Diagnosed a no heat condition on 13-year-old Lennox furnace that was repeatedly tripping on high limit due to severely restricted airflow from completely plugged filter caused by portable humidifier dust. Replaced filter, restored proper airflow, and furnace stopped limiting out. Visual inspection of heat exchanger revealed heavy heat damage and burners displaying dancing/flashing orange patterns during blower operation, indicating possible heat exchanger breach; scheduled service advisor follow-up for further evaluation.
