[SOLVED] Question about Push/Pull

[DarthBatman]

Hey guys question for you about the Push/Pull set up. Currently I run a Noctua U12s chroma, with two NF-F12 fans, one as intake and one as exhaust on the heat sink.

I have both fans connected via a Noctua splitter cable connect to the CPU header. My temps are pretty good. My idle is about 35-36C .

I run a 3700x CPU. I would like to upgrade that down the line to a 12 core processor. Probably a 3900x-5900x. But I absolutely love this heatsink and want to keep using it. I know I can run this cooler with those chips but it is not exactly the best for that processor.

My question is, can I increase my cooling capabilities by running my pull fan, as a separate connection to the motherboard? Like run it as a system fan on PWM mode and lets say I run it at 70% all the time instead of when the CPU decides the push fan needs to ramp up?

So in my theory the pull fan would be forcing air out of the heatsink at a constant rate, almost as precaution. While the front push fan (the essential fan) will run as the CPU sees fit?

Im a newb so please feel free to tell me how this wouldn’t work.

 

[tennis2]

Manually setting the pull fan to a constant 70% is unnecessary.

The mobo is going to adjust the CPU fan(s) rpm to a set CPU temp. By nature, a more power-hungry CPU swap will automatically result in higher fan speeds to maintain the same target CPU temp then...

I see no reason to assume a cooler replacement is necessary on a 5900X until you actually install it and try. It's free to try with the cooler you have. It's not like the U12S isn't going to work at all. There just may be other options that are better/quieter/cooler. Noctua coolers are pretty commonplace in cooler reviews. I'm sure you can find a cooler review that uses a 5900X as well.

 

[curley81]

many years back i read that push pull config needs to have both fans running at the same speeds to keep good airflow and static pressure

 

[rubix_1011]

I think people overthink push+pull beyond what they need to.

Ideally, both fans would run at same speed, but that's never achievable because the push fans will always have more resistance than pull fans due to the radiator fins they're trying to force air though. There will always be a difference. Also, every fan is slightly different, so even if you have all push fans on the same splitter, they will still each be slightly different because of how the signal is processed for every fan and how every fan motor responds. They might all be set to 1200rpm, but the reality will likely be that one is at 1185, another at 1240 and the last at 1170 (just random examples). They will 'try' to reach the set PWM speed but fans aren't really able to hold an exact speed like the cruise control of your car, by comparison, so they 'pulse' up and coast down (hence pulse width modulation or PWM).

There is also an odd fear that push and pull fans which are not exactly the same will cause a burn out of bearings on one side or the other. The reality is, there is probably little to zero issue here unless you have fans of tremendous speed difference, i.e. 400rpm and 2400rpm, but mostly this has to do with airflow volume in CFM and static pressure in mm/h2o. In the event that there are differences, these can be negated (in push+pull) by having the stronger fans (in mm/h20 and possibly CFM on the push side) with the pull fans being the lighter duty fans. The radiator fins are going to dissipate much of this energy, leaving the pull fans with the job of simply getting the heat away from the radiator.

 

[Paperdoc]

tennis2 has the right approach. Leave the two fans you have in push/pull connected as they now are, and let them continue to work together. As tennis2 says, the automatic fan control system will adjust the actual fan speeds to whatever it takes to keep the new CPU at its optimal operating temperature. The ONLY thing to watch for as a possible indication of trouble is if you find that at very heavy workloads the fan speeds max out and the CPU temp still keeps climbing. Even for that, don't freak out unnecessarily. Look up what the normal temp in that new CPU is, and what is likely for heavy load, and what is the max recommended. You'll find it is NORMAL for the CPU internal temp to get into the 80's (Celsius), and the upper limit is higher.

By the way, the meaning of PWM and the "pulsing" behaviour described above is wrong. It is true that a fan speed may cycle up and down a bit under certain circumstances. The automatic speed control system decides what speed the fan should run based on a measured temperature. It does that in steps: for a certain range, it will run one speed, and if temp goes up to the next range interval, it will increase the speed by a noticeable step up. SOMETIMES it is operating near that transition zone, so when the fan speeds up and cools the hot item, the temp drops back down to the lower temp range and the fan slows down. Then re-heating causes a speed-up. etc. If that gets to bother you, you can simply change the "fan curve" so the temp for a step change is a little higher or lower than the pre-set values, and the cycling can be stopped.

PWM means Pulse Width Modulation - true. It does NOT mean the fan speed pulses. A 4-pin computer fan uses a 12 VDC power supply source, and also receives a PWM signal from Pin #4. The PWM signal operates around 20 to 22 kHz frequency, and it is an ON or OFF signal - nothing in between. ON is 5 VDC, Off is zero. So in one cycle of this "wave" (50 microseconds period for 20 kHz) the signal turns On for a certain time period, then turns Off for the remainder of that cycle. For example, 30 µs On followed by 20 µs Off (and continuing to repeat this) would be a "60% On" PWM signal. Inside the fan motor there is a small chip that uses this PWM signal to act like a switch that turns the 12 VDC feed to the motor windings on and off just as the PWM signal dictates. So the motor does not get full power all the time - it gets full power only part time - and hence it runs at less than full speed. The PWM signal can be sent out anywhere in the range from 0% On to 100 % On, although most systems will not send out a signal less than about 30% On - a very low speed signal can cause a fan motor to stall and not re-start without some other action. So the PWM signal operates a "switch chip" that controls the feed of power to the motor, and hence its speed. Within the single cycle of the repeating PWM signal, the "On part" is often called the "On Pulse", and the width of that Pulse (or % Time On) can vary; thus the signal is called a Pulse Width Modulation signal.

 

[tennis2]

I'm not all that concerned about variable fan speeds in a push/pull config. There's plenty of airflow resistance through a rad that (like rubix said) the only time you even START to see a situation where 1 fan is accelerating the other is at MASSIVE differences (like the 400/2400 example). In general, it's better to have the pull fan running slightly higher rpm/cfm/sp than the push because (again, as mentioned) there's so much "splashback" air happening on the push fan, that creating that low pressure zone through the rad with the pull fan can actually make the push fan more efficient/effective.