Lennox furnace Alert code 116: High 24 volt secondary voltage

Alert 116 means the 24 volt control supply is too high. Lennox gives the acceptable window as 18 to 30 volts AC, and the fault points at the line voltage feeding the transformer rather than at the transformer itself.

This code is not a homeowner repair. Faults in this group involve the gas valve, the burner compartment, or the heat exchanger — the parts whose failure can put combustion products, including carbon monoxide, into the air the house breathes. Do not reset it repeatedly to keep the heat on. Have it looked at by a licensed technician, and make sure there is a working carbon monoxide alarm on every level of the home, including the level the furnace is on. The conditions that mean stop now.

Applies to: Lennox communicating systems on an S30, S40 or comparable communicating thermostat. Reported by the furnace or the air handler, whichever is the indoor unit.

What it means

The 24 volt secondary is what powers the thermostat, the gas valve and every relay on the control. Lennox monitors it and reports at both ends of the range — alert 115 when it is low, alert 116 when it is high — using the same 18 to 30 volt window. High secondary voltage is the less common of the two and the more revealing, because a transformer produces its secondary in fixed proportion to what it is fed. If the secondary is high, the primary usually is too. That is why Lennox's own instruction is not to look at the transformer but to check the line voltage reaching the equipment, naming 120 and 240 volts as the values to verify against.

Most common causes

Line voltage to the equipment running high, which raises the secondary in proportion.
A transformer wired to the wrong primary tap — 208 volt equipment fed 240, for example.
A replacement transformer with the wrong turns ratio fitted during an earlier repair.
A supply problem affecting the whole property rather than the furnace circuit.

How to diagnose it

Note whether other equipment in the house has been failing unusually often, which is what a chronically high supply produces.
Note whether the furnace or air handler was recently serviced, since a mis-tapped transformer arrives with a person.
Measuring line and secondary voltage means working in a live cabinet and belongs to a technician.
Lennox notes the code clears when the control sees the voltage back in range, so it is a live reading rather than a stored fault.

How to fix it

Have the incoming line voltage checked first — Lennox's instruction is to verify proper line voltage to the equipment, not to replace anything.
If the line voltage is correct and the secondary is still high, the transformer tap or the transformer itself is the next step.
If the supply is genuinely high at the meter, that is a matter for the utility rather than an HVAC repair.

Notes

Alert 115 is the same measurement at the other end of the range. Between them the system is acting as a supply monitor, which is information most furnaces cannot give you.

All codes for this manufacturer

Lennox furnace alert codes — every code, with what differs between models.

Sources

Lennox Residential Communicating Systems Alert Code Guide, document 100017, March 2023 — Table 2, Alert Codes and Troubleshooting

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.