Lennox E110

What does Lennox E110 mean?

E110 indicates that the AC line voltage supply to the furnace or air handler is below the acceptable operating range. The control board monitors incoming line voltage continuously; when it falls too low, E110 fires as a Service Soon/Urgent alert. This may occur during utility brownouts or when line voltage is chronically below the nameplate rating. The alert clears automatically when voltage returns to the correct range.

Symptoms

• E110 or 'Low AC Line Voltage' displayed on the thermostat.
• Furnace or air handler may operate erratically or fail to start reliably.
• Alert may appear simultaneously on multiple system components.
• Symptoms worsen during periods of high grid load (hot afternoons, cold mornings).

Common causes

• Utility brownout or supply voltage below nameplate rating (typically below 108VAC on a 120VAC system or below 216VAC on a 240VAC system).
• Undersized or overloaded branch circuit serving the equipment.
• High-resistance connections at the disconnect, breaker, or equipment terminal block.
• Long branch circuit run with undersized wire gauge causing significant voltage drop under load.
• Shared circuit with large loads (compressors, motors) causing voltage sag.

Diagnostic steps

1. 01

   Measure line voltage at the equipment

   With the system running at full load, measure voltage at the equipment terminal block or unit disconnect. Compare to the nameplate rating. Voltage should be within ±10% of nameplate.

2. 02

   Measure voltage at the circuit breaker

   If voltage at the equipment is low, also measure at the breaker in the panel. A significant difference between the panel and the equipment indicates high-resistance wiring or connections on the branch circuit.

3. 03

   Inspect disconnect and terminal block connections

   With power off, inspect all wiring connections at the outdoor disconnect, indoor unit terminal block, and junction boxes for loose lugs, corroded conductors, or undersized wire. Tighten and clean as needed.

4. 04

   Check if other high-load equipment shares the circuit

   Identify any other loads on the same circuit or phase. Large motor loads starting on the same branch can cause momentary voltage sag sufficient to trigger E110. The furnace should be on a dedicated circuit.

When to call a professional

Chronic low line voltage requires an electrician to measure panel voltage, assess branch circuit sizing, and correct any wiring deficiencies. Utility-side voltage problems require contacting the utility company. Equipment should not be operated on sustained low voltage — motor windings running below design voltage draw higher current and overheat, shortening equipment life.