Tuning Fan Speeds for Positive/Negative/Neutral Net Flow

Cooling Fan Tuning
the nerd 389
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One question that frequently pops up around these forums involves tuning fans to achieve positive, negative, or neutral pressure or flow.

These three distinct flow conditions each have their own advantages. First, we will discuss what each of the three conditions are, and when you might want to use them. Next, we will discuss one inexpensive and reliable way to achieve the flow condition of choice with air coolers. Third, we will discuss a significantly more involved method for tuning fan speeds when using an AIO cooler. Finally, we will discuss how to tune the fans for a fully water cooled build that includes both a GPU and CPU in the cooling loop.

Throughout this tutorial, I will refer to the flow conditions as positive flow, neutral flow, and negative flow. My choice to use the term "flow" instead of the more common "pressure" is simply out of convenience. In this article, flow and pressure may be freely interchanged without altering the concepts presented.

Understanding the Three Flow Conditions
What is a flow condition?
These flow conditions describe the balance of forced airflow between the exhaust fans and intake fans. Positive flow means that the intake fans push more air into the case than the exhaust fans force out of the case. Negative flow means the opposite. Neutral flow means that the exhaust fans force the same amount of air out of the case as the intake fans force into it.

The difference between the intake and exhaust flow due to the fans is balanced by any gaps that do not have fans on them. In positive flow conditions, the excess intake flow will pass out of the case through these openings. In negative flow conditions, air will enter the case through these openings. See the image below for reference.

Most cases feature more passive openings on the rear of the case than the front. There are a number of reasons for this. This type of design can be used to great effect with AIO coolers and positive flow.

Positive Flow or Positive Pressure
Positive flow is a condition that exists when the intake fans in a case move more air than the exhaust fans. The result of this condition is that air is forced out of any available openings where ever an intake fan is not present.

This means that dust can only enter the case through an intake fan. If the intake fans are filtered, then all air entering the case will be filtered as well. This is the most commonly cited advantage of positive flow, but certainly not the only one.

The other advantages of positive flow are more subtle, but can be just as important.

Another advantage manifests itself if a case is arranged with intake fans exclusively on the front and bottom of a case, and exhausts on the rear and top. In this configuration, the heat sources within the case are all near the exhaust fans, not the intake fans. As such, heated air will leave the rear or top of the case, and cannot be drawn back into the intake fans easily. This results in appreciably lower case temperatures than the opposite orientation, and if the area between the top of the GPU and the left side panel is open to airflow, it will exhaust GPU heat more efficiently.

Builds with top-mounted radiators benefit the most from positive flow. In all flow conditions, the heat generated by the GPU is pulled through the CPU cooler to varying degrees. In positive flow conditions, a large portion of the GPU heat can be evacuated through the rear of the case instead of the CPU cooler, mitigating the disadvantage of using the CPU cooler as an exhaust.

The main disadvantage of positive flow is that the intake fans generally must run faster than the exhaust fans, and thus create more noise. This results in a higher noise level in front of the case, where most people sit when using the computer. Some case designs place sound absorption around the front fan ports specifically to mitigate this issue. When properly employed, this absorption can significantly reduce intake noise.

A minor disadvantage of positive flow is that dust filters on the intake fans will clog up more quickly, and require more frequent cleaning. This disadvantage is offset by the fact that less dust enters the case itself, leading to reduced maintenance of the inside of the computer.

Neutral Flow or Neutral Pressure
Neutral flow is a condition where the exhaust fans move the exact same amount of air as the intake fans, and there is no flow in openings that do not have a fan.

Like positive flow, all air entering the case must move through the intake fans. If those fans are filtered, then all air entering the case will be filtered.

Neutral flow is generally the quietest of the three flow conditions when there are the same number of intake and exhaust fans. This is simply due to the fact that running all fans at the same speed results in lower noise levels than running fans at different speeds to produce the same net flow.

Neutral flow has quite a few disadvantages, though.

Specifically, the dust filters will clog up over time. As this happens, the intake fans will gradually move less air, and the flow condition will become progressively more negative. As such, the dust filters must be cleaned extremely frequently to maintain neutral flow.

