By the way you'll often see equal component values used when the values aren't super critical, as
manufacturing costs go up with the number of different components used on a board. The fact
R2 = R3 is not indicative that 2k2 is the ideal value for either.
And because this is designed for a heater its probably never intended for PWM. The free-wheel
diode is probably a "just in case" component, heaters are not usually particularly inductive, but
the designer of the circuit guarded against the user of the circuit sticking in a relay between this
board and the actual heater.