HVACopedia

Lennox · Air handler

36 Heating when not requested

High severity

What the control is telling you

Heat has run 15 minutes or more with no call for heat. S30/S40 alert code 36.

Applies to: Lennox communicating air handlers and EIM-equipped indoor units in S30/S40 smart thermostat systems

Likely causes

  • Stuck gas valve, relay or electric heat sequencer
  • Miswiring
  • Other active alert codes

Test and repair steps

  1. Run diagnostics and compare with what the equipment actually does (S30: Dealer Control Center > Diagnostics > Start Diagnostics; S40: Service Support Center > Diagnostics > Select All > Start).
  2. Look for other alert codes that may be driving operation.
  3. Check all heating equipment for the cause of the heat (stuck relay, gas valve, sequencer).
  4. Cycle power. 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.11. 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.