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Lennox · Heat pump

E115 Low secondary (24 VAC) voltage

High severity

What the control is telling you

24 VAC to a component control is below the required 18 to 30 VAC range. S30/S40 thermostat alert code 115 (shown as E115 on the unit control display).

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

Likely causes

  • Overloaded transformer (power-robbing accessories)
  • Low line voltage
  • Loose wiring

Test and repair steps

  1. Measure 24 VAC at the control and correct it.
  2. Look for extra accessories loading the transformer.
  3. A larger or additional VA transformer may be needed.
  4. Damper module runs non-zone mode until voltage is restored.
  5. 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.16. 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.