How the Case Rear Fan Improves CPU Cooling
By Rafael Otto Coelho on October 31, 2011


Introduction

It is common sense that installing an exhaust fan at the rear panel of the case helps to cool all the components of the computer, including the CPU.

However, most people don’t know simple facts about this subject, such as do you need a rear fan if you keep your case open? Do twin-fan tower coolers benefit from a rear case fan as much as one with a single fan? How much better does a high-rpm fan perform than a silent one?

In order to answer those questions, we ran some tests, measuring the temperature difference between the CPU core and room temperature in several different conditions. We will present the test methodology and results in the following pages.

How We Tested

We tested different situations using the same testbed system that we currently use to test CPU coolers and thermal compounds, which is fully described below. Our Core i7-860 (quad-core, 2.8 GHz) CPU, which is a socket LGA1156 processor with a 95 W TDP (Thermal Design Power), was overclocked to 3.3 GHz (150 MHz base clock and 22x multiplier), and we kept the standard core voltage (Vcore).

We used a Corsair A70 CPU cooler, which is a typical tower cooler that can be mounted with one or two fans.  We used the thermal compound that comes with this cooler.

The fan used in the rear of the case is the SilverStone FM123. We chose this fan because it is very strong and it has a potentiometer where we can set its rpm to any value we choose. We repeated each test four times with the fan turned off, at 1.000 rpm, at 2.000 rpm, and at 2.700 rpm, which is the maximum speed for this fan.

Case Rear Fan
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Figure 1: Fan used in our tests

Room temperature measurements were taken with a digital thermometer. The core temperature was read with the SpeedFan program (available from the CPU thermal sensors), using an arithmetic average of the core temperature readings.

Figure 2 shows our test system, with the rear fan installed and only one cooler fan. Figure 3 shows it with both cooler fans installed.

Case Rear Fan
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Figure 2: Cooler with one fan

Case Rear Fan
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Figure 3: Cooler with two fans

Hardware Configuration

Operating System Configuration

Software Used

Error Margin

Since both room temperature and core temperature readings have 1 °C resolution, we adopted a 2 °C error margin, meaning temperature differences below 2 °C are considered irrelevant.

Tests with the Case Closed

We tested the effect of the rear fan on the CPU core temperature, keeping the left panel of the case closed. There was no case fan other than the rear one. The left panel itself was solid, with no mesh or air intake. This means that, in these tests, the case ran with “negative pressure configuration,” which means that there were only exhaust fans (the case fan and the power supply fan), with no fan blowing air into the case.

The temperatures were taken with the case fan turned off, and then with it at 1,000 rpm, 2,000 rpm, and 2,700 rpm (full speed).

We repeated the tests with only one fan installed on the CPU cooler, and then with both fans installed.

The following table shows our results.

Rear Fan StateRoom Temp.CPU Temp. with Single FanCPU Temp. with Dual Fan
Off16 °C60 °C55 °C
1,000 rpm16 °C57 °C55 °C
2,000 rpm16 °C55 °C54 °C
2,700 rpm16 °C53 °C52 °C

In the graph below, you see the temperature difference between the CPU core temperature and the room temperature with the case closed and with different rear fan speeds. Green bars are the results with two cooler fans; red bars represent the measures taken with only one fan at the CPU cooler.

Case Rear Fan

The measures show a crystal clear reality: the rear fan helps the CPU run some Celsius degrees colder. But it also shows that the temperature drop is more dramatic when the CPU used only one fan (seven degrees) than when we used a two-fan cooler (three degrees).

When using the CPU cooler in push-pull fan configuration, the temperature drop was smaller and actually only appeared when the rear fan was set at high speeds. In this case, the rear fan running at 1,000 rpm didn’t make any temperature difference from when it was turned off.

Notice that the same result obtained with one fan on the cooler and the rear fan running at 2,000 rpm was achieved with push-pull fans on the cooler with the case fan turned off. This means that the second cooler fan can assume the same role as the case’s rear fan, helping the CPU cooler to perform better.

Tests with the Case Open

We repeated the tests with the case open, i.e., without the case left panel. Again, the only fan on the case was the rear one.

The temperatures were taken with the rear fan turned off, and then with it at 1,000 rpm, 2,000 rpm, and 2,700 rpm (full speed). We repeated the tests with only one fan installed on the CPU cooler, and then with both fans installed.

The following table shows our results.

Rear Fan StateRoom Temp.CPU Temp. with Single FanCPU Temp. with Dual Fan
Off16 °C53 °C53 °C
1,000 rpm16 °C52 °C51 °C
2,000 rpm16 °C51 °C50 °C
2,700 rpm16 °C50 °C50 °C

In the graph below, you see the temperature difference between the CPU core temperature and the room temperature, with the case open, with different rear fan speeds. Green bars are the results with two cooler fans, and red bars represent the measures taken with only one case fan.

Case Rear Fan

This graphs show that, without the left panel of the case, the rear fan also can improve the CPU cooling. In tests with both one and two fans on the CPU cooler, the rear fan dropped the temperature in three Celsius degrees when at full speed, comparing to the measure with this fan turned off.

Conclusions

The first conclusion we can make is that keeping the left panel of the case open can reduce the CPU temperature. The best temperature we measured with the case closed is almost the same as the worst temperature achieved with the case open.

We can also say that, even with the left panel of the case open, a rear fan can still improve the CPU temperature.

Our data also show that, if you keep the case closed, the rear fan can take the same role as having a CPU cooler with a push-pull fan configuration. This makes sense, since the rear fan helps to direct the airflow that leaves the CPU heatsink to the exterior of the case. The data also show that a low-rpm (which means a silent) fan helps to improve temperature in single-fan tower coolers, but it needs a more powerful fan to decrease temperature if you use a twin-fan CPU cooler.

Please keep in mind that those conclusions apply to the kind of case configuration we used. In other configurations (such as in cases where the power supply is located on the bottom or in cases with large meshed areas), the results can differ from the ones we presented here.

Originally at http://www.hardwaresecrets.com/article/How-the-Case-Rear-Fan-Improves-CPU-Cooling/1416


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