A03
Herz
Safety Temperature Limiter (STB) Tripped / Overheat
Boiler locks out shortly after start, display shows A03, overheat LED may be lit
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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 on Herz boiler, failed circulation pump, closed system isolation valves, faulty safety temperature limiter sensor
How to Fix / Troubleshooting
Safety: Switch off power and allow the boiler to cool completely. Do not reset repeatedly without finding the cause.
- Check system pressure: Verify that the pressure gauge shows the correct cold pressure (typically 1.0–1.5 bar, check Herz manual). Refill via the filling loop if low.
- Verify circulation: Ensure all radiator and isolation valves near the boiler are open. Listen for the Herz circulation pump running when there is a heat demand.
- Bleed air: Bleed air from radiators and the automatic air vent near the boiler to restore proper flow.
- Reset STB: Locate the manual reset safety thermostat on the Herz boiler (usually on the flow pipe or heat exchanger). Press the reset button if accessible.
- If A03 returns: The primary heat exchanger may be scaled or the pump may be seized. Contact a Herz service partner for descaling or pump replacement.
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Repair Difficulty
Hard
4/5
Required Part
Herz circulation pump
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