Heil furnace LED flashes 8 times; furnace may not start or may shut down unexpectedly, often related to low-voltage issues.

Safety: Turn off power before accessing the control board. Low-voltage circuits can still cause damage if shorted.

Steps to check:

• Inspect low-voltage fuse: Locate the automotive-style blade fuse on the Heil control board. If it appears blown (melted link), replace with the same amperage rating only.
• Check thermostat cable: Inspect the thermostat cable where it enters the furnace cabinet. Look for insulation cuts from sheet metal. Repair damaged sections and secure with grommets or tape.
• Disconnect accessories: Temporarily disconnect wires going to humidifiers, air cleaners, or outdoor condenser at the control board R, C, Y, W, etc. Replace the fuse and test. If the fuse no longer blows, reconnect accessories one at a time to find the shorted device.
• Test transformer output: With power on and fuse intact, measure between R and C on the board; you should see around 24 VAC. If not, the transformer may be faulty.

When to call a pro: If fuses continue to blow or you cannot locate the short, or if the transformer has failed, have a technician perform a full low-voltage circuit diagnosis and replace components safely.