A05
IVT
Primary Circuit Overheat / Safety Thermostat Trip
Boiler locks out with A05, overheating, may make boiling or kettling noises before shutdown
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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 primary heat exchanger, Failed or seized circulation pump, Airlock in primary circuit, Faulty overheat thermostat or NTC sensor
How to Fix / Troubleshooting
Safety: Overheating can cause scalding and damage. Turn off power and allow the boiler to cool completely before touching any components.
User actions:
- Check that all radiator valves are open to allow circulation.
- Ensure that system pressure is within the correct range (usually 1.0–1.5 bar cold).
- Perform a single reset once the boiler is cool. If A05 returns, call a technician.
Technician actions:
- Verify operation of the circulation pump (check for voltage, rotation, and free movement). Replace the pump if seized or weak.
- Bleed air from the pump and high points in the system.
- Check the primary heat exchanger for scale or sludge blockage; power-flush or replace if necessary.
- Test the overheat thermostat and flow NTC sensor for correct resistance and replace if faulty.
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Repair Difficulty
Hard
4/5
Required Part
Circulation Pump
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