Gas Furnace — Full Sequence of Operation

For homeowners

Here is what happens, in order, when your thermostat calls for heat:

1. Thermostat senses room temperature below setpoint, closes its internal R-to-W contacts.

2. 24V signal arrives at the furnace control board on the W terminal.

3. Board powers up the draft inducer motor. The inducer spins up and starts pulling air through the heat exchanger.

4. Negative pressure inside the inducer housing pulls the diaphragm in the pressure switch, closing its contacts. Board sees the pressure switch closed.

5. Board verifies the rest of the safety chain — high-limit and rollout switches all show closed (no overheating, no flame escape from a previous run).

6. Board energizes the hot surface igniter. Element starts glowing, heats from cold to 1800°F+ over the next 30–60 seconds.

7. Board opens the gas valve. Gas flows through the regulator to the burner manifold and out the burner orifices.

8. Gas-air mixture contacts the glowing igniter element. Flame ignites at the first burner and propagates from burner to burner along the manifold within a fraction of a second.

9. Flame sensor — a rod sitting in one flame — sees rectified current flow through the ionized flame. Board reads the microamps and confirms flame is present.

10. Board de-energizes the igniter (job done) and runs steady — burners on, inducer running, heat exchanger heating up.

11. After a 30–60 second “blower-on delay” (giving the heat exchanger time to warm up so the first air out the vents is already hot), the blower starts at heat speed.

12. System runs in steady state. Burners produce heat, blower moves warm air through the house, thermostat is satisfied as room temperature climbs.

13. Thermostat reaches setpoint, opens R-W contacts. 24V to the board drops.

14. Board immediately closes the gas valve. Burners go out within a fraction of a second.

15. Inducer continues running for 15–60 seconds (post-purge) to clear residual combustion gases from the heat exchanger.

16. Blower continues running for 60–180 seconds (blower-off delay) to cool the heat exchanger and harvest residual heat into the home.

17. Both motors stop. System is at rest, waiting for the next call.

This sequence is your diagnostic map. If something is wrong, the sequence stops somewhere — and which step it stops at tells you which component to investigate.

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For technicians

Detailed timing. Most modern residential gas furnaces follow this approximate timing, though specific values vary by manufacturer and model:

Step	Action	Typical timing
1	Thermostat call	Instant
2	Signal reaches board	Instant
3	Inducer energizes	Instant
4	Inducer ramps to speed; pressure switch closes	5–15 seconds
6	Hot surface igniter warm-up	30–60 seconds
7	Gas valve opens	Instant
8	Ignition occurs at burners	1–3 seconds after valve opens
9	Flame sensor proves flame	1–2 seconds after ignition
10	HSI de-energizes	Immediately after flame proven
11	Blower-on delay, then blower starts	30–60 sec after burners light
14	Gas valve closes	Instant
15	Inducer post-purge	15–60 seconds
16	Blower-off delay	60–180 seconds

Diagnostic uses of the sequence.

Sequence stops at step 3 (inducer doesn’t start):

• No 24V at the inducer from the board → board fault, or board not receiving thermostat signal
• 24V at the inducer but no rotation → failed inducer motor, failed run capacitor
• Some boards check rollouts and limit before energizing inducer — verify all safeties closed

Sequence stops at step 4 (pressure switch doesn’t close):

• Inducer running but no negative pressure to the switch → blocked vent, blocked combustion intake, disconnected hose
• Switch failed mechanically → replace switch
• Hose tap port plugged at the inducer

Sequence stops at step 5 (safety chain not proven):

• Tripped rollout switch (manual reset required, investigate root cause first)
• Tripped/failed high-limit switch
• Broken wire in the safety chain

Sequence stops at step 6 (igniter doesn’t warm up):

• No voltage from the board to the igniter → board fault
• Voltage present but no current draw → open igniter element (cracked, failed)
• Current draw but no visible glow → element severely damaged or wrong voltage type

Sequence stops at step 7 (gas valve doesn’t open):

• No 24V at the gas valve → board fault, possibly safety chain dropped after initial check
• 24V at valve but no gas flow → failed valve (one or both solenoids stuck closed)

Sequence stops at step 8 (gas valve opens but burners don’t light):

• Gas pressure issue → manifold pressure too low, gas supply weak, wrong fuel/orifice combination
• Igniter not actually hot enough to ignite → end-of-life igniter, wrong placement

Sequence stops at step 9 (burners light but flame sensor doesn’t prove):

• Flame sensor dirty (most common — clean with steel wool)
• Flame sensor cracked at ceramic insulator → replace
• Flame sensor not in flame (bent rod or misaligned position)
• Burner not grounded (missing return path for flame current)
• Failed flame-sense circuit on the board

Burners light then go out immediately, repeatedly: classic flame sensor problem. Board doesn’t see proof of flame within its timeout window (typically 4–7 seconds), closes the gas valve, attempts again. Three to five attempts then locks out.

Sequence stops at step 11 (burners run but blower never starts):

• Failed blower motor or capacitor
• Failed board blower-control circuit

Furnace runs but cycles on limit during operation:

• Reduced airflow problem (filter, blower, ductwork — see static pressure and return air sizing)
• Oversized firing for the system
• Failed limit switch

Trial-for-ignition logic. When the gas valve opens (step 7) and the flame sensor fails to detect flame within the timeout window (step 9), the board enters retry behavior:

1. Close gas valve immediately
2. Run inducer through a short post-purge to clear unburned gas
3. Wait briefly
4. Return to step 6, attempt ignition again
5. After typically 3–5 failed attempts, enter “ignition lockout”

Ignition lockout means the board has given up and is waiting for either:

• Power cycle (turn the thermostat off, then back on; or open and close the door switch)
• Lockout timeout (typically 1 hour) automatically retries

The lockout fault code is one of the most common diagnostic findings. The next step is to power-cycle and watch a full sequence with eyes on the equipment to see exactly where ignition is failing.

Two-stage and modulating furnaces add complexity:

• Two-stage: most calls start in first stage (low fire, ~65% of rated input). If the thermostat is still calling after a programmed delay (typically 5–15 minutes), the board commands second stage — opens the second gas valve solenoid, increases inducer speed. Second stage continues until thermostat satisfies.

• Modulating: continuously variable firing rate from typically 40% to 100% of rated input. The control board adjusts manifold pressure and inducer speed continuously. Smoother operation, less cycling, more efficient.

Power-loss recovery. If utility power fails during operation:

• Gas valve closes (no 24V to hold it open)
• Inducer stops
• Blower stops
• Heat exchanger cools naturally

When power returns, if the thermostat is still calling, the board starts a new sequence from step 1. This is one reason gas furnaces are considered fail-safe: the energized state is “running” and the de-energized state is “off.” Loss of control, loss of power, loss of any safety signal — all default to “off.”