F22 displayed, no ignition, often after a water supply interruption.

Safety first: Turn off power before opening any water connections.

Steps:

• Check water supply: Ensure the main water valve is fully open and that cold water is available at other taps.
• Clean inlet filter: Remove and clean the filter on the cold water inlet to the heater.
• Bleed air: Open hot taps to purge air from the system.
• Pressure switch: On models with a water pressure switch, check wiring and replace the switch if it does not close at normal pressure.

Warning: Do not operate the heater without water. This can destroy the heat exchanger.