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The correct way to adjust fan speed, thermal mass and heat transfer

It boggles the mind how many PC products have no idea how to automatically adjust fan speed, when you get a motherboard it defaults to CPU fan speed = CPU temperature, and that’s just stupid, recent motherboard have improved this slightly by giving you an option to slowly ramp the fan over a few seconds, but that’s still just bad.

There just two very simple principles you need to know, to see how to make a proper fan curve:

1. The greater the difference in temperature, the much much better the heat transfer.
2. Cooling systems themselves have thermal mass, they take energy to warm up gradually, rather than instantly.

So for example we just have a CPU and a heat sink on it, if you tie the fan speed to CPU temperature, what’s going to happen is if you run something and the CPU boosts to 4.5Ghz, the temperature will shoot up like 20 degrees instantly, and your fan speed will also shoot up. Not only is this annoying, it’s wasted fan speed.

Because of principle 1, we know that for a given fan speed, the hotter the heat sink, the more cooling you are doing. So for example even at a constant 1000RPM, if the heat sink is at an average of 30 degrees, you might only be doing like 50w of cooling. However if the heat sink is at 50 degrees, you could be doing 150W of cooling. All this extra cooling is completely free, no extra noise, no extra power.

So what you really want to do, is just ignore the CPU temperature and monitor the heat sink temperature instead. Even though the CPU just shot up 20 degrees, for a few seconds, there’s really nothing you can do about that even if you shoot up the fan speed, so why generate the extra annoyance and noise pollution? The only way you can reduce the temperature difference between the heat sink and the CPU, is by improving your thermal interface, so like better paste, delid, stuff like that, fan speed is not one of them.

There’s also principle 2, the cooling system is like a leaky bucket, the more you fill it up, the more pressure, and the more it leaks. But don’t forget that it is still, a bucket and it will take some time to fill up, exactly how long obviously depends on the situation. High power CPU + air cooler would only take a few seconds, lower power CPU + big custom loop could take over half an hour. However, in either case, things like browsing the web or using most casual applications where you are only getting spiky loads, it can be totally necessary to spin up the fans at all. The CPU temps will spike all over the place but again there’s nothing you can do anyway, just let it fill the bucket and it will leak away during the down times without any effort from the fans. But if you foolishly link fan speed with CPU temps, it’ll be an annoying work session.

I see many modern motherboards with thermal probe function, this is great, you can just use a thermal probe, stick it in your heat sink/radiator, and control the fan speed with that, this would be almost exactly the ideal way to control it. If you’re going to generate noise by spinning up the fans, you want the noise to be worth every db, so you really only want to do that when it can achieve the maximum increase in cooling power, and that only happens when the radiator/heatsink is already hot, regardless of what temperature the CPU is at. I guess that’s a good thing about AIOs, is they can measure liquid temperature and control the fans by itself, where as air coolers can’t do this correctly by itself.

I mean sure, motherboards can’t automatically detect the temperature of your heat sinks, you’re gonna have to set this up manually. What I don’t get is why laptop and desktop OEMs have no idea how to do this, You’re making a laptop you design and control every aspect of it! Is it too hard to put a thermal probe on the PCB where the heat pipe passes over? Yet it’s 2019 and there are still tones of PC laptops that just spin the fan up and down based on CPU temperature, or God forbid, CPU load, it’s so just so lazy.

Apple laptops, well you know they often have minimum cooling capacity, we’re not talking about that, we are only taking about the fan speed management. They’re often good in that they base fan speed on the thermal mass of the entire chassis, however the problem is they don’t spin hard enough, until it’s too hot, and then they spin like crazy, this is what I don’t get, it’s so close to the right curve but not quite. So for example the fans in the MBP are silent at 3000, audible at 4000 and loud at 6000, often what happens is if the thing just spun the fans at 3500 all the time it would be sufficient for the work session. But what actually happens is it tries to stay at 2000 for as long as possible, and then it’s like oh shit we’re too hot, and it goes to 6000 for a couple of minutes before going back down to 2000, which is just more annoying than necessary.

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