[SOLVED] Extra exhaust fan.

Aug 21, 2019
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I have a Corsair 780T case which is shipped with three 140mm fans: 2 at the front and 1 at the back of the case. This is my new system based on AMD TR. The computer will be used mainly for work and heavy computations. I would like to add one extra fan (fans in future) because I am planning to buy an extra video card and more memory and I have a couple questions.

Is top of the case a good place for exhaust?
My case has options to add extra fans at the top or at the bottom. The recommendations in manual are 120mm or 140mm fans:
  • 3x120mm or 2x140mm at the top
  • 3x120mm or 2x140mm at front
  • 1x120mm or 1x140mm at back
  • 2x120mm at bottom
Which and how many fans would you recommend for the complete system (2 x video card)?
I came across parameter like m^3/h or CFM and I was wondering if there is anything like a total air flow and total value of m^3/h I should aim for.
As to the brand, Noctua is usually recommended and has best reviews. I was going through their website trying to choose the right fan with PWM. There is a number of items, which are grouped by applications (CPU or case fans). The faster fans have more air flow, higher m^3/h but are but louder. On the other hand, CPUs fans are quiet, slower, with less air flow.

Currently the main question I ask is do I need higher air flow?
If yes, I need to accept more dB/A and choose either NF-A14 industrialPPC-2000 PWM or NF-A14 industrialPPC-2000 IP67 PWM.
If no, I would go for NF-A15 PWM.

I will be honest I am quite confuse what to select and would be grateful for comments on that.

Thanks
 
Reactions: Kiril1512
You have told us about your three case ventilation fans, but not your CPU cooler. IF your cooler is a liquid-filled loop with a pump on the CPU and a radiator with fans, then it also contributes to case air flow. If not (i.e, if the CPU cooler is just a fan and heatsink on the CPU) ignore this point.

In general you want fans arranged for AIR FLOW balanced between intake and exhaust. But you should aim for slightly MORE intake capacity than exhaust for two reasons. First, the intake fans always should have dust filters (often foam mesh) in front of them (they need periodic inspection and cleaning) and these reduce the air flow somewhat so they don't deliver the spec'd air flow. Secondly, my preference (not everyone agrees on this) is to have real intake slightly more than exhaust so the the interior of the case is very slightly at higher pressure that the outside room; that way, any air leakage through case cracks will be outwards, preventing intake of dusty room air.

Any fan's air flow output is reduced by anything in the air flow path that interferes. That could be dust filters, case components, wires, mobo items sticking up, added cards in the PCIe slots, etc. Most of those things are considered to have small influence on air flow. On the other hand, forcing air through small spaces like the gaps between the fins of a heat exchanger on the CPU, or the spaces between fins of a radiator, definitely reduces air flow. For those cases you want a fan able to "push harder" - that is, able to generate higher air pressure to force the flow. When you read the specs for a fan, most tell you two important items - Air Flow and Pressure - and you need to understand them. Both are given for when the fan is running at max speed. Air Flow is the max air flow this fan can generate for NO flow impediments in its path - free air condition. It is given in CFM or m³/hr, and the conversion is: 1.0 CFM = 1.699 m³/hr, or 1 m³/hr = 0.5886 CFM. As more resistance to air flow is introduced, this creates backpressure that reduces air flow. The Air Pressure specs in units of mm of water is the maximum pressure this fan can generate and still produce air flow; at higher backpressures there will be effectively zero air flow. If you sketch out a graph of air flow versus backpressure, the line between these two limits is VERY roughly a straight line. You can make a graph like this and sketch the lines for several fans on the same graph to compare.

Now, a fan can be designed to maximize air flow at low pressure (good for case ventilation with little interference), or for good flow at higher backpressures (good for CPU heatsinks and radiators). Other factors being equal, one optimized for use in high backpressure applications will also deliver less air flow at low backpressure than another fan optimized for low pressure. That is why, when you compare specs, the high-pressure ones recommended for CPU coolers have lower max air flow specs. In fact, if you do that rough graph thing and plot the lines for some Air Flow models and some Pressure models, you'll likely see that they cross each other. At low backpressures the Air Flow ones deliver better flow, but at some higher backpressure they become less able and the Pressure-optimized one do better.

