F.28 ignition fault; water heater attempts to ignite several times but fails, no stable flame

Critical safety: Work on gas components must only be carried out by a qualified gas engineer. Do not attempt to adjust gas valves or burner settings yourself.

• Basic user checks: Ensure the gas isolation valve to the heater is open and that other gas appliances in the property are working. If there is a general gas supply issue, contact your gas supplier.
• Visual inspection (engineer): With power isolated, remove the burner cover and inspect the ignition electrode and cable for cracks, carbon build-up, or damage. Clean the electrode with a non-abrasive pad and check the gap to Vaillant specification.
• Check gas valve operation (engineer): Verify that the gas valve receives the correct voltage during ignition and that inlet gas pressure is within spec. Replace the gas valve if it does not open correctly.
• Check flue and air supply: Ensure the flue is not blocked and that combustion air inlets are clear. Blockages can prevent proper ignition.
• Test ignition transformer/PCB: If there is no spark at the electrode, test the ignition transformer output and PCB control signals. Replace the faulty component.
• Reset and test: After repairs, reset the appliance and perform several ignition cycles to confirm reliable starting.