Gas heat call, hot surface ignition and retry

What happens, in order

1. Thermostat W closes and the inducer starts pre-purge
2. Pressure switch closes
3. 5 s after the pressure switch closes the hot surface igniter heats for 30 s to full temperature; inducer runs for the whole heat cycle
4. After the 30 s warm-up the gas valve opens for the 8 s trial for ignition
5. Igniter stays on for the first 7 s after the valve opens
6. Flame sensor must prove flame for 1 s at the 8 s mark
7. Main blower starts about 20 s after burners light
8. If no flame valve closes, igniter off, 10 s post-purge, then control confirms the pressure switch reopened
9. Next try 30 s pre-purge, 30 s igniter warm-up, valve opens, up to 8 s to find flame
10. After a failed second try the control runs the blower and inducer together for 3 minutes (self-healing) before the next attempt
11. Up to 4 attempts (first try plus 3 retries); 2nd and 4th attempts use a 30 s pre-purge plus 30 s warm-up to protect the igniter
12. After the 4th failure the control enters a 1 hour soft lockout and ignores the W call
13. After the hour the whole sequence repeats until flame is proven or the call ends
14. Reset lockout by cutting power at the unit or thermostat for 5 to 10 s, or by dropping and re-making the W call
15. Call for heat ends valve closes, inducer runs 10 s post-purge, blower runs out the heat off delay

Timings and thresholds

Models

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