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Lennox · Furnace

29 Over temperature protection (indoor above 90 F)

High severity

What the control is telling you

Indoor temperature is above 90 F (factory default) or rose above it during a heating or cooling call; heating is blocked until it drops below 90 F. S30/S40 thermostat alert code 29.

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

Likely causes

  • Heating equipment stuck on (e.g. reversing valve)
  • Thermostat sensor reading wrong
  • Very hot space

Test and repair steps

  1. Check that no heating equipment is stuck ON (gas valve, electric heat, reversing valve).
  2. Verify the thermostat's temperature sensor reads correctly.
  3. Use cooling mode to bring the space below 90 F.
  4. Clears automatically once fixed.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.10. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

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.