Another disadvantage is that all heated air must pass through the exhaust fans. When using an AIO cooler, this can increase the CPU temperature significantly. This also subjects the fans to higher temperatures than other flow conditions.

AIO coolers present another disadvantage that's quite significant. Neutral flow conditions require extremely precise tuning. AIO coolers vary the speeds of either the exhaust or intake fans based on CPU temperature. This means that if the cooler is an exhaust, some of the intake fans need to be tied to the CPU temperature, and vice versa. This also means that all of the fan curves for those fans need to be precisely tuned for all operating conditions in order to maintain neutral flow. This is incredibly time consuming, and GPU heat can disrupt the balance quite easily.

Negative Flow or Negative Pressure

Negative flow is a condition where the exhaust fans force more air out of the case than the intake fans force into it. This results in air entering the case at all openings that do not have a fan.

Unlike neutral and positive flow, negative flow does not allow all air entering the case to be filtered, and accumulates dust inside of the case more quickly than the other flow conditions when intake filters are used. There is no difference in dust accumulation between the three flow conditions if no intake filters are employed.

Negative flow has a few advantages in specific situations.

Specifically, when an AIO is used as a front intake, negative flow can, in some instances, reduce the case temperature by allowing cool air to enter where ever it can. This can never completely offset the temperature increase associated with intake radiators, but it can mitigate the effects to some degree.

Negative flow can also result in a quieter build than the other conditions when the computer is placed underneath a desk. This is a result of the intake fans running more slowly than the exhaust fans, and the fact that the user is shielded from the exhaust fans by the desk.

Negative flow does have many disadvantages, though. So many, in fact, that it normally best to avoid it altogether in most builds.

The first and most significant disadvantage in air cooled builds stems from the PSU heat being drawn back into the case very close to the GPU. This is known as recirculation, and it basically means that some of the air is used to cool two separate components or cooling the same component twice. It results in less effective cooling on the second pass. Since GPUs tend to operate very close to their thermal limits, the impact of this can perceptibly reduce gaming performance of a build.

In more aggressive negative flow conditions, the exhaust flow from the upper rear fan can also be drawn back into the case, effectively trapping a portion of the GPU heat in an eddy. As such, it is usually advisable to forgo any rear exhaust fans in builds that make use of negative flow conditions. Note that a rear intake fan does not improve matters, as the additional suction will create an eddy between the rear intake and the top-rear exhaust. The result is basically the same, but in a different location. Instead of increasing the GPU's temperatures, it will increase the CPU's VRM temperatures.

Tuning Airflow
The simplest approach to tuning airflow is the incense method. There are several video guides on the basic incense method available. The methods below describe a more comprehensive procedure that can properly tune the most complex builds.

The methods presented below are intended to provide a rigorous and reliable approach to tuning that will always result in the flow condition of your choice, provided that your system is physically capable of achieving that flow condition. As a result, they can take a good deal of time to perform on more complex builds. The third method, intended for the most complex of water cooling builds, will require the better part of a day to complete in some cases. On the plus side, though, these methods will get you consistent and reliable results at all operating conditions.

The basic incense method requires these materials:
  • Fully assembled computer that supports fan speed control on all fans
  • Several sticks of incense
  • A lighter
  • Patience
In addition, these materials are required for the more advanced tuning methods covered later:
  • Pencil
  • Paper
  • Flexible thermometer, such as a thermocouple
Each of the tuning methods presented below builds on the previous methods. For instance, step 2.2 includes performing steps 1.1 through 1.4, and step 3.2 includes performing steps 2.1 through 2.5, and therefore also includes steps 1.1 through 1.4. The steps are in this format to reduce the extreme redundancy of the procedure required for complex builds.

Basic Method
Step 1.1
Turn on the computer and open the fan control software of your choice. Set all fans to manual control.

Pick a setting for all fans to start with. If you're not using temperature control, this should be around 90-100%.

Step 1.2
Light a stick of incense, and hold it near an opening that doesn't have a fan associated with it.

Look at the trail of smoke rising from the incense and note it's direction. The direction that the smoke moves serves as a clear indicator of your present flow condition.

If the smoke is pulled into the case, the current settings are producing negative flow.
If the smoke rises in a straight line, neither pulled toward the case or push away from it, your current settings produce neutral flow.
If the smoke is pushed away from the case, your current settings produce positive flow.

