This is purely educated guesswork by the way, I may be totally wrong as I don’t have the latest ASHP firmware…
- H FAN SPEED 0 = 5c and above
- H FAN SPEED 1 = below 5°c
- H FAN SPEED 2 = below ≈ -4°c
- H FAN SPEED 3 = who knows, my fan has never gone 800rpm so it must be really low
Other have fiddled with the pump speed (and yes this would be what affects flow rate) and can help, @Dave_Storey has done so (see his post here), so he could probably help you. As you can see mine are 0 and are being ignored.
With the fan speed, I assume basically this is how econet is set up by default and if you have the newer firmware you get a raw deal (by default), but that’s guesswork 