Lennox E115

What does Lennox E115 mean?

E115 indicates that the 24VAC secondary power to a communicating system component has fallen below the required range of 18–30VAC. The Lennox control monitors secondary voltage continuously; when it drops below 18VAC, E115 fires as a Service Soon/Urgent alert. Zone damper modules revert to non-zone mode until voltage is restored.

Symptoms

• E115 or 'Low Secondary (24VAC) Voltage' displayed on the thermostat.
• Zone damper control module reverts to central (non-zone) operation.
• System may operate intermittently or fail to respond to thermostat demands.
• Other voltage-related alert codes may appear simultaneously across multiple components.

Common causes

• Control transformer VA rating exceeded by total accessory load connected to the secondary.
• Long low-voltage wiring runs with undersized wire gauge causing voltage drop under load.
• Loose or high-resistance connection in the 24VAC circuit.
• Failing transformer with reduced secondary output under load.
• Short-to-ground or partial short in the 24VAC wiring increasing current draw.

Diagnostic steps

1. 01

   Measure 24VAC at the component reporting E115

   With the system under full load demand, measure voltage at the R and C terminals of the reporting component. Voltage below 18VAC confirms the low-secondary condition is reaching the device.

2. 02

   Measure transformer secondary voltage under load

   Measure voltage at the transformer secondary terminals while all system accessories are energized. If voltage is below 18VAC at the transformer output, the transformer is overloaded or failing.

3. 03

   Audit all accessories connected to the transformer secondary

   List every device drawing 24VAC from the transformer: thermostats, zone controls, humidifiers, UV lights, Equipment Interface Modules. Sum the VA loads and compare to transformer VA rating.

4. 04

   Replace transformer with a higher VA unit or add a supplemental transformer

   If the load exceeds the transformer's VA rating, install a higher-rated transformer or add a dedicated supplemental 24VAC transformer for the accessories. This is the most reliable long-term fix.

5. 05

   Inspect 24VAC wiring for resistance and partial shorts

   Check for loose wire nuts, corroded terminals, or wires pinched by access panels. Measure resistance in the wiring runs. High resistance or partial shorts increase current draw and lower delivered voltage.

When to call a professional

Transformer replacement involves working with line-voltage wiring and, on communicating systems, requires system recommissioning after the repair. If a partial short or ground fault is suspected, a licensed technician with test equipment is needed to isolate the fault without damaging the control boards.