Having correct airflow through your PC case is critical to your system running properly. Heat buildup in a computer case can cause lockups, slowdowns and even shutdowns. You should carefully consider the direction of the air when building a PC or when modifying an existing computer.

Which way should the case fans blow?
The CPU fan attached to the heat sink should blow down onto the heat sink and processor. Fresh air can be pulled in this way, but you will want additional fans installed to help bring cooler air into the case. An 80mm case fan installed in the front of the computer pulling air in will help the processor run cooler. You may also want to add an additional fan at the rear of the PC that blows the air out of the back. This additional fan in the rear may not be necessary if the power supply has an intake fan inside the case that can pull that warm air out.

Improving airflow in the case
You can greatly increase the airflow through your PC by swapping out the standard, flat IDE cables with the rounded type. Because the rounded cables take up less space, air can easily move through the case without being trapped inside. Depending on your setup, you can also rearrange the power cables so that extra wires can be coiled and tucked away near the top.

Balancing PC airflow
Another thing to consider when adding fans is the effect it will have on the interior of the case. If you have a lot of fans pulling air in and not enough moving air out, the interior of the PC will become filled with hair, dust and other debris. You should also not hear one fan working harder that the others. This means the airflow is not balanced properly. For instance, if the rear case fan makes a whirring sound every few seconds, this could indicate there is too much air trying to escape and it cannot keep up.

Whatever you do to improve airflow, you should open the case after a week to make sure everything is clean. You can also download monitoring tools to test the case and CPU temperatures before and after to make sure air is circulating properly and keeping the CPU cool.