BoilerFaultCodesPotterton P20 Fault Code
The Potterton P20 fault code means: your boiler has too much water in the system, which puts extra stress on the internal components and may cause it to shut down for safety.. It appears on 1 Potterton model in our database.
Technical description: High water pressure (above 2.8 bar)
What causes the Potterton P20 fault?
This fault occurs when the water pressure inside your system rises beyond safe operating levels, often caused by a filling loop being left slightly open or a failure in the internal expansion vessel. When the vessel can no longer absorb the natural expansion of heated water, the pressure spikes and triggers an automatic safety shutdown. In some cases, it can also be caused by a faulty internal valve allowing fresh mains water to constantly leak into the heating circuit.
This fault is more common in winter when the heating is first turned on for long periods, as the increased heat causes water to expand and exposes any underlying issues with the expansion vessel.
DIY fix possible
Check that the silver braided filling loop valves are fully closedLocate a radiator and bleed water from it using a radiator key to lower the pressureMonitor the pressure gauge until it returns to between 1.0 and 1.5 barPress the reset button to clear the fault code
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Gas Safe Register →Potterton models that show the P20 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Potterton P20 fault code information on this page is sourced and cross-checked against the official Potterton service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026