Don't kn="peng, post: 712876, member: 7644"]I think they may be adequate protection for premature damage due to overheating from not having adequate natural ventilation. Denon is vague about what is good ventilation. In the manual they only advised "Please re-install this unit in a place having good ventilation."
The build in fans appeared to have been factory set to turn on when the heatsink temperature is so high that the unit may be allowed to run hot enough to shorten its life without the fans ever reach the set point. That is just based on my educated guess, having measured how "warm" the unit could get by shooting my infrared temperature gun onto the warmest spot on top and inside of the enclosure that the light beam could reach. So to me, an external fan is for longevity reason if nothing else.
Don't know why they don't allow manual control, like on lowest rpm whilst on and 20 mins after putting into standby[/QUOTE]
Cost!