Lennox SLP98UHV Error Code E 311: Heat Rate Reduced - Cutback Mode

What Does Code E 311 Mean?

On the Lennox SLP98UHV, E 311 means the furnace has entered heat-rate cutback mode: the SureLight control has automatically lowered the firing rate to match the airflow the blower can actually move. Importantly, the furnace is still running and still heating your home — just at less than full capacity — so this is a protective response rather than a shutdown.

The SLP98UHV is a modulating furnace that varies its firing rate over a wide range, and its ECM blower continuously tracks airflow. When airflow slips below the optimal level but is not yet critically low, the board trims the gas input so the furnace does not overheat. This makes E 311 an early, milder member of this board's airflow family. If the restriction worsens, it can escalate to E 291 (measured airflow below the minimum firing rate) or provoke the overheating codes E 250 (limit switch open), E 274 (soft lockout after repeated limit trips), and E 252 (discharge air too hot). Its cooling-side counterpart is E 312, where the blower cuts back in cooling or fan mode.

The most common homeowner-fixable cause is a dirty air filter that keeps the blower from moving enough air. A less common cause is undersized or restricted ductwork limiting the maximum airflow the system can reach. Left alone, a filter that keeps loading up will push E 311 toward E 291 as airflow drops further.

What You'll Notice

• The furnace runs but the home heats slowly and struggles to reach the setpoint
• Heating cycles run longer than usual for less warmth
• Airflow from the registers is softer or cooler than you are used to
• The 7-segment display shows E 311 when you recall codes
• The air filter looks dirty or overdue for replacement

Common Causes

How This Is Diagnosed

Because E 311 is a low-airflow cutback rather than a shutdown, the filter is checked first, then the registers, then the ductwork. If output stays reduced after airflow is restored, a technician evaluates duct sizing and blower performance.

How to Fix It: Replace the Air Filter

What You'll Need

• Replacement air filter (matching size) 🛒 Find at FiltersFast · 🛒 Find at Amazon

Steps

1. Turn off electrical power at the breaker and shut off the gas supply valve Switch the furnace breaker to OFF and turn the manual gas shutoff valve to the OFF position (handle perpendicular to the pipe). If you smell gas, leave immediately and call your gas company.
2. Replace the air filter Find the filter in the blower compartment or return duct, remove the dirty filter, and install a new one of the correct size with the airflow arrow pointing toward the blower. If you are running a high-MERV filter (MERV 13 or above), consider a lower-MERV filter as recommended by Lennox, since denser filtration adds airflow resistance.
3. Open every supply register and return grille Check each room to confirm all vents are open and unobstructed. On a modulating furnace, even a few closed registers can reduce airflow enough to hold the furnace in cutback mode.
4. Restore power and gas, then test Turn the gas valve to ON (handle parallel to the pipe) and switch the breaker to ON. Set the thermostat to call for heat; with airflow restored, the furnace should ramp back up toward its normal firing rate.

When to Call a Professional

Contact a licensed HVAC technician if:

• E 311 persists after replacing the filter and opening all vents
• The furnace also shows E 291, meaning airflow has dropped to critically low
• You suspect the ductwork is undersized for the furnace capacity
• The blower wheel looks dirty or damaged
• Heating output stays low even after airflow restrictions are addressed

✓ Verified against manufacturer service manual — March 2026