Display shows E8 and unit shuts down, often accompanied by rumbling or boiling sounds

Safety first: Turn off power and, for gas units, close the gas valve. Allow the unit to cool completely. Do not continue to operate the heater with this error.

Steps to check:

• Verify that all isolation valves on the inlet and outlet are fully open and that there are no closed valves downstream restricting flow.
• Check inlet filter screens and clean any debris.
• Bleed air from the system by opening multiple hot water taps until a steady stream of water flows.
• For tankless gas models, perform a chemical descaling of the heat exchanger using a pump and descaling solution per CTC instructions.
• For electric models, inspect the heating elements for signs of dry-firing (blistered or deformed sheath). Replace any damaged elements.

Note: Persistent E8 errors after descaling and flow checks indicate a possible faulty temperature sensor or control board and should be evaluated by a technician.