Lennox EL296UHV Error Code E 115: Low 24V Power
What Does Code E 115 Mean?
Code E115 on the Lennox EL296UHV means the 24V control-circuit voltage sagged below the acceptable window, which this board holds between 18 and 30 volts. This is a low-voltage-side issue, separate from the incoming 120V mains that E110 watches. The 24V supply comes from a transformer inside the furnace and powers the SureLight control and the accessories tied to it, so when it droops the control flags E115 and will restart once the voltage recovers.
The most common cause is simply too much load on that one transformer. Powered humidifiers, electronic air cleaners, zone-damper controllers, and communicating thermostats all draw from the 24V circuit. As accessories are added over the life of the system, their combined draw can exceed what the original transformer can hold up, pulling the voltage down under load.
The usual remedies are to move some accessories onto their own transformer or to fit a larger-VA transformer sized for the total load — both of which are technician tasks. E115 is the low counterpart to E116 (24V too high); together they bracket the healthy 18-30V range, and distinguishing them tells a technician whether to look at excessive load (E115) or an incorrect incoming supply voltage (E116).
What You'll Notice
- "E 115" appears and the furnace restarts, especially when accessories kick on
- The trouble began after a humidifier, air cleaner, or zone controller was added
- The system works most of the time but fails during peak accessory demand
- Heating drops out when the humidifier or air cleaner energizes with a heat call
- The furnace runs but occasionally resets for no obvious mechanical reason
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Too many accessories drawing power from 24V transformer | Most common | ✗ Call a pro → |
How This Is Diagnosed
A technician measures the 24V control voltage while the system and its accessories are energized, watching whether it sags below the 18-30V range under load. A reading that is fine at idle but drops when a humidifier or air cleaner engages points to an overloaded transformer.
From there they tally the VA draw of everything on the circuit against the transformer's rating and decide whether to offload some accessories to a separate transformer or install a larger-VA unit. Because E115 clears when voltage recovers, verification is confirming the 24V stays in range with all accessories running.
When to Call a Professional
This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:
- E115 appears and the furnace restarts repeatedly
- A humidifier, electronic air cleaner, or zoning accessory was recently added
- The system fails specifically when accessories energize during a heat call
- E115 shows up together with E116 or other supply-voltage codes
Frequently Asked Questions
What does the 24V power actually run?
The 24V transformer feeds the furnace's control board and the accessories wired to it, such as a humidifier, electronic air cleaner, zone dampers, and the thermostat. When their combined draw is too high, the voltage sags and E115 appears.
Did adding a humidifier cause this?
It can. Each accessory adds load to the same transformer, and enough of them can pull the 24V below the acceptable range. A technician can add a larger-VA transformer or power some accessories separately to fix it.
Is E115 the same as low house voltage?
No. E115 is about the furnace's internal 24V control power, while low incoming 120V line voltage shows up as E110. They are different circuits and are diagnosed differently.
✓ Verified against manufacturer service manual — March 2026