HVACopediaLennox · Furnace · Heating
Two-stage furnace heating with single-stage thermostat
What happens, in order
- Dip switch 1 is set to ON for a single-stage thermostat
- First stage runs as normal self-check, low-speed inducer, 15 second pre-purge, 20 second ignitor warm-up, low fire ignition
- Blower starts on low heat speed after the 30 second on-delay
- The control also starts a second-stage on delay timer
- If the call is still active when the timer ends, inducer goes to high, the high fire pressure switch is checked, the high fire valve opens and the blower goes to high heat speed
- When the call ends, gas valve closes, 5 second low-speed post-purge runs, and the blower runs for the selected off delay at low heat speed
- Inducer, HUM and ACC drop after post-purge; blower stops at the end of the off delay
Timings and thresholds
| Item | Value |
|---|---|
| Second-stage delay | 7 min (factory) or 12 min, dip switch 2 |
| Pre-purge | 15 s |
| Ignitor warm-up | 20 s |
| Post-purge | 5 s |
| Blower off delay | 60, 90 (factory), 120 or 180 s |
Applies to: Two-stage variable-speed Lennox furnaces with dip switch 1 ON
Models
More Lennox furnace sequences
- Single-stage furnace ignition retries and watchguard · Ignition
- Two-stage furnace ignition retries and watchguard · Ignition
- Single-stage furnace gas heating cycle · 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.