Lennox furnace Alert code 252: Discharge air temperature high

Alert 252 means the air leaving the furnace is too hot. It is an early warning of the condition that eventually opens the limit switch, caught before the switch has to act.

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

Lennox's communicating furnaces measure discharge air temperature directly, which lets the control notice overheating developing rather than waiting for a limit switch to open. That makes this alert more useful than the limit switch alert it precedes: the furnace is still running, the heat exchanger has not yet reached the point where the safety must act, and the underlying cause is the same. Lennox's guidance names three things to check — temperature rise, airflow, and input rate — plus dirty air filters. Treated as a prompt, this alert prevents the limit trips that damage heat exchangers over time.

Most common causes

• Dirty air filters, which Lennox names explicitly.
• Airflow lower than the furnace requires.
• An input rate set too high for the airflow available.
• Temperature rise outside the range on the furnace's rating plate.
• Restrictive or undersized ductwork.
• Closed or blocked registers.
• Zone dampers reducing airflow through the furnace.

How to diagnose it

1. Check the air filter, which is Lennox's first named cause and the most common.
2. Walk the house and check registers and the return grille.
3. Note whether the alert appears on long runs, which fits a marginal airflow condition.
4. Note whether limit switch alerts have also appeared, since this alert normally precedes them.
5. Lennox references Service and Application Note ACC-14-01 for further detail.

How to fix it

1. Replace the air filter with the correct size and type and open closed registers. This addresses the most common cause and is homeowner work.
2. Temperature rise measurement, airflow adjustment, and input rate correction are technician tasks.
3. Act on this alert while it is only a warning. It is reporting the condition that eventually cracks heat exchangers, before the safety has had to intervene.
4. If it recurs after a filter change, have the ductwork and airflow assessed rather than continuing to change filters.

Notes

This is one of the genuine advantages of a communicating system. A conventional furnace has no way to report that it is running hot but not yet unsafe — it simply runs hot until the limit opens, and every one of those trips is wear on the heat exchanger.

Related

• Alert code 251 — Primary limit switch open
• Alert code 274 — Primary limit open — Watchguard

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