F24 error code on valliant thermo boiler?

F24 = dry fire protection (temp gradient of flow NTC too steep)

Possible causes: air in boiler or system, pump blocked or defective, pump output too low, blocked or partially blocked heat exchanger, sludged up system.

Basically, what it is saying, when the boiler fires, the water temp on the flow pipe increases so quickly the boiler sees something is wrong with either the volume of water (air in system) or the flow of water not being enough (pump / heat exchanger / sludged system)

T
April 2008