Display shows E04 and the heater will not start; may occur immediately on power-up.

Safety: Allow the unit to cool completely. Turn off power before removing covers. Overheat conditions can indicate serious circulation problems.

Steps:

• Check system pressure and circulation (if connected to a system circuit): Ensure the system pressure gauge (if fitted) is within the recommended range. Verify that isolation valves on the flow and return are open.
• Reset overheat thermostat: Locate the manual reset overheat thermostat on the Baxi heat exchanger (often a small button). Press firmly until a click is felt. Do not repeatedly reset without finding the cause.
• Check for limescale: If the heater is used in a hard water area, limescale can cause local overheating. Listen for kettling noises. Descaling of the plate heat exchanger or primary circuit may be required.
• Verify pump operation (if applicable): On Baxi system water heaters with an internal pump, check that the pump runs when there is demand. Listen for vibration or feel the pump body. A seized pump can cause rapid overheating.

Professional intervention: If E04 recurs, a technician should check flow rates, clean or replace the primary heat exchanger, verify pump performance, and replace the overheat thermostat if defective.