Heater displays F13; unit may start but shuts down quickly without heating.

Safety first: Turn off power and gas. Hot exhaust components can cause burns; allow the unit to cool.

• Check exhaust outlet: Inspect the flue outlet for obstructions such as nests, leaves, or covers. Clear any blockage and ensure proper clearances.
• Inspect flue sensor: Locate the exhaust temperature sensor on the flue collector. Check wiring for damage and ensure the sensor is firmly attached.
• Clean heat exchanger (visual): Look through inspection ports for heavy soot or scale. Excessive buildup can overheat exhaust gases and should be professionally cleaned.
• Verify venting: Confirm that vent pipe size, length, and termination meet Thermeau installation requirements. Incorrect venting can trap heat.
• Replace flue sensor if needed: If wiring is intact but F13 persists, replace the flue temperature sensor with the correct Thermeau part.