High Pressure
Grant
System Over-Pressurisation
Boiler shows high pressure warning, gauge above 3 bar, pressure relief valve discharging water outside.
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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
Overfilled system via filling loop left open, Failed expansion vessel diaphragm, Faulty pressure relief valve not reseating
How to Fix / Troubleshooting
Safety: High pressure can cause leaks. Do not tamper with safety valves unless competent.
User checks:
- Ensure the filling loop valves are fully closed and the flexible hose is disconnected if the design allows.
- Bleed a small amount of water from a radiator air vent to reduce pressure to around 1.5 bar when the system is cold.
Engineer actions:
- Check the expansion vessel pre-charge using a tyre gauge with the system depressurised; recharge with air/nitrogen to the specified pressure or replace the vessel if the diaphragm has failed.
- Inspect and test the pressure relief valve; replace if it continues to drip after pressure is normalised.
- Verify that the filling loop is correctly installed with non-return valves and is not allowing mains water to pass when closed.
Persistent high pressure requires professional diagnosis to prevent damage to the Grant boiler and system.
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Repair Difficulty
Medium
3/5
Required Part
Pressure relief valve
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