A18
Herz
Overpressure / Expansion Fault
Boiler shows A18, pressure rises rapidly during heating, safety valve may discharge
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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
Failed Herz expansion vessel (loss of air charge), blocked expansion connection pipe, faulty pressure relief valve, overfilled system
How to Fix / Troubleshooting
Safety: Hot water discharge from the safety valve can cause burns. Allow system to cool before working on it.
- Check cold pressure: Ensure the system is not overfilled. If pressure is above recommended cold value, safely drain some water from a drain point.
- Test expansion vessel: With system pressure at zero, measure the air charge at the Schrader valve of the Herz expansion vessel. Recharge to the value specified in the manual using a pump.
- Inspect connection: Make sure the pipe between the system and expansion vessel is not blocked or isolated.
- Replace vessel if failed: If water comes out of the Schrader valve or the vessel will not hold pressure, replace the Herz expansion vessel.
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Repair Difficulty
Medium
3/5
Required Part
Herz expansion vessel
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