or furnace logic based on run time and temperature.
• Modulating: Adjusts gas valve and blower continuously to match load. Communicating controls manage the sequence, airflow, and firing rate for precise comfort.

In staged/modulating units, blower speed and ignition timing may differ between stages to maintain a target temperature rise and quiet operation.

Typical Timing And Airflow Numbers

Exact timings vary by model. These ranges are typical references for many residential furnaces.

Airflow should keep temperature rise within the furnace nameplate range. Use the heat equation to estimate required CFM.

Formula: CFM ≈ Output BTU / (1.08 × Temperature Rise °F)

Always verify temperature rise at the supply and return plenums and compare to the nameplate range, commonly 35–70°F.

Troubleshooting The Sequence: Symptom-Driven Checks

No Response When Heat Is Called

• Likely causes: No 24 VAC at R, open door switch, blown low‑voltage fuse, broken thermostat wire, miswired smart thermostat lacking C wire.
• Checks: Verify 120 VAC to furnace and 24 VAC at R‑C. Jump R to W at the board. If it starts, the issue is upstream at the thermostat or wiring.

Inducer Runs But Ignition Never Starts

• Likely causes: Pressure switch not closing, blocked vent or intake, cracked or misrouted pressure tube, condensate trap blocked (90+%), weak inducer.
• Checks: Use a manometer at the pressure switch port. Inspect vent terminations for ice/debris. Clean the pressure port on the inducer housing.

Ignitor Glows Or Sparks, But No Flame

• Likely causes: Closed manual gas shutoff, failed gas valve, no gas supply, plugged orifices, insufficient inlet pressure.
• Checks: Confirm gas cock is parallel with pipe. Measure inlet/outlet gas pressure with a manometer. Verify 24 VAC reaches the gas valve during trial for ignition.

Flame Starts Then Drops Out After A Few Seconds

• Likely causes: Weak flame signal, dirty or mispositioned flame sensor, poor burner ground, low gas pressure, cross‑lighting issues.
• Checks: Clean the flame sensor with a non‑abrasive pad. Measure microamps in series; aim for 2–6 μA. Inspect burner carryover ports.

Burners Run But Blower Never Starts Or Trips High Limit

• Likely causes: Failed blower motor or relay, incorrect blower tap/ECM speed, clogged filter or coil, collapsed return duct.
• Checks: Command fan in “ON” mode at the thermostat. Measure motor amperage vs. nameplate. Inspect the evaporator coil for debris.

Short Cycling (Frequent Starts And Stops)

• Likely causes: Oversized furnace, high static pressure, dirty filter, restrictive registers, incorrect cycles‑per‑hour settings.
• Checks: Measure temperature rise. If above nameplate, increase blower speed or correct airflow restrictions. Review thermostat settings.

Random Lockouts Or Intermittent Heat

• Likely causes: Loose low‑voltage connections, marginal ignitor, wet or corroded board, intermittent pressure switch.
• Checks: Reseat Molex connectors. Inspect for water intrusion from a clogged condensate line. Perform wiggle tests on harnesses.

Tip: Watch the entire sequence and note where it stops. The failed “proof” at that point is your best diagnostic clue.

How To Safely Observe And Document The Furnace Sequence

Heating appliances produce combustion byproducts, including carbon monoxide. If there is any sign of venting issues or flame rollout, stop and involve a licensed pro immediately.

• Power And Access: Turn off power, remove the burner door, restore power, and call for heat. Keep hands clear of moving parts and flame.
• Record Timings: Note time stamps for inducer start, pressure switch close, ignitor glow, gas valve open, flame prove, blower on, and off‑delays.
• Measure Draft And Pressure: Use a manometer on the pressure switch port and verify the rated setpoint printed on the switch.
• Verify Flame Signal: Place the meter in microamp mode in series with the flame sensor lead. Compare to manufacturer specs.
• Check Temperature Rise: Measure return and supply temperatures after steady operation. Compare with nameplate rise to assess airflow.

Clear documentation makes intermittent issues easier to solve and supports warranty or service decisions.

Maintenance That Keeps The Sequence Running Right

• Filters: Replace on schedule. High‑MERV filters can raise static pressure; size the filter area accordingly.
• Flame Sensor: Clean annually with a fine pad; avoid sandpaper. Ensure good ground from burner to chassis.
• Condensate System (90+%): Flush trap, verify slope, and inspect for cracks. Clear vent screens to prevent pressure switch faults.
• Blower And Coil: Clean the blower wheel and evaporator coil. Airflow deficiencies drive high‑limit trips and short cycling.
• Vent/Intake: Check terminations for leaves, nests, or snow. Confirm proper clearances per the installation manual.
• Gas Train: Verify leak‑free connections with an approved leak detector. Confirm inlet pressure under load.
• Electrical: Tighten low‑voltage terminals and reseat harnesses. Inspect for heat‑stressed or discolored connectors.

Routine maintenance improves safety, efficiency, and comfort while minimizing nuisance lockouts.

