Herz Post-Purge / Control Stuck

Boiler fan runs continuously even when there is no heat demand

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

Stuck relay on Herz PCB, incorrect parameter for post-purge time, faulty flue gas sensor keeping fan on, external control signal stuck

Safety: Continuous fan operation is usually not dangerous but indicates a fault.

Check demand: Ensure there is no hidden demand from a frost protection or anti-legionella program.
Power cycle: Turn off power for several minutes and restore. If the fan immediately starts without demand, suspect a PCB fault.
Check sensors: Verify that flue gas and temperature sensors are reading normally; a false high reading can keep the fan running.
PCB replacement: If a relay is stuck closed, the Herz main PCB will need replacement.

Hard 4/5

Herz main control PCB

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