Thermocontrolled Fan, ver 3

In previous circuit, the temperature turn on the fan instantly. The regulation period is very short. To make it more wide just add only one resistor (R5):

While adjusting sensitivity of your schematics use potentiometer instead of R5. It must me about 100K … 1M range. After fine tuning, measure the resistance of potentiometer and replace with simple resistor with same nominal value.

Don’t forget to add capacitors near error inputs. Circuit is quite sensitive and various interferences can start the fan.