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Improving System Cooling Part 1 How and why to add case fans to your system

Improving System Cooling Part 1
How and why to add case fans to your system

Adding a Rear Case Fan

I have discussed the necessity of adequate system cooling over the years in my book Upgrading and Repairing PCs as well as in my article Keep Your Computer Cool In Any Season. However, as system speeds continue to rise, its worth a second look at the topic. My previous article laid the groundwork for improving system cooling by describing the reasons for doing it and discussing some of the types of cooling you can install. In this article, I want to show you how to add more cooling power to your system by adding a rear case fan.

No matter how many internal fans you have in your system (processor, North Bridge/MCH), graphics card, drive bay), they wont accomplish much unless the hot air they move away from the components they protect is vented out of the system. While the power supply does pull air through the system, this alone isnt enough to aid proper cooling, especially with faster memory and processors creating more heat than ever before. Most systems have a front case fan lined up with the expansion slots to help the power supply draw air into the system, and most full-size ATX cases have one or more rear openings designed for cooling fans (see Figure 1).

Figure 1 This system has several heat sources (processor 1, North Bridge chip 2, and memory 3), but no rear case fan to move hot air out of the system. Two openings (4) are available for installing case fans.

If your system doesnt have a rear case fan, or if you have only one fan and have an opening for a second case fan, you should install one. This is especially necessary if you have installed more memory, a faster processor or a faster 3D graphics card. All of these components add to the thermal load inside the computer.

Features of Typical Rear Case Fans

Typical rear case fans range in size from 60mm to 120mm. The openings visible in Figure 1 are each 80mm wide (see Figure 2).

Figure 2 Choose a fan that fits the opening(s) in your case.

Case fans, like other fans, are rated in two ways:

  • CFM, which measures airflow (higher provides better cooling)
  • dBA (decibels), which measures noise (lower is quieter, but quieter fans usually provide less cooling)

If you are trying to cool a system which is located in an office or used as part of a home-theater setup, I suggest that you look for case fans with a low dBA rating, even if the airflow (CFM) value isnt as good as a noisier fan. However, if maximum cooling is the most important factor, look for high CFM values.

Low-cost case fans typically use sleeve bearings. Sleeve bearings are noisy and can fail. I recommend ball-bearing fans for quieter operation and more durability, especially models with dual ball-bearing mechanisms.

Case fans can be powered in one of two ways:

  • With a direct connection from the motherboard
  • With a spare 4-pin Molex power supply connector (normally used for hard drive and optical drive power)

The motherboard connection has the benefit of enabling you to monitor fan performance with the BIOS hardware monitor feature available in many recent computers. However, plugging fans directly into the motherboard increases the electrical requirements on the motherboard, and could cause system instability if your power supply is marginal. If you connect the fan to a Molex connector, you usually have a pass-through option which enables you to power another device (such as another fan or a drive) with the same connector. However, with a fan kit such as the one shown in Figure 3, you lose the ability to monitor fan performance in the BIOS if you use the four-wire Molex connection. For this reason, some vendors now provide a different type of connector, which enables the fan to draw power directly from the power supply with a Molex connector while also connecting to the motherboard so the BIOS can monitor the fan.

Figure 3 A typical case fan which can be connected directly to the motherboard (1) or can be adapted to draw power from a Molex power connector (2). A pass-through connection enables the fan to share power with another device with a Molex power connector (3).

Installing the Rear Case Fan

Follow this procedure to install a new case fan or replace a case fan that has failed:

1. Shut down the computer and unplug it.

2. Take precautions against electrostatic discharge (ESD): touch the case before touching any components and use a wrist strap grounded to the case. See Chapter 22 of my book, Upgrading and Repairing PCs for additional details.

3. Remove the computer case and locate the opening you plan to use for the case fan. I recommend you use the opening nearest the processor, North Bridge chip and memory to help exhaust as much heat as possible from the system interior.

4. Hold the fan in place and fasten it with a single screw.

5. Repeat by fastening the opposite corner into place from the outside of the case, then fasten the rest of the screws (see Figure 4).

Figure 4 Fastening the case fan into place.

6. Connect the fan to the power supply or motherboard.

7. Restart the system and verify that the fan is running (see Figure 5).

Figure 5 This case fan (1) is connected to the motherboard (2) for power and system monitoring. A second case fan can be added to the opening (3) next to the new fan.


Adding a rear case fan helps the power supply fan move the hot air from increasing numbers of sources out of the system. With typical rear case fans selling for less than $15 in the USA, this upgrade is relatively simple, but can pay big dividends in improving reliability.

In a future story, Ill show you how to install cooling devices on your motherboards North Bridge and South Bridge chips.

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