Bosch tankless water heater makes loud banging or knocking noises when operating

Safety first: Turn OFF power and gas before internal inspection. Loud bangs from combustion can be dangerous.

• Differentiate noise: Determine if the noise is from pipes (water hammer) or from inside the heater (combustion or scale).
• Water hammer: Install water hammer arrestors near quick-closing valves (dishwasher, washing machine) and secure loose piping with proper clamps.
• Scale noise: Rumbling or kettling sounds often indicate scale. Perform a full descaling of the heat exchanger.
• Mounting: Ensure the heater is firmly mounted to the wall with all screws tight and appropriate backing.
• Combustion bangs: If you hear a loud boom at ignition, shut down the heater and have a qualified technician inspect the burner, gas valve, and ignition timing.