Which way does the rear fan go in a compaq computer tower

The proper installation of a CPU fan is often the subject of debate. Though there are conflicting theories as to which direction the CPU fan should face, there are methods to ascertain which CPU fan installation best suits your needs. The determining factors of your CPU fan installation direction include the manufacturer specifications, your case design and the specific design of your heat sink
Horizontal Installation

• The installation of a CPU fan horizontally to the plane of the motherboard is the most common installation orientation there is. Your options when installing your CPU fan are to blow cool air onto the heat sink or to pull cool air through the heat sink from the motherboard. Horizontal installations show negligible advantages in either installation direction. The differences in temperature usually vary by only 1 or 2 degrees Celsius. The main point to consider when installing a horizontal CPU fan is the location of the rear exhaust fan on the case. If the rear exhaust fan on the case is parallel to the processor, it is advantageous to have the fan pull cool air from the motherboard and push it out above the heat sink. This is because the case fan will quickly draw that hot air away from the CPU and out of the case, allowing for quick cooling.
  

• Vertical, or tower, heat sinks rise high enough off the motherboard to allow for a fan to be installed perpendicular to the plane of the motherboard. This allows for cooling to go from the front to the back of the case, as opposed to going from the bottom to the top. Optimal installations for vertical heat sinks are blowing toward either the back or the top of the case. If there is no exhaust fan in the top face of your computer's case, it is more effective to orient your fan on your vertical heat sink so that it blows toward the rear exhaust fan of the case. If there is an exhaust fan on the top face of your computer case, it is almost always more effective to mount your CPU fan so that it blows air up toward the top of your case. The reasoning behind this is that hot air rises and will more naturally flow off the top of your case. Also, top-mounted fans are usually larger and allow for faster airflow out of the case.
  

• It is often the case with nonstock heat sinks that the manufacturer has designed the heat sink's fan to be mounted in a limited number of orientations. This is usually evidenced by limited mounting notches on the heat sink itself. Most aftermarket heat sinks use technologies such as heat pipes that have specific cooling needs to operate appropriately. Because of this, it is important that in any CPU fan installation you follow the manufacturer's installation instructions.