BoilerFaultCodesSaunier Duval F21 Fault Code
The Saunier Duval F21 fault code means: your boiler has stopped working because the water pressure inside the heating system has dropped too low to operate safely.. It appears on 3 Saunier Duval models in our database.
Technical description: Low water pressure
DIY fix possible
Locate the external filling loop, which is usually a flexible silver braided hose connecting two pipes under the boiler.Open the valves at both ends of the loop to allow mains water into the heating system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (typically between 1.0 and 1.5 bar).Close both valves tightly and remove the filling loop if required by your specific model setup.
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Gas Safe Register →Saunier Duval models that show the F21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Saunier Duval F21 fault code information on this page is sourced and cross-checked against the official Saunier Duval service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026