Responded to no heat call for Lennox furnace displaying error code 291 (restricted airflow). Diagnosed heavily clogged filter with fine white dust likely from ultrasonic humidifier as the cause of the airflow restriction. Removed and replaced filter, which cleared the fault code. Tested system operation for 15 minutes at maximum heating rate with proper 60-degree temperature rise, confirming normal function. Advised customer to discontinue humidifier use temporarily and replace filter monthly.
