BoilerFaultCodesYour boiler has detected that the water isn't moving through the system correctly, causing it to heat up too quickly and shut down for safety.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Dry fire protection (difference between flow and return NTC's too large) | No water in appliance, pump cable defective, pump blocked or defective, pump output too low
What causes this fault?
This fault occurs when the boiler detects a massive temperature gap between the pipes leaving the unit and those returning, suggesting that water has stopped circulating. The most common reason is a failed or jammed internal pump, but it can also be caused by a severe blockage in the system or air being trapped within the heat exchanger.
This issue frequently appears at the start of autumn when heating systems are switched back on after being idle all summer, which can cause the pump to seize.
DIY fix possible
Check if the central heating valves under the boiler are fully openCheck the boiler pressure gauge and top it up to 1.5 bar if it is too lowEnsure all radiator valves (TRVs) are turned to their highest setting
Find a Gas Safe engineer
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Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like F.23 — no call-out fees, no surprises.
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Vaillant Thermocompact is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
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