Boiler displays E20; burner shuts down shortly after lighting, and primary temperature rises too quickly.

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Blocked primary heat exchanger, Sludge in system, Closed isolation valves, Incorrect pump speed

Safety: Allow the boiler to cool. Surfaces and water may be very hot.

User checks:

Ensure all radiator valves are open and that no isolation valves near the boiler have been closed.
Check system pressure and bleed radiators.

Engineer checks:

Inspect the Keston primary heat exchanger for signs of blockage or poor circulation (large temperature difference between flow and return).
Powerflush the heating system and install a magnetic filter if heavy sludge is present.
Check pump performance and speed setting; replace the pump if it cannot maintain adequate flow.
Verify that any bypass valve is correctly set to allow minimum flow through the boiler.

Hard 4/5

Primary Heat Exchanger

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