Cabinet fan systems that contain only one fan should have it configured to exhaust hot air out the top of the cabinet. There must be ventilation holes located elsewhere on the cabinet, preferably towards the bottom to act as an air intake. Using fans to push air out will cause negative or lowered pressure within the cabinet. Because air moves naturally from high pressure to low pressure areas, outside air will flow into the cabinet from the ventilation holes. Poorly designed systems include set-ups where there are one or more fans to exhaust air, but no other fans or ventilation holes to act as the air intake.