Heil furnace LED flashes 13 times; furnace may short-cycle or shut down on limit frequently.

Safety: Turn off power before accessing internal components.

Steps to check:

• Confirm filter and airflow: Replace dirty filters and ensure all registers are open. Check for obvious duct restrictions near the furnace.
• Check blower speed setting: On the Heil control board, verify that the HEAT blower speed tap is not set to the lowest speed if the furnace is tripping on limit. Increasing blower speed can reduce temperature rise.
• Inspect limit wiring: Ensure all limit switches in series (main limit and any auxiliary limits) have tight connections and no burnt wires.
• Reset lockout: Power cycle the furnace to clear the lockout and monitor operation.

When to call a pro: If the furnace continues to trip on limit with proper airflow, a technician should measure temperature rise, static pressure, and evaluate duct sizing and furnace capacity. Limit switches may also need replacement.