All of this, of course, is also affected by the power and speed of the motor, and the overall design of the blades. For any given motor and a single speed, changing the blade design can optimize for low or high backpressures. On the other hand, for one particular blade design, running on a faster motor will raise the output, but will also need more power and likely will generate more noise. Clever blade design can reduce noise generation while maintaining air flow output.

The Noctua line has a reputation of higher air flow at lower noise levels than many competitors for similar design specs, and long life, but they are a bit more expensive. Further, some do not like their colour choices, and they do not make any lighted fans. They make fans for many applications. In most cases the have virtually matching models for 3-pin (older Voltage Control Mode, aka DC Mode) and 4-pin (newer PWM Mode). The Pressure specs will tell you whether the model is designed to optimize for CPU cooling or case ventilation - usually Pressure under 2 mm is for cases, and higher ones for CPU's. Do not worry much about the fan speed spec - you are interested more in the Air Flow specs, which correlates with speed roughly.

The industrial PPC line from Noctua is their high-performance line, offering higher air flow and higher backpressure ratings because their motors deliver more power, and the result is higher noise at max speed. There are two different speed models for this. Most people do not need the very high air flow these can deliver, but if you do, they are up to the job.

Remember an important fact. The automatic fan speed control systems in mobo headers are based on the actual TEMPERATURE from a sensor. For the case cooling fans (SYS_FAN or CHA_FAN headers) this sensor normally is one mounted on the mobo by its maker (not the one inside the CPU chip). The control system reduces the fan speed (and hence air flow) to whatever it requires to maintain the temperature at the proper target value on that sensor, and that also reduces the noise generated. So a fan with very high air flow rating may NOT actually be used to generate that air flow in most situations. However, when your workload increases a lot and more heat is generated, a higher-output fan has the reserve capacity to speed up (and generate more noise) enough to keep up with that thermal load.

Now back to your particular question. For better case cooling, particularly if you are adding a second video card, more fans are a good approach. I caution you about four parts of this. First, you may or may not need a very high flow fan like the iPPC ones from Noctua. (Some of their lesser fans may do enough.) Next, if you add a second exhaust fan (and yes, the top is a good location) very similar to the ones you have now, you will get to a rough balance of intake and exhaust flows, with slightly more exhaust because the dust filters reduce the intake a little. But if you use a high-performance iPPC model, that certainly will make exhaust capacity higher than intake. So next thought: consider adding BOTH and intake and exhaust fans, with slightly higher intake air flow rating if possible, to ensure a real increase in total air flow. And finally, examine HOW your video cards are cooled. IF they are ones that draw air from the (inside the case) surroundings and then blow the hot air out the back through a vent screen, then they add to the total air exhaust capacity. In that case you would want to have the group of fans dedicated to case ventilation to have MORE intake capacity that exhaust.
 
Reactions: franekw

Kiril1512

Honorable
Jun 4, 2014
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For your build I would recommend to see other fans with higher airflow and lesser static pressure. Those noctuas fans have a impressive static pressure but this is really help if you need them for and water cooling radiator. I believe you can look for something more simple and less expensive and get the results you need. So look for higher m^3/h than static pressure.

About your second GPU, obviously you will generate a lot of more heat so add more fans will help a lot.
I also recommend you to check about the positive/negative pressure on the case.

Video about positive/negative pressure.

1 year utilization with positive/negative pressure.
 
Aug 21, 2019
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Thanks for the links. The videos were quite interesting to watch and I did not realise things I've learned from them. I am still confused about what the difference is between air flow and static pressure and how it is related to fans.
 
You have told us about your three case ventilation fans, but not your CPU cooler. IF your cooler is a liquid-filled loop with a pump on the CPU and a radiator with fans, then it also contributes to case air flow. If not (i.e, if the CPU cooler is just a fan and heatsink on the CPU) ignore this point.

In general you want fans arranged for AIR FLOW balanced between intake and exhaust. But you should aim for slightly MORE intake capacity than exhaust for two reasons. First, the intake fans always should have dust filters (often foam mesh) in front of them (they need periodic inspection and cleaning) and these reduce the air flow somewhat so they don't deliver the spec'd air flow. Secondly, my preference (not everyone agrees on this) is to have real intake slightly more than exhaust so the the interior of the case is very slightly at higher pressure that the outside room; that way, any air leakage through case cracks will be outwards, preventing intake of dusty room air.

