BoilerFaultCodesSaunier Duval F26 Fault Code
The Saunier Duval F26 fault code means: your boiler has detected that the water is getting hot too quickly or the temperature difference between pipes is too wide, often caused by poor water flow.. It appears on 2 Saunier Duval models in our database.
Technical description: Maximum delta temperature (flow - return). Water circulation fault. Check central heating flow and return connection
What causes the Saunier Duval F26 fault?
This fault occurs when the boiler's internal sensors detect that heat isn't being carried away from the burner fast enough, causing the unit to overheat rapidly. The most common reason is a blockage in the system or a pump failure, which prevents water from circulating through the radiators as it should.
This issue often surfaces during the first cold snap of autumn when heating systems are turned on after sitting idle for months, which can cause internal pumps to seize.
DIY fix possible
Check that all radiator valves and isolation valves are fully openEnsure there is enough water pressure by checking the boiler gauge (target 1.5 bar)Bleed all radiators to remove trapped air that might be blocking flowReset the boiler to see if the fault clears
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Gas Safe Register →Saunier Duval models that show the F26 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Saunier Duval F26 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