Most newer computer cases locate the Power Supply Unit (PSU) on the bottom of the case. However, when I started building PCs the PSU was usually located at the top of the case. This caused a problem the first time I used a case with the PSU on the bottom of the case. I liked to place the computer on the floor, which is carpeted. I learn the hard way that a case with a PSU on the bottom should never be placed on carpet. Why? I'm glad you asked. Newer PSUs have a fan at the bottom, which draws in air across the PSU components and exhausts the air out the back. That is why cases have a dust filter where the PSU fan draws in air. This type of PSU on carpet acts like a vacuum cleaner filling the dust filter in no time at all, which results in an overheated PSU. Luckily, most newer PSU have overheating protection. It was just strange to have the computer shutdown at what I thought were random times. Since I wasn't going to rip out the carpeting, the simple solution was to put thin wooden boards under the case. Problem solved and the cost was minimal.