HVACopediaLennox · Furnace · Heating
Single-stage furnace gas heating cycle
What happens, in order
- Thermostat calls for heat (W) and the control runs a self-check
- Control confirms the primary and secondary limit and the rollout switch are closed, and that the combustion air pressure switch is open
- If the pressure switch is already closed, the control will not start the cycle
- Inducer starts
- When the pressure switch closes, a 15 second pre-purge starts
- If the pressure switch has not closed after 2.5 minutes the control goes into pressure switch watchguard for a 5 minute wait, then restarts the sequence
- Ignitor warms for 20 seconds; the gas valve opens at the 19 second point and the 4 second trial for ignition begins
- Ignitor stays on for the first 3 seconds of the trial
- Flame is proven and the gas valve stays open for the rest of the call
- Indoor blower starts at heating speed 30 seconds after flame is proven
- Thermostat satisfied gas valve closes, inducer runs a 5 second post-purge, and the blower keeps running for the selected off delay
- Line voltage below 70 V stops the cycle (EL195 flash 13) until voltage is back above 75 V
- Rollout open gas valve off, inducer and blower run, and the control holds until the rollout closes and power or the thermostat call is reset
- Primary limit open gas valve off, blower runs on the limit
Timings and thresholds
| Item | Value |
|---|---|
| Pre-purge | 15 s |
| Ignitor warm-up | 20 s |
| Gas valve opens | 19 s into warm-up |
| Trial for ignition | 4 s |
| Ignitor on during trial | first 3 s |
| Pressure switch prove time | 2.5 min, then 5 min wait |
| Blower on delay | 30 s after flame (not adjustable) |
| Post-purge | 5 s |
| Blower off delay EL195 | 90 s factory, jumper adjustable |
| Blower off delay ML180 | 120 s factory, 2-position dip switch, 4 choices |
| Low voltage lockout | below 70 V stops, resumes above 75 V (EL195) |
Codes you may see during this
EL195 flashes: 1 polarity reversed; 2 poor ground; 7 limit open; 8 rollout open; 9 pressure switch failed to close or opened during call; 10 pressure switch opened 5 times in one call; 11 pressure switch stuck closed; 12 flame without gas valve; 13 low line voltage. ML180: E110, E111, E112, E200, E223, E224, E227, E241, E250
Applies to: EL195UHNE, ML180UHEK gas furnaces with single-stage integrated control
Models
More Lennox furnace sequences
- Single-stage furnace ignition retries and watchguard · Ignition
- Two-stage furnace ignition retries and watchguard · Ignition
- 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
Take it to the job
Sequences of operation, error codes and tips work offline in the HVACopedia app.
Source: Lennox EL195UHNE service manual Corp 1801-L1 (rev 04-2024), Sequence of Operation pp.9-10 and 49-54; Lennox ML180UHEK service manual 100166, Heating Ignition Sequence p.11 and Sequence of Operation pp.36-40. 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.