HVACopediaLennox · Furnace · Ignition
Two-stage furnace ignition retries and watchguard
What happens, in order
- Warm-up ends and the gas valve opens for the 4 second trial
- No flame in 4 seconds control does a 35 second inter-purge and warm-up, then tries again
- Five trials in total, then watchguard flame failure mode
- After 60 minutes the control restarts the ignition sequence
- The built-in watchguard also breaks and remakes the thermostat demand after one hour of continuous demand to reset an ignition lockout
- Soft lockouts for too many retries or recycles clear when a later heat call finishes successfully
Timings and thresholds
| Item | Value |
|---|---|
| Trials for ignition | 5 total |
| Inter-purge and warm-up between trials | 35 s |
| Watchguard reset | 60 min |
Codes you may see during this
Applies to: Two-stage variable-speed Lennox furnaces
Models
More Lennox furnace sequences
- Single-stage furnace ignition retries and watchguard · Ignition
- Single-stage furnace gas heating cycle · Heating
- Two-stage furnace heating with single-stage thermostat · Heating
- Two-stage variable speed furnace heating with two-stage thermostat · Heating
- Cooling call on single-stage furnace blower · Cooling
- Two-stage variable speed cooling call · Cooling
- Continuous fan with heat or cool calls · Standby/shutdown
Related
Lennox error codes · Lennox tips · Lennox parts · All Lennox sequences
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Source: Lennox SL280UHV installation manual 508245-01 (04-2024), Heating Sequence of Operation p.46; Lennox SL280UHNV service manual Corp 1601-L8 (rev 04-2024), Sequence of Operation; Lennox SL297UHNV installation manual 507760-02 (02-2018), Heating Sequence of Operation pp.51-52; Lennox EL296UHV installation manual, Ignition and Two Stage Operation pp.27-28. Written in our own words; timings can differ by model and software version, so check the manual for your exact unit.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.