A temperature isolation duct in a computer system comprising a chassis securing a circuit board and a fan system that draws air through the chassis, and a heat-generating component is mounted on the circuit board and exposed to the air flow. The hot air duct passively directs air heated by the heat-generating component from a single hot air duct inlet in direct downstream alignment with the heat-generating component to a single hot air duct outlet. A thermal sensor is secured within, or in direct alignment with, the hot air duct near the duct outlet for sensing the temperature of air flowing through the hot air duct and generating a temperature signal. A controller is in electronic communication with the thermal sensor for receiving the temperature signal and in electronic communication with the fan system for sending a fan speed control signal.