Combustion Fan Speed is Constant. There is no adjustment.
Just bump your MIN up a couple points, this will raise the idle temp up a bit. What it is set at currently? I think I have mine around 8 or 10.
What is you draft at Idle? Are you monitoring it with a draft gauge? Mine will get down around .01-.02 at idle. As long as you don't have any CO leakage and it is getting some draft, you should be okay. It all depends on your chimney, outside temps, etc.....
Always check and set your draft with a draft gauge. Also, have working CO detectors.