Hi Most people don't understand air flow from muffin fans, including the system designers. When the fan is pulling, it pulls air relatively evenly from all directions. When it blows, it has a funny character. The air flow will adhere to the closest surface it will also tend to want to move away, at a right angle to the output direction of the fan. This strange behaveour is the result of two things. One is that the air flow has some rotation from the fan. The other is the good old Bernulli principle. Once the air flow goes over a surface, it will form a low pressure on the surface that will cause the airflow to follow the surface. If there is a filter on the output instead of the input, the airflow will no longer be rotating when it exits. This solve the problem of it taking of at right angles but doesn't solve the problem of sticking to the first surface that it finds. I have even measured air flowing back towards the fin's output, right in front of a fan and not much more than 8 inches away. Before understanding the flow, most would have expected that to be right in the center of the airflow. It would have been, had the fan been in a completely open area. If you want relatively even flow, sucking the air will do better. If you know where the air is going to go, you can expect the best cooling on the outlet side. The trick is to predict where to flow would go. Dwight