Lennox furnace E115: Low 24 volt supply
E115 means the 24 volt control supply is low. Lennox gives the acceptable range explicitly — 18 to 30 volts — and flags that added equipment may have outgrown the transformer.
Applies to: Lennox gas furnaces with a seven-segment diagnostic display on the control board, such as the ML193UHEK, per installation instructions 508565-01.
What it means
The 24 volt circuit powers the thermostat, the gas valve, the relays and anything else wired into the low voltage side. Lennox publishes the range it expects, 18 to 30 volts, which turns this from a judgement into a check anyone with a meter can make. The genuinely useful part of their guidance is the cause they name that most troubleshooting misses: additional power-robbing equipment connected to the system. Humidifiers, ventilators, zone panels, air cleaners and smart thermostat accessories all draw from the same transformer, and they accumulate over years of small additions until the transformer that was correctly sized for the original furnace no longer is. Lennox says so directly, and notes a larger VA transformer may be needed.
Most common causes
- Accessories added over time — humidifier, ventilator, zone panel, air cleaner — all drawing from one transformer.
- A transformer that is failing or was undersized from the start.
- A partial short in the low voltage wiring drawing current without blowing the fuse.
- A long or undersized thermostat wire run causing voltage drop.
- Corroded or loose low voltage connections.
How to diagnose it
- List what has been added to the system since it was installed. That list is usually the answer.
- Note whether the fault started after an accessory was fitted.
- Note whether the thermostat behaves oddly, since it shares this supply.
- Measurement against the 18 to 30 volt range is technician work.
How to fix it
- A technician should measure the 24 volt supply under load, not at rest — the difference is the whole point.
- If accessories have outgrown the transformer, Lennox's own remedy is a larger VA transformer.
- A partial short should be ruled out before the transformer is upsized, or the new one will be loaded the same way.
- The code clears after the fault recovers.
Notes
This is the sort of fault that arrives gradually and gets blamed on whatever failed first. The published 18 to 30 volt range is what makes it checkable rather than arguable.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
The 40VA transformer and the 75VA fix
The same 508565-01 manual quantifies the outgrown-transformer story this code tells. The ML193UHEK ships with a 40 VA transformer, and the manual's electrical section states directly that when the furnace is matched with zoning, dual-fuel or other 24-volt accessories, the factory transformer should be replaced with kit 27J32 — a 75 VA transformer — precisely “so you do not overload the original 40VA transformer.” That is the manufacturer conceding, in print, that the as-shipped low-voltage budget covers the furnace and a thermostat and not much else. An E115 on a system that has grown a zone panel and a humidifier since installation is that sentence coming true, and the remedy was named before the fault ever appeared.
What the fix costs
This is low-cost territory as furnace repairs go. A diagnostic visit at $75–150 covers measuring the 24-volt supply under load against Lennox's published 18-to-30-volt range; a transformer swap is a modest parts-and-labor job on top of that. The trap worth paying attention to is on the control board cost page: low-voltage problems get misread as board failures, and boards run $400–900 installed — while the board's low-voltage fuse alone is $5–150. If a partial short is dragging the supply down, find it before replacing anything, or the new transformer inherits the same load and the same fate.
DIY or call a pro
Leave this one to a technician, for a reason specific to the circuit involved: the 24-volt side may sound harmless, but it powers the gas valve, and E115 diagnosis is done live and under load — the reading at rest can look fine while the supply collapses the moment the system actually works. Distinguishing an overloaded transformer from a partial short from a corroded splice is meter work through the same wiring that opens and closes gas flow. The homeowner contribution that genuinely shortens the visit: write down every accessory added since the furnace went in — humidifier, ventilator, zone panel, air cleaner, powered thermostat — with rough dates. If the code started after the most recent addition, say so; that sentence often is the diagnosis.
What this looks like
From the outside, this usually shows up as there is no heat but nothing looks obviously wrong — that walkthrough covers what to check in order, and when it stops being a homeowner job.