The temperature sensors, and the limits set for them, as well as the various fan speeds, are not accessible by users. If your PC continues to run system heavy tasks, the fans will also run to stabilize the temperature. The chassis of your laptop or your PC are designed so that air will flow out which is why it is very important you never block the vents. They have sensors built-in that periodically monitor the temperature of your system, and when it reaches a certain threshold, they start to spin to cool it down. The fans know when to start running faster, and when to slow down.
