Engine fan over-running

[Bagpuss12]

My wife has a 2003 Fiesta with the 1.4 petrol engine. The car previously belonged to her parents and prior to her owning it, the whole engine was changed. Prior to this change and since (so happened with both engines) the car's heater embarrassingly overruns whenever the engine is turned off no matter how short the journey? The air con works and is used regularly plus the car never overheats? Any ideas?

[Titanium]

I am not an expert but my last 3 fiestas have done this. My latest Fiesta has 2000 miles on the clock if I was worried I would have taken it back to the dealership.