Once you have taken note of the current flow conditions, set the incense down in a safe place where it will not touch anything flammable.

Step 1.3
If your flow conditions are not currently what you want, adjust your settings.

If the current setup is more positive than you want (I.E. you want to have neutral or negative flow, and you read positive flow in the incense test), make one of the two following adjustments:
  • Increase the speed of the exhaust fans, or
  • Decrease the speed of the intake fans
If the current setup is more negative than you want (I.E. you want to have positive flow, and you read negative or neutral flow in the incense test), make one of the following two adjustments:
  • Increase the speed of the intake fans, or
  • Decrease the speed of the exhaust fans
For the best results, alternate your adjustments between increasing and decreasing speeds.

Step 1.4
Repeat steps 1.2 and 1.3 until you achieve the flow condition that you want.

Fully Tuning Flow Conditions with an AIO CPU Cooler
Tuning an open loop that makes use of fan curves is largely the same as the AIO method, except instead of using the CPU temperature, you use the coolant temperatures.

Step 2.1
Choose a minimum fan speed setting that you want the cooler fans to spin at when idle. Dial in this speed using the fan control software of your choice. Make sure that temperature control is disabled, as varying fan speeds will make it impossible to tune the rest of the fans.

Step 2.2
Tune the non-cooler fan speeds using the basic method outlined (steps 1.1 through 1.4) above. When making adjustments, make sure you alternate between increasing and decreasing fan speeds to achieve the flow conditions that you want.

When you achieve the flow conditions you want, write down all of the settings used for the fan speeds.

Step 2.3
Repeat steps 2.1 and 2.2 for the maximum fan speed setting, and at least one speed between the maximum and minimum settings.

Step 2.4
Using the values that you gathered for minimum, maximum, and all intermediate speeds you collected, dial in the appropriate fan curves for all non-cooler fans.

Make sure all of the fan curves use the same temperature sensor.

Make sure that the temperature of each point is exactly the same across all fans (including the cooler fans) for a given speed. (All have the same temperature setting at which they reach maximum speed, etc.)

Step 2.5
Use the basic incense method to monitor the flow conditions, and run several tests of varying intensity (I.E. they result in different CPU temperatures). Watch the incense and make sure the flow condition is always what you want.

If the flow condition changes, then take note of what temperature resulted in the change. Look at the fan curves to determine what settings were produced at that temperature. Use those settings as the starting point for another iteration of steps 2.1, 2.2, and 2.4.

Also run some GPU stress tests in order to verify that the GPU doesn't change the flow conditions. If it does, you won't be able to tune the build for neutral flow. If you want positive pressure, you may need to make a few adjustments to make the flow more positive in order to compensate for the added exhaust flow from the GPU. If you want negative flow, the GPU can only make the flow more negative.

Once you have finished making any necessary corrections, write down all settings that were used in case you ever need to reformat the machine or reset the fan curves.

Tuning an Open Loop that Includes Multiple Components and Multiple Temperature Sensors in the Cooling Loop
Step 3.1
If your loop has multiple temperature sensors, and you wish to base some fan speeds on one, and other fan speeds on another, you can tune the fans in groups.

Divide the fans into groups based on what temperature sensors the fan curves are based on. If you want to run any fans without any fan curves, treat all such fans as their own group.

In order to ensure that the build will always achieve the flow condition you want, you will need at least one intake and one exhaust fan in each group.

Step 3.2
Choose a group to start with and disconnect all fans that are not in that group.

Perform the AIO tuning method (Steps 2.1 through 2.5) with only the fans in the current group running.

Step 2.5 may require you to use a cup of heated water to fool the temperature sensor, as it may not be safe to turn the computer on with certain groups operating in isolation. This is where you want to have a spare thermocouple on hand.

Step 3.3
Repeat step 3.2 with each remaining fan group until all groups have been tuned.

Step 3.4
Reconnect all fans and double check that all fan curves are set up according to your notes.

If all groups are tuned to achieve the a the same flow condition, then all combinations of those groups will achieve the same flow condition. If all groups achieve neutral flow, for example, at all points on their fan curve, then combining the different groups at different speeds will still achieve neutral flow.