Responded to no-heat call for 6-year-old furnace displaying high limit lockout code due to overheating. Diagnosed restricted airflow caused by severely clogged 16x20x1 filter, dirty blower wheel, and contaminated secondary heat exchanger. Performed blower wheel cleaning and installed electronic air cleaner to resolve the issue. Recommended maintaining either monthly replacement of MERV-8 (or lower) pleated filters or continued use of electronic air cleaner to prevent recurrence.
