A04
Herz
Fan / Air Pressure Fault
Boiler does not start, display shows A04, no fan operation
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Safety Warning
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.
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Possible Causes
Defective Herz flue fan, blocked flue or air intake, faulty air pressure switch, loose wiring to fan connector
How to Fix / Troubleshooting
Safety: Disconnect power before removing the boiler front cover. Do not run the boiler with the combustion chamber open.
- Check flue terminal: From outside, ensure the Herz flue terminal is not blocked by debris, snow, or bird nests.
- Listen for fan: Restore power and call for heat. If the fan does not start, switch off again and inspect the fan connector and wiring harness.
- Inspect fan wheel: Remove the fan cover and check for dirt buildup or foreign objects preventing rotation. Clean carefully.
- Check air pressure tubing: Ensure the silicone tubes from the fan/venturi to the air pressure switch are not kinked, split, or blocked with condensate.
- Persistent A04: The fan motor or air pressure switch may need replacement. This should be done by a qualified Herz technician.
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Repair Difficulty
Hard
4/5
Required Part
Herz flue fan assembly
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