How To: Properly Plan And Pick Parts For An Air-Cooled PC, Part 1

Power Supply: Mounting Location And Chassis Selection

PSU at the Base of the Unit

Many modern PC enclosures put the power supply down under the motherboard. This installation option has many advantages, so we strongly recommend a chassis configured this way. As you can see from the diagram, a fan sucks in cool air from the floor level through the power supply's own intake, uses it to cool active components inside the PSU, and expels it at the rear of the device.

Advantages to Bottom-Mounting:

• Steady supply of cool air from the floor outside the chassis
• Direct expulsion from the case
• Lower fan speed
• Cooler operation allows for greater PSU efficiency
• Less heat stress on the components, longer life-span
• Lower center of gravity
• No hanging power cable

Disadvantages:

• Chassis must have a bottom opening
• It should also have an air filter
• Possible impact on acoustics, depending on the floor material

Despite minor drawbacks, the above configuration is preferable to some of the other installation options that follow, and you should always look for a case that accommodates it. But it's also possible to make a mistake here:

Don’t install the PSU with its opening facing up in the chassis. You should go this route only with passively-cooled "silent" PSUs, so that the warm air can rise. Otherwise, you're fighting the forces of convection, and possibly creating a situation where a screw or other loose part could fall into the power supply.

PSU at the Top of the Unit

Older PC cases manufactured according to the ATX specification put the power supply just under the case's top. Air is sucked in from inside the chassis and then expelled. Supposedly, this improves dissipation and prevents heat from building up. However, it also results in the power supply absorbing much of the waste heat generated by the graphics cards and processor. Consequently, you get inefficient operation from the PSU, whereby maximum power and efficiency are almost impossible to reach at temperatures exceeding 40°C (as they are normally based on an operating environment of around 25°C.) The longevity of components inside the power supply also suffer.

Advantages to Top-Mounting:

• Helps with cooling in some systems
• Shorter cable necessary for 12 V connections

Disadvantages:

• Higher PSU temperatures
• Inefficient and loud operation
• System ages faster

The Perfect Chassis...

...doesn’t exist. However, large, well-designed towers like the Corsair Graphite 600T come very close. There is nothing to restrict airflow inside the structure. Space, rear cable management, and lots of fans and air filters combine to create a solution that approaches what we'd consider to be ideal.

As much as possible, you should stick to enclosures that don't restrict airflow from bottom to top. If you want to include a particularly long graphics card in your configuration, employ a chassis with as much depth as possible. Otherwise, the card will split the airflow. Thick cables should always be at the rear. Also, anything that rattles around will significantly disrupt the airflow.