Lennox furnace Alert code 291: Heating airflow below minimum — Watchguard

Alert 291 means the furnace measured heating airflow below the minimum it requires and shut down into Watchguard mode. It is the alert that names the airflow problem directly rather than reporting it as an overheat.

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

Most furnaces discover an airflow problem indirectly, by overheating until the high limit opens. A Lennox communicating system knows what airflow its variable speed blower is actually producing, so it can report the shortfall itself, before the limit trips. That is a genuinely more useful diagnosis: a limit trip says the furnace got too hot and leaves the cause open, while this alert says the air moving across the heat exchanger is below the minimum the furnace was designed around. Lennox's troubleshooting is short and points where it should — dirty filters and other restrictions first, blower performance second.

Most common causes

• A dirty or clogged air filter, which is the first item on Lennox's own list.
• Closed, blocked or covered supply registers and return grilles.
• Undersized, crushed or disconnected return duct.
• A blower wheel loaded with dirt, which reduces airflow without any obvious noise.
• A blower motor running below commanded speed.
• Excessive duct static from a closed zoning system or an added restriction.

How to diagnose it

1. Check the air filter first and replace it if it is dirty. This is the single most common cause and it is homeowner work.
2. Walk the house and confirm supply registers and return grilles are open and unobstructed, including under furniture and rugs.
3. Note whether the alert appeared after new filters were fitted — a filter with a much higher MERV rating than the system was designed for restricts airflow.
4. Note whether it coincided with duct work, a zoning system change, or a closed damper.

How to fix it

1. Replace a dirty filter and open any blocked registers, then see whether the alert returns.
2. If it returns with a clean filter and clear registers, the restriction is in the duct system or the blower, and that is technician work.
3. Expect a technician to check blower performance and static pressure rather than replacing parts.
4. Resist raising the blower speed to make the alert go away — it treats the symptom and leaves the restriction.

Notes

This alert and the primary limit alerts describe the same physical problem at different stages. 291 catches it as a measurement; 251 and 274 catch it after the furnace has already overheated.

Related

• Alert code 251 — Primary limit switch open
• Alert code 252 — Discharge air temperature high

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