A05
Herz
Air Pressure Switch Not Proving
Boiler locks out with A05, fan runs but no ignition sequence starts
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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
Faulty air pressure switch, blocked venturi on Herz fan, disconnected pressure tubing, incorrect flue installation causing insufficient draft
How to Fix / Troubleshooting
Safety: Isolate power before accessing internal components.
- Inspect pressure tubes: Check the clear/silicone tubes from the fan/venturi to the Herz air pressure switch for splits, blockages, or water. Drain any condensate and reattach securely.
- Clean venturi: Remove the venturi from the fan and clean any dust or debris that may affect pressure sensing.
- Check switch operation: With power off, gently tap the air pressure switch body; sometimes stuck contacts can free temporarily, indicating a failing switch.
- Verify flue integrity: Ensure the flue is correctly assembled and sealed as per Herz instructions; incorrect lengths or slopes can affect pressure.
- If A05 persists: Replace the Herz air pressure switch and test combustion with a flue gas analyser (technician task).
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Repair Difficulty
Hard
4/5
Required Part
Herz air pressure switch
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