Boiler displays E16; internal temperature rises rapidly and boiler shuts down

Safety first: Turn off power and allow the boiler to cool fully. Do not continue to reset an overheating boiler without finding the cause.

Steps to check:

• Check system valves: Ensure all isolation valves around the boiler and on the heating circuit are fully open.
• Clean strainers: Locate and clean any Y-strainers or filters on the return line to the boiler.
• Verify pump operation: Confirm that the circulation pump runs when there is a heat demand and that the flow/return temperature difference is reasonable.
• Assess for limescale: If the boiler is in a hard water area and has not been treated, the heat exchanger may be scaled. This often requires professional chemical descaling or replacement.
• Reset and monitor: After addressing flow issues, reset the boiler and monitor operation. If E16 recurs, stop using the boiler and seek service.

When to call a technician: Internal descaling and heat exchanger replacement should be done by a qualified technician.