Passively-cooled CPU Thermals

The Completely Silent Computer I built relies entirely on passive cooling.  Some folks are keen to know how well (or poorly) it works in the real world.  Can it adequately cool a loaded system, or does thermal throttling make it pointless?  Well, it’s time to find out.

DB4n

As can be seen in these images, the Ryzen 5 1600 that I installed in the DB4 is cooled by heat pipes that transfer heat from the block on the CPU to spreaders attached to the aluminium walls of the case.  Since I installed the optional LH6 Cooling Kit, there are a total of six heat pipes that connect to three spreaders on two walls of the case.

DB4o

The walls are 13mm-thick extruded aluminium plates.  They weigh a lot.  Heat conducts from the spreaders inside to the exterior surface.  Grooves that run the full height of the exterior walls provide a huge surface area that allows heat to be transferred to air that then flows up and out the top of the plates.

DB4p

Vents have been machined into the inside of each plate — one towards the bottom and one towards the top — which allow cool air to be drawn into the case, and hot air to flow out.

DB4q

That’s about it.  Hot air rises due to buoyancy, flows out of the case, creating a negative pressure zone inside the case, which then sucks cool air in through the bottom and sides of the case.  Rinse.  Repeat.  Doesn’t get much simpler.  Totally passive.

Enough ‘theory’ — let’s put this thing to the test!

I searched around a bit and ended up installing a nice little program called Psensor because it would let me produce some clear graphs of how system load affects CPU and GPU temperatures over time.

Note:  This post only deals with CPU thermals.  GPU thermals will come later.

In a passively-cooled system, instantaneous temperatures aren’t actually that useful.  The walls of the case will initially soak up a lot of heat and then radiate/convect some of that heat back into the case, which will heat up the internal components.  What that means is that it takes a lot longer to reach an equilibrium temperature in a passively-cooled system than it does in an air-cooled system (or even a water-cooled system).

To establish a baseline, I monitored CPU temperatures from the moment the computer was turned on, and let it idle for a couple of hours.

Test 0 — CPU idling at an ambient temperature of 20⁰C

Thermals 120m at IDLE

At idle, the 3.2GHz (stock base clock) Ryzen 5 1600 (with 6 cores and 12 threads) — housed in the completely passively-cooled DB4 — reached a temperature of 31⁰C.  That’s 11⁰C above ambient.

If I’d left the test running for a couple more hours the CPU temperature probably would have gotten 1⁰C warmer, but my house was heating up at the same time and the increase in ambient temperature would have impacted on the results — so I didn’t bother extending the test.  Let’s accept 12⁰C above ambient.

Having established a baseline, I then decided to give the CPU a variety of fixed workloads and let them run for as long as it took for the CPU temperature to stabilise.  That ended up being about an hour.

Test 1 — CPU loaded to 25% at an ambient temperature of 21⁰C

Thermals 60m at 25%

At 25% load, the 3.2GHz Ryzen 5 1600  — housed in the passively-cooled DB4 — reached an equilibrium temperature of 47⁰C.  That’s 26⁰C above ambient.  No thermal throttling occurred.

The hottest part of the hottest exterior wall was 37⁰C (as measured with an IR thermometer).  Very warm to the touch, but nothing at all to be worried about — cats would love it.

Test 2 — CPU loaded to 50% at an ambient temperature of 20⁰C

Thermals 60m at 50%

At 50% load, the Ryzen 5 1600 — housed in the passively-cooled DB4 — reached an equilibrium temperature of 52⁰C.  That’s 32⁰C above ambient.  No thermal throttling occurred.

The hottest part of the hottest exterior wall was 40⁰C.  Toasty — borderline hot even — but I was able to press my hand firmly against it for as long as I liked without feeling any discomfort.

Test 3 — CPU loaded to 75% at an ambient temperature of 22⁰C

Thermals 60m at 75%

At 75% load, the Ryzen 5 1600 — housed in the DB4 — reached an equilibrium temperature of 58⁰C.  That’s 36⁰C above ambient.  No thermal throttling occurred.

The hottest part of the hottest exterior wall was 42⁰C.  Hot, and if I pressed my hand firmly against it for more than about 10s it became uncomfortable.

Test 4 — CPU loaded to 100% at an ambient temperature of 22⁰C

Thermals 60m at 100%

At 100% load, the Ryzen 5 1600 in the DB4 reached an equilibrium temperature of 60⁰C.  That’s 38⁰C above ambient.  No thermal throttling occurred.

The hottest part of the hottest exterior wall remained 42⁰C.  Hot to touch, and uncomfortable after a while, but not painful.

So, the first set of real-world results are in!

If we normalise the results for an ambient temperature of 20⁰C then we have the following equilibrium temperatures for a stock 3.2GHz Ryzen 5 1600 in a Streacom DB4 with the optional LH6 Cooling Kit installed:

  • Idle (0%):  32⁰C
  • 25% load:  46⁰C
  • 50% load:  52⁰C
  • 75% load:  56⁰C
  • 100% load:  58⁰C

For the folks that can’t get enough charts:

Thermal Results 1

Analysis

So, what do these results tell us?

