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Lennox · Heat pump

E105 Wiring issue / communication problem

High severity

What the control is telling you

Low-voltage wiring between components is compromised and a device cannot communicate (RSBus message errors, usually electrical noise). S30/S40 thermostat alert code 105 (shown as E105 on the unit control display).

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

Likely causes

  • Unused thermostat wires left floating
  • Loose terminals or poor splices
  • Low-voltage wiring run beside high-voltage wiring
  • Poor grounding
  • Float switch or zoning wired into the comm circuit
  • Induced voltage on the cable

Test and repair steps

  1. Check notification details in the dealer control center to see which device has the problem (review active and cleared alerts).
  2. Step 1 bundle all unused wires and run them to C at the indoor control; tighten terminals (3/32 in. / 2.4 mm slotted screwdriver); fix bad splices; separate low voltage from high voltage in walls/ceilings/floors; check grounding of line voltage, transformer and equipment.
  3. Step 2 disconnect all devices except thermostat-smart hub-indoor unit and add them back one at a time, recommissioning each time.
  4. If zoning is fed from the thermostat, run the wires from the zone control straight to the indoor control instead. For a float switch, use an isolation relay, and take the switch out of the circuit while testing.
  5. Measure each wire to C in AC volts 0.3 VAC or less is fine, up to 0.7 VAC moderate success, up to 1.2 VAC occasional success, over 1.2 VAC must be fixed.
  6. Step 3 run new wiring - 18/2 for R and C plus 18-22/2 shielded for i+/i- (shield/drain to indoor C only), or two separate 18/2 unshielded cables (one for R/C, one for i+/i-). S30 hub-to-display: same idea for +12V/-12V and A/B.
  7. Keep communication wiring at least 15 ft (4.6 m) from house wiring and large appliances when possible.
  8. 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.14. Summarized in our own words; check the original manual for your model.

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