Any fan's air flow output is reduced by anything in the air flow path that interferes. That could be dust filters, case components, wires, mobo items sticking up, added cards in the PCIe slots, etc. Most of those things are considered to have small influence on air flow. On the other hand, forcing air through small spaces like the gaps between the fins of a heat exchanger on the CPU, or the spaces between fins of a radiator, definitely reduces air flow. For those cases you want a fan able to "push harder" - that is, able to generate higher air pressure to force the flow. When you read the specs for a fan, most tell you two important items - Air Flow and Pressure - and you need to understand them. Both are given for when the fan is running at max speed. Air Flow is the max air flow this fan can generate for NO flow impediments in its path - free air condition. It is given in CFM or m³/hr, and the conversion is: 1.0 CFM = 1.699 m³/hr, or 1 m³/hr = 0.5886 CFM. As more resistance to air flow is introduced, this creates backpressure that reduces air flow. The Air Pressure specs in units of mm of water is the maximum pressure this fan can generate and still produce air flow; at higher backpressures there will be effectively zero air flow. If you sketch out a graph of air flow versus backpressure, the line between these two limits is VERY roughly a straight line. You can make a graph like this and sketch the lines for several fans on the same graph to compare.

Now, a fan can be designed to maximize air flow at low pressure (good for case ventilation with little interference), or for good flow at higher backpressures (good for CPU heatsinks and radiators). Other factors being equal, one optimized for use in high backpressure applications will also deliver less air flow at low backpressure than another fan optimized for low pressure. That is why, when you compare specs, the high-pressure ones recommended for CPU coolers have lower max air flow specs. In fact, if you do that rough graph thing and plot the lines for some Air Flow models and some Pressure models, you'll likely see that they cross each other. At low backpressures the Air Flow ones deliver better flow, but at some higher backpressure they become less able and the Pressure-optimized one do better.

All of this, of course, is also affected by the power and speed of the motor, and the overall design of the blades. For any given motor and a single speed, changing the blade design can optimize for low or high backpressures. On the other hand, for one particular blade design, running on a faster motor will raise the output, but will also need more power and likely will generate more noise. Clever blade design can reduce noise generation while maintaining air flow output.

The Noctua line has a reputation of higher air flow at lower noise levels than many competitors for similar design specs, and long life, but they are a bit more expensive. Further, some do not like their colour choices, and they do not make any lighted fans. They make fans for many applications. In most cases the have virtually matching models for 3-pin (older Voltage Control Mode, aka DC Mode) and 4-pin (newer PWM Mode). The Pressure specs will tell you whether the model is designed to optimize for CPU cooling or case ventilation - usually Pressure under 2 mm is for cases, and higher ones for CPU's. Do not worry much about the fan speed spec - you are interested more in the Air Flow specs, which correlates with speed roughly.

The industrial PPC line from Noctua is their high-performance line, offering higher air flow and higher backpressure ratings because their motors deliver more power, and the result is higher noise at max speed. There are two different speed models for this. Most people do not need the very high air flow these can deliver, but if you do, they are up to the job.

Remember an important fact. The automatic fan speed control systems in mobo headers are based on the actual TEMPERATURE from a sensor. For the case cooling fans (SYS_FAN or CHA_FAN headers) this sensor normally is one mounted on the mobo by its maker (not the one inside the CPU chip). The control system reduces the fan speed (and hence air flow) to whatever it requires to maintain the temperature at the proper target value on that sensor, and that also reduces the noise generated. So a fan with very high air flow rating may NOT actually be used to generate that air flow in most situations. However, when your workload increases a lot and more heat is generated, a higher-output fan has the reserve capacity to speed up (and generate more noise) enough to keep up with that thermal load.

