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Lennox · Heat pump

21 Incompatible equipment detected

High severity

What the control is telling you

A component is not compatible with the rest of the system, e.g. a single-stage furnace without variable-speed motor paired with a modulating outdoor unit. S30/S40 thermostat alert code 21.

Applies to: Lennox communicating heat pumps incl. variable-capacity inverter units in S30/S40 smart thermostat systems

Likely causes

  • Incompatible indoor/outdoor combination

Test and repair steps

  1. Check the model numbers of indoor and outdoor units against Lennox matchups.
  2. Replace with compatible equipment.

Settings for these units

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.9. 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.