  1. The most obvious thing is that either the Ryzen 5 1600 is a very efficient CPU, the DB4+LH6 is a very effective cooling combination, or both!  To max out at 58⁰C under full load is excellent for a completely passively-cooled system — much better that what I was expecting.
  2. There will clearly be no problem running compute-heavy overnight jobs on this system.  A 100% load for 4–6 hours at a time should be a walk in the park.
  3. Ryzen CPUs thermally throttle north of 90⁰C and shut down at 95⁰C so there is ample room for overclocking the Ryzen 5 1600 to 1600X levels, upgrading to the new Ryzen 5 2600 (and moderately overclocking that), or even stepping up to something like a Ryzen 7 2700 (although I doubt you’d get more than a light overclock out of that).
  4. If a Ryzen 5 1600 or 2600 gives you as much performance as you need (and the 1600 does for me), then I’m confident you could get away with just the stock cooling solution that ships with the DB4 — no need to get the optional LH6 Cooling Kit like I did.  Four heat pipes and a single spreader would provide adequate cooling, but your CPU temps would be noticeably higher (probably in the high 60s or low 70s).
  5. Exterior wall temperatures can get hot, and even uncomfortable after extended contact, but never painful and certainly not dangerous.  No need to worry about the safety of pets or small children (if you have them) — at least not with a stock Ryzen 5 1600 (or 2600).  If you overclock or put in a Ryzen 7 2700 then you’d want to monitor the exterior temperatures yourself to see if they ever climb to hazardous levels.

Changes

With the benefit of this real-world testing and hindsight, would I do anything differently if I was building a passively-cooled DB4 system today?  Yep, sure would.

  • I’d get a Ryzen 5 2600 — a little bit more performance for about the same heat.
  • I wouldn’t bother buying the optional LH6 Cooling Kit — it would be overkill for even a moderately-overclocked 2600.
  • Not having to fit the longer LH6 heat pipes would mean that pretty-much all of the component-clearance issues would disappear, and I’d have many more motherboards to choose from.  I’d pick one with two NVMe M.2 slots to cut down on cable clutter.

Apart from that, everything else would stay the same.

Future plans

  • Conduct some load tests on the GPU in isolation.
  • Torture-test the DB4 by doing things like:
    • Loading both the CPU and GPU to 100% at the same time.
      • Not very realistic, but a ‘worst-case’ scenario that deserves to be tested — For Science!
    • Blocking vents while the system is under load.

If you have any questions about the results or methodology above, or you’d like a certain type of thermal test performed, just ask in the comments.

Auf Wiedersehen!

14 thoughts on “Passively-cooled CPU Thermals

  1. Thanks for sharing your build! I have a question for you (though it may sound stupid). You said that you if you were to do this again, you would get the AMD Ryzen 5 2600 but not get the LH6 Cooling Kit. Does that mean you would use the Ryzen with the fan that comes with it, buy some other heat-sink, or just go without anything attached to the CPU?
    Thanks

  2. Whoops, never mind. I just saw that the DB4 chassis comes with a cpu heat sink… Sorry! Feel free to delete.

  3. Not a stupid question at all! The DB4 is a purely fanless chassis — you would have to start drilling holes into it if you wanted to start installing fans so no, you don’t use any fans (included or after-market). When you buy the DB4 case you get four (short) heatpipes that let you bond the CPU to a single wall of the case. Those four pipes and a single wall can cool a CPU with a TDP of 65W or less. The LH6 Cooling Kit includes three (long) heat pipes that let you bond the CPU to a second wall of the case. If you install both the stock heat pipes and the LH6 heat pipes then you can theoretically cool a 105W CPU.

    Since I thought I might need to overclock the Ryzen 5 1600 to get the performance I needed, I purchased and installed both sets of heat pipes — because an overclocked 1600 can output ~95W. As it turns out a stock (3.2GHz) Ryzen 5 1600 is more than adequate for my needs, so there’s no real need for me to overclock it. I’ll do it anyway — For Science! — but I’ll probably just restore the 1600 back to stock when I’m done. It’s really a very nice CPU.

    The 2600 is a bit faster than the 1600 (about 8% or so) but still has the same 65W TDP. So you get a bit more performance for about the same heat. Having done a bunch of testing on the 1600 I am now 100% confident that the four stock DB4 heat pipes (that you get with the case), bonded to a single wall, are all you would need to comfortably cool a stock Ryzen 5 2600. You don’t need to buy and install the optional LH6 Cooling Kit to adequately cool that CPU.

    Hopefully that clears things up?

    PS: Streacom have designed a prototype chassis that is similar to the DB4 but which can be actively cooled (i.e. by fans). If you do an image search for “Streacom DA4” you can see it. I have no interest in that product because fans make noise and the whole point of this build was to be completely silent. But I guess for folks that like the look of the DB4, and don’t mind a bit of noise, the DA4 may be an option. Not sure if/when it will be available, though.