Smart Thermostats, Controls, And The Sequence

Modern controls can influence the sequence through staging logic, fan profiles, and cycles per hour. Compatibility matters.

• C Wire: Smart thermostats typically require a common wire. Without it, the furnace board may see erratic W calls or nuisance resets.
• Staging Control: Two‑stage furnaces can be staged by the thermostat (W1/W2) or internally (time/temperature based). Coordinate settings to avoid rapid staging.
• Fan Profiles: ECM blowers can ramp gently at ignition to avoid flame disturbance and adjust speed to maintain temperature rise.
• Dehumidify/Comfort Modes: Some controls reduce blower speed to increase supply temperature or dehumidify. Verify modes align with duct capacity.
• Communicating Furnaces: Brand‑specific protocols manage the entire sequence. Use matched thermostats and updated firmware.

Tip: If heat issues start after a thermostat change, review wiring, system type selection, and cycles‑per‑hour settings first.

Common Fault Codes And What They Point To

Each brand’s blink codes differ, but the meaning is similar across many control boards.

• Pressure Switch Open: Inducer running but switch never proved. Check venting, condensate, hoses, and inducer ports.
• Ignition Lockout: Failed ignition after multiple trials. Suspect ignitor, gas supply, grounding, or control board.
• Flame Sensed With Gas Off: Possible stuck gas valve, shorted sensor, or stray flame.
• Limit Or Rollout Open: Overtemperature or unsafe combustion. Inspect airflow and heat exchanger before resetting.
• Polarity/Neutral Fault: Line/neutral reversed or poor ground. Correct wiring to ensure proper flame rectification.

Always consult the exact model’s service manual for code definitions, test points, and timing diagrams.

Quick Reference Table: Gas Furnace Sequence And Proofs

Safety And Compliance Considerations

Combustion appliances must be installed and serviced to code. Correct venting, gas pressure, and airflow are essential to safe operation.

• Codes: National Fuel Gas Code (NFPA 54) governs installation and venting requirements.
• Manufacturer Instructions: Installation and Service Instructions take precedence for model‑specific procedures and settings.
• Combustion Air: Ensure adequate combustion air per code or provide sealed combustion with proper intake sizing.
• CO Alarms: Install UL‑listed carbon monoxide alarms outside sleeping areas and on each floor.

If corrosion, heat damage, or suspected heat‑exchanger cracks are present, do not operate the furnace until fully inspected by a qualified technician.

Pro Tips To Speed Up Diagnostics

• Watch The LEDs: Read fault codes before cycling power. Photograph the code chart on the blower door for reference.
• Measure, Don’t Guess: Use a manometer, microamp meter, and thermometer. Document readings next to the rated values.
• Staging Logic: Disable W2 temporarily to see if problems only occur at high fire, pointing to airflow or gas supply limitations.
• Grounding Matters: Flame rectification needs clean ground. Tighten burner screws and verify neutral/ground integrity.
• Vent Ice: In cold snaps, inspect PVC terminations for frost. Clear restrictions to restore draft.

Frequently Asked Questions

Can A Dirty Filter Really Stop Ignition?

Yes. On marginal systems, reduced airflow increases heat exchanger temperatures, tripping the high limit during warm‑up. The control board may never reach the programmed blower‑on delay, appearing as an ignition fault.

What Microamp Value Is “Good” For A Flame Sensor?

Many boards are stable above roughly 2 μA DC, but consult the service manual. Low signals often trace to grounding or sensor contamination—not just the sensor itself.

How Many Ignition Retries Before Lockout?

Common boards allow 2–5 trials per heat call. After lockout, some auto‑reset after an hour; others require a power cycle or thermostat reset.

Is It Normal For The Inducer To Run After Heat Ends?

Yes. A short post‑purge clears combustion products and protects the heat exchanger and pressure switch from residual moisture.

Why Does My Two‑Stage Furnace Jump To High Fire Quickly?

Thermostat staging, outdoor temperature, or internal algorithms can promote W2 rapidly if the space is far from setpoint. Adjust cycles‑per‑hour or staging delay if comfort or noise is an issue.

Field Checklist: Verifying The Full Sequence

• Call for heat received (W energized) and 24 VAC confirmed at board.
• Inducer starts; pressure switch proves within the rated setpoint.
• Ignitor glows or sparks; gas valve receives 24 VAC.
• Flame proves; microamp signal within spec and stable.
• Blower starts within expected delay; temperature rise within nameplate.
• No nuisance limit trips; smooth burner carryover across all tubes.
• Blower off‑delay and any inducer post‑purge complete properly.
• LED status normal; no stored fault codes after cycle.

Helpful Resources And Further Reading

• U.S. Department of Energy: Furnaces And Boilers
• ACCA Quality Installation Standard
• NFPA 54: National Fuel Gas Code
• Manufacturer Service Manuals (Carrier, Trane, Lennox, Rheem, American Standard, Goodman/Amana) for model‑specific sequences and codes.