The fan goes down to 1/2 speed after 30 seconds since the last operation of it.
If the temperature is over a certain point the pump and the fan will run for a bit longer than this to clear the residual heat from the heat exchanger when the boiler isn't being called for heat.
This stops the heat in the heat exchanger building up excessively due to the "inertia" left behind from the burner flames.
In a way it is like a old car that overheats when it is stopped suddenly right after a long motorway journey. Modern cars have an electric fan that keeps running after the engine is switched off the get rid of the resisdual heat build up in the cylinder block.
Mark Lawton, February 2010
