Unit locks out with F5, often accompanied by noisy fan or poor combustion

Safety first: Turn off power and gas. Risk of carbon monoxide if flue is blocked.

Steps:

• Inspect flue route: Check that the Saunier Duval concentric flue is correctly assembled, with no sagging sections or signs of water leakage. Ensure terminal clearances are respected.
• Clean condensate trap: If the heater is condensing, remove and clean the condensate trap and siphon. Blockage can cause backpressure in the flue.
• Check air pressure switch: Verify that the switch clicks when gently applying suction/pressure to the tubing (with power off). Replace if it does not operate reliably.
• Check fan speed feedback: Some models monitor fan speed via a tachometer signal. If the fan is noisy or slow, the fan assembly may need replacement.

Warning: Flue system work must comply with local regulations. If you suspect flue gas leakage, stop using the appliance and call a professional immediately.