Lennox furnace Alert code 274: Primary limit switch open more than three minutes — Watchguard
Alert 274 means the limit switch stayed open beyond three minutes and the system entered Watchguard mode. Lennox notes that in Smart Zoning systems this places the system into central mode.
Applies to: Lennox communicating gas furnaces on systems using the S30, S40 or comparable communicating thermostats, as documented in the Lennox Residential Communicating Systems Alert Code Guide.
What it means
This is the escalated form of the primary limit alert, triggered when the switch remains open past three minutes. Lennox rates it Service Urgent, a step above the non-escalated version, which is appropriate — a limit that opens briefly may be a marginal condition, while one that stays open for minutes means the furnace is substantially overheating. The zoning note matters practically: on a Lennox Smart Zoning system, a limit trip of this kind puts the whole system into central mode, so the symptom a householder notices may be that zoning has stopped working rather than that the furnace faulted.
Most common causes
- High gas pressure, which Lennox lists first.
- Low supply air from a plugged filter or a blockage in the duct work.
- An incorrect firing rate on the furnace.
- Non-functioning zone dampers restricting airflow.
- Closed or blocked supply registers.
- A failing blower motor or a blower wheel packed with dust.
- Undersized or restrictive ductwork.
How to diagnose it
- Check the air filter first — Lennox gives a dirty filter as its example of restricted supply air.
- Walk the house and check registers and the return grille.
- If the house has zoning, note whether it has reverted to central mode, which Lennox documents as a consequence of this alert.
- Check whether zone dampers appear to be operating, since Lennox lists non-functioning dampers as a cause.
- Lennox references Service and Application Note ACC-14-01 for corrective actions.
How to fix it
- Replace the air filter and open closed registers.
- Zone damper faults, firing rate adjustment, and gas pressure work are technician tasks.
- Do not keep resetting the furnace. Sustained overheating is what cracks heat exchangers, and this alert means the overheat lasted minutes rather than seconds.
- Have the zoning system checked as well as the furnace, since a stuck damper causes this without any filter or duct being obviously wrong.
Notes
Zoning is an easily overlooked cause of overheating. A damper that has failed closed restricts airflow through the furnace in a way that no amount of filter changing will fix, and the householder's experience is simply that some rooms are cold.