Boiler shows A21, burner output limited, fan speed abnormal

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

Defective Herz fan tachometer sensor, blocked fan wheel, loose fan connector, faulty fan control circuit on PCB

Safety: Isolate power before removing the fan assembly.

Check fan wheel: Remove the fan cover and ensure the wheel spins freely and is not fouled by debris.
Inspect tachometer wiring: Verify that the small signal wires from the fan to the PCB are intact and firmly connected.
Clean fan: Carefully clean dust from the fan blades and housing to reduce load.
Replace fan if needed: If the fan does not reach commanded speed or the tach signal is missing, replace the Herz fan assembly.

Hard 4/5

Herz modulating fan

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