Now back to your particular question. For better case cooling, particularly if you are adding a second video card, more fans are a good approach. I caution you about four parts of this. First, you may or may not need a very high flow fan like the iPPC ones from Noctua. (Some of their lesser fans may do enough.) Next, if you add a second exhaust fan (and yes, the top is a good location) very similar to the ones you have now, you will get to a rough balance of intake and exhaust flows, with slightly more exhaust because the dust filters reduce the intake a little. But if you use a high-performance iPPC model, that certainly will make exhaust capacity higher than intake. So next thought: consider adding BOTH and intake and exhaust fans, with slightly higher intake air flow rating if possible, to ensure a real increase in total air flow. And finally, examine HOW your video cards are cooled. IF they are ones that draw air from the (inside the case) surroundings and then blow the hot air out the back through a vent screen, then they add to the total air exhaust capacity. In that case you would want to have the group of fans dedicated to case ventilation to have MORE intake capacity that exhaust.
 
Reactions: franekw
Aug 21, 2019
24
1
15
0
HI Paperdoc,
Thank you for the exhausted and detailed explanation. This is exactly what I needed. I really appreciate your help and effort because your post is so well structured and detailed and very easy to get through and understand. I am sure it took time to write this. Very appreciated. Finally the numbers make sense and I can make much better decisions on what I need :). Kiril1512's links to videos explained a couple of things which made sense but there was still one or two questions why and your post gave answers to that :)

You also mentioned headsets (SYS_FAN and CHA_FAN). My motherboard has a number of them:
  • 4-pin CPU_FAN
  • 4-pin CPU_OPT
  • 3-pin COV_FAN
  • 4-pin AIO_PUMP
  • 4-pin W_PUMP+
  • 4-pin M.2_FAN
  • 4-pin CHA_FAN1-3
  • 5-pin EXT_FAN
I have an air cooler with two fans and a cable, a joiner, to connect both fans to one CPU_FAN headset. If I have CPU_OPT, shouldn't I use it for a second fan, instead? I have 3 x 4-pin CHA_FAN headsets which i believe are for fans mounted to case . If I buy two extra fans, where do you think they should be connected?
Thanks
 
Several of those fan headers are extra special-purpose ones you will not need. The 5-pin EXT_FAN one is a special ASUS design, and it is NOT for a fan, exactly. It is where you can plug in an ASUS custom optional (extra cost) expansion card specifically used to create several more standard CHA_FAN headers for more fans. Few people need these, and you don't, so ignore it.

All fan headers use some temperature sensor to guide them, and the header's automatic control system manipulates the speed of its fan according to the temperature measured there. All mobos have at least two sensors. One always in built into the CPU chip by its maker, and that is always used for the CPU_FAN and CPU_OPT headers, and MAY be available for use by others. Then the mobo maker also has built in a general sensor on the motherboard, located near a component they consider is vital to monitor and cool, and that normally is used by the CHA_FAN headers. Your mobo has more than that. It has a couple extra ones (not clearly shown or named in the manual) on the mobo, plus others.

Some ASUS graphics cards are equipped with a non-standard feature to feed their own internal temp sensor readings back out to the mobo on one card pin, and the mobo has the ability to use those sensor readings to guide selected CHA_FAN headers. So, IF you choose to, you can select a particular case fan that is aimed at the graphics card locations and have its speed guided by that card's cooling needs, rather than by the general mobo cooling needs. I believe the Asus GeForce RTX 2080 Ti 11 GB ROG Strix Gaming card you have includes this feature so you MAY choose to do that. But of course, this means that ONE of your CHA_FAN headers will be configured that way (with its fan suitably positioned), so that header would NOT be used for other general case ventilation control.

So let's go through the main headers. The CPU_FAN and CPU_OPT headers generally behave exactly the same, and both are guided by the internal CPU chip temp sensor. Your idea is right. For the two-fan Noctua air cooler on the CPU, the Splitter included is intended when you have only one CPU_FAN header to use. But since you have both, you can plug those fans separately into those headers. The only real advantage to that is that then the speed signals of BOTH can be shown and monitored for possible failure. Just be sure in BIOS Setup that those two headers are configured identically so they do the same thing. By the way, some Noctua products are shipped with a little device called a Low-Noise Adapter. It is just a small resistor that can be inserted into the cable connecting the fan to its power source. All it does is reduce the fan's voltage supply, and thus its speed, air flow output and noise. It is useful only when you are forced to connect the fan to a fixed 12 VDC supply directly from the PSU and thus have no other way to control the fan speed. When you use the fan with an automatic speed control system on the mobo headers, do NOT install that LNA module in the wiring.

