A05
De Dietrich Air Pressure Switch / Flue Safety Fault

Boiler shows A05, fan may run briefly then stop, no ignition

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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

Blocked flue or air intake on De Dietrich concentric flue, Faulty air pressure switch, Blocked condensate trap affecting flue pressure, Damaged silicone tubes between fan and pressure switch

How to Fix / Troubleshooting

Safety: Switch off power. Do not operate the boiler with a suspected flue fault; risk of flue gas spillage and carbon monoxide.

Checks:

  • Inspect flue terminal: Outside, ensure the flue outlet and air intake are clear of obstructions (ice, nests, leaves). Remove any blockage without damaging the terminal.
  • Check condensate trap: Open the boiler casing and locate the condensate trap. Remove and clean it of sludge and debris. Refill with clean water before refitting to maintain the water seal.
  • Inspect pressure switch tubing: Ensure the small silicone hoses from the fan or flue collector to the air pressure switch are not split, kinked, or disconnected. Replace damaged hoses.
  • Reset and test: Restore power and reset the boiler. Listen for the fan starting and check if the error clears.

Technician tasks: If A05 persists, the air pressure switch or fan speed control may be defective. A qualified De Dietrich technician should test switch operation, fan speed, and flue integrity.

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Repair Difficulty

Hard 4/5

Required Part

Air Pressure Switch
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