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Lennox · Furnace

40 HP heating lockout

Low severity

What the control is telling you

The heat pump could not raise room (or zone) temperature 0.5 F toward set point in 30 minutes, so it was locked out and the furnace / electric heat took over (dealer information). S30/S40 thermostat alert code 40.

Applies to: Lennox communicating gas furnaces (e.g. SL280UHV/SL280UHNV, EL296UHV) in S30/S40 smart thermostat systems

Likely causes

  • Low airflow to the zone(s)
  • Heat pump undersized for conditions or low charge
  • Balance points set far apart
  • Thermostat out of calibration

Test and repair steps

  1. Check airflow to the zone(s) and the discharge air temperatures.
  2. Check room thermostat calibration.
  3. Check HP Heating Lockout Time (default 120 min, range 60-240): S30 Dealer Control Center > Equipment > Smart Hub; S40 Service Support Center > Equipment Settings > Thermostat.
  4. Dual fuel set high and low balance points close together (about 3 F apart, e.g. 25 F high / 22 F low).
  5. Check heat pump operation (charge, defrost) if lockouts are frequent.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.13. Summarized in our own words; check the original manual for your model.

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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.