The COV_FAN header is an unusual one, apparently intended to blow cooling air over a cover somewhere. I suspect that one is intended for the metallic cover over the area at top rear of the mobo where the external connection panel is. IF your case has a place in that area to mount a fan aimed there, you could use that, but it's not commonly done. NOTE that this particular header is a 3-pin one, so if you decide to buy and install a fan for this, you can use a 3-pin model, although a 4-pin one would work. But I suspect this header is fixed to use a temp sensor in that area, so it is not suited for use for general case fans.

The AIO_PUMP and W_PUMP headers are intended for powering the pump units of liquid-cooled systems. The AIO_PUMP one particularly is aimed a the All-In-One complete systems, whereas the W_PUMP one is for the pump component of a DIY self-designed system. You have neither, I believe, although your system list does include a hint that you are considering a BeQuiet Dark Rock Pro AIO system instead of the Noctua air cooler. You MAY be able to re-purpose one or both of these headers for general case fans, but I'm not sure (see below).

The three CHA_FANn headers are the ones intended for general case ventilation fans. Your Corsair 780T case includes three 140 mm fans (2 front, 1 rear) of the AF140L type. Corsair does not provide the max current consumption spec for these, but I am sure they are less than 0.3 A each. Since a mobo standard fan header can supply up to 1.0 A max to its total load, you CAN connect all three of these (perhaps even 4 of them, if their max currents are more like 0.2 A each) to a single CHA_FAN header. Although they may come pre-connected to the case's fan speed control system in the top, that system allows NO automatic control of their speeds by the mobo; it is strictly a manual system for which YOU are the controller who decides when and how to set their speeds. You are better to disconnect those three from the case system and connect them instead to a mobo fan header. In doing that you need to make two adjustments because these are THREE-pin fans that must have a header that uses the older DC Mode of control. That is not the same as the newer PWM Mode, so it would be ideal to create two fan groups based on control Mode. Use a simple SPLITTER for these three on one CHA_FAN header configured to use DC Mode, and then connect other 4-pin case fans to a different CHA_FAN header configured to use PWM Mode. Here's an example of a simple Splitter with four output arms. You could use a second one of this same device to connect a group of 4-pin fans to the other CHA_FAN header.

https://www.amazon.com/Computer-Splitter-Connector-Sleeved-Adapter/dp/B00DYQSZ68/ref=sr_1_4?keywords=computer+fan+splitter&qid=1568910278&s=gateway&sr=8-4

Lastly the mobo has a special-purpose M.2_FAN header. Now, you are getting one Samsung NVMe SSD module, and your mobo has two possible places to mount it. See manual p. 1-2 items 6, p. 1-24, and p. 2-11. I suspect the SSD you have can be mounted in either place. The difference I see is that the M.2_1 socket near the bottom places the SSD under a heatsink (must be removed and re-installed) to help cool the SSD. (Note if you do this, there appears to be a tape to remove so that some pre-installed thermal paste on the heatsink can contact the SSD.) IF you choose this location, that seems to be the one for which there is a special temp sensor to guide the M.2_FAN header so any fan connected there and directed at that heatsink location can be guided by the SSD's temperature, and not the main mobo temp.

On many mobos, for several of the fan headers (especially the CHA_FAN and sometimes the AIO_PUMP or W_PUMP ones), there are options that are NOT shown in the manual. So for these you'd have to search inside BIOS Setup to see whether they are available. For the pump headers, you may find you can specify whether they are to be used for a pump of for a plain fan. The plain fan option then re-purposes that header for use with a case vent fan IF you also can choose the correct temp sensor. .For many of these you are offered the option to select exactly which temperature sensor is used by this header for control guidance. If you have that option, select the general mobo sensor for any general case ventilation fans. IF you have fans you plan to dedicate to particular components (e.g. the rear panel of connector sockets and its associated mobo circuits, the graphics card, the M.2 SSD unit). set the relevant header this way. Since the mobo has several special-purpose temp sensors, I am surprised the manual makes no mention of these.
 
Reactions: franekw

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