Boiler displays E21; burner output is limited, and the error may be linked to an excessively high return temperature.

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

Oversized boiler for the system, Pump speed too high, Bypassed flow (automatic bypass valve stuck open), Undersized system circuit.

Safety first: Allow the system to cool before adjusting valves.

Check bypass valve: If an automatic bypass valve is installed near the ACV boiler, ensure it is not fully open. It should be set according to the system design (usually by a technician).
Open radiators: Ensure enough radiators or heating circuits are open to absorb the boiler's output.
Controller settings: Lower the maximum flow temperature on the boiler to reduce stress on the system.
Professional balancing: A technician should balance the system, adjust pump speed, and set the bypass correctly.

Medium 3/5

Automatic Bypass Valve

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