  4. Nah, I’ll leave it up. It’s actually a good question/topic and one that I didn’t go into a heck of a lot of detail in the post. May as well leave the answer here in the comments for others to read. Thanks for asking!

  5. Thanks Tim! That’s great information. I’m seriously considering a build very close to this, but without the LH6 kit and a motherboard with 2 NVMe M.2 slots like you mentioned. I’m also thinking of going down to 2 x 500 GB SSDs instead of 2 x 1TB since it saves about a $1k, and I don’t have huge storage needs.

    I have been using a laptop for most of my work (Dell XPS 13), and it’s relatively quiet. Recently I fired up my old budget gaming desktop, and wow, that thing is LOUD. Reading this has inspired me to build a silent pc! Thanks again.

  6. Hi, very interesting analysis. Thanks.
    I am building a very similar system these days based on a Ryzen 5600X. It is a similar power envelope like what you use, however, my thermals are very different. When loading the CPU to 100% I see basically immediate jumps into the mid 70C range und then from there a slow climb. After like 20 minutes it reaches 90C and starts to throttle. I use the standard cooling kit and for the CPU extension kit I bent my own Heatpipes just to go around the electronics on the mainboard.
    I tried re-seating the cooler already to be sure I do not have issues on the thermal contact, but that’s all fine.
    For CPU load I use Prime95 or MPrime under Linux in blend mode. The CPU is consuming straight 75W during the run.
    The chassis side panels are heating up quite nice. The 2 witch have Heatpipes attached reach 47C during the stress test. So all seems normal.
    Wonder what is the difference between our setups.
    If you are interested to discuss feel free to get in touch.
    Thanks and best regards
    Andreas

  7. Hi Andreas. Your 5600X probably draws 26–28W more than my 1600 under full load, so it’s no surprise that the temps ramp up faster. Your 5600X is closer to a 1600X than it is a 1600 in the thermals department.

    That said, I’m quite sure that a DB4 with the optional LH6 Cooling Kit (which I assume is what you mean when you talk about a “CPU extension kit”?) can handle even that amount of power.

    Have you made sure that all of your heat pipes have a steady downward slope from their tips back to the CPU block end? When steam condenses at the tip of a heat pipe, the (now liquid) water has to flow back to the CPU block end so it can get heated and up and converted to steam again. The sintered copper that lines the inner walls of the heat pipe does wick this water, but that job is made much harder if it has to try wick water up a hill. A steady downward slope, with no ‘traps’, greatly enhances the wicking action.

    (If you look closely at the second image in this post, you will see that all of the heat pipes slope gently downwards.)

  8. Hi Tim, thanks for the quick reply.

    I am not too familiar with older Ryzens, so
    I just cross checked the power consumptions of 1600 vs 1600x. What I saw the 1600 is in the 65W department while the 1600x is in the 95W department.
    The 5600x even though it is an x is in the 65W department again. AMD seems to have decided to always put the x. There is no non- x. Also direct measurement shows that under constant full load it draws 75W quite steadily.

    With the CPU extension kit I meant indeed the LH6 from Streacom. I had to add customized heat pipes with 3 bends to actually route those to the side wall. It is a good hint to cross check the slopes on those. Indeed they might be oriented rather leveled.

    Still the standard heat pipes that come with the chassis have a nice slope.

    One thing that I noticed when looking at your pictures it seems for thermal connections of the blocks to the side wall it uses paste. Is that correct? I used the large thermal pads that came with the chassis and the LH6. There are no specs with them so I wonder whether they are causing issues in my setup.

    Best regards
    Andreas

  9. Andreas, I hadn’t noticed that AMD stopped making non-X CPUs — thanks for pointing that out. As for power draw, my entire system (CPU, GPU, mobo, RAM, 2xSSDs, keyboard, mouse — all of it) draws 75W at the wall plug when the CPU is under 100% load (and the GPU is doing nothing noteworthy). I’m not sure if your 75W is full system, or just the CPU.

    As for bonding the heat spreaders to the side walls, I used Noctua NT-H1 thermal paste. In fact, I used that for everything. The thermal pads Streacom ship with the DB4 and the LH6 are very mediocre. When doing my initial research I came across many posts that discussed the conductivity of the pads. Everyone who used them, and replaced them, saw improvements. I suggest you do the same.

    Given that it takes you 20 full minutes to throttle under a torture test load, I don’t think you need to make many changes at all to your system to get it thermally stable. Maybe replacing the pads and checking the slope of the heat pipes is all you need to do to bring temps back under control. I hope so!

  10. Hi Tim, thanks for your reply.
    That clarifies it. I measured 75W only at the CPU. My full box draws 130W under load from the DC side of the PSU.
    Thanks also for the hint on the thermal paste. This is very valuable input.
    Well I feel also it will run fine under a realistic load with minor changes already. But then as you say “just for science “ I want to see how far it can go.
    Thanks again and have a nice day
    Andreas

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