random things and thoughts … and bad ideas

the other kind of stack

I love using passively cooled low power hardware. No noise, no wear that leads to maintenance, no added power consumption. On the other hand modern computer hardware is so powerful that even the smallest, cheapest and most energy-efficient processors are more than enough to handle many day-to-day computing jobs. So when I first thought about building my own router it already was clear that it had to run completely silent and with no spinning parts like a hard disk or a fan. I got the most energy-saving consumer grade hardware available at the time I built myself a compact PC. It did indeed run without additional fans but I was not satisfied with the temperatures it reached. Thinking about what could be done to reduce the temperature even just a bit without adding fans led me to one of the more silly things I did to my computers …


As someone interested in everything technology or physics I thought if I couldn’t just make use of convection and thus the stack effect. Warm air rises up because its density is lower than that of cold air. Inside a chimney or stack with a heat source at the bottom the whole chimney is filled with air of roughly the same density while cold air outside the chimney has a higher density. This difference in density or air pressure causes a draft from the outside to the inside of the chimney. There is a nice illustration of this on Wikipedia. The relevant factors this draft depends on are the height of the chimney, the temperature difference between inside and outside and the cross-sectional area of the chimney. That means with higher temperature of the heat sink the draft of cold air would be stronger too. Which is a convenient thing for passive cooling.

For that router I used an Intel D945GSEJT mainboard which had an integrated Atom N270 CPU and contrary to its bigger brothers used a mobile chipset with much reduced TDP. The board already shipped without fans but I chose to replace one of the stock heat sinks with a bigger custom one. Still running it without further modification made it climb up to around 60°C which was too high for my tastes considering it should run 24/7. My first try at using the stack effect was rather simple. I just cut out a piece of paper, bent it over both heat sinks which were right next to each other and taped it to the sinks. As the case had a stand and openings for ventilation right next to the heat sinks my theory was the paper and the board itself would form something like a chimney and drawing up air from below the case. To my surprise it actually worked reducing the maximum temperature by a few °C. The effect was not huge but it was noticeable.

the stacked stack

This setup has been running for a few years but sadly it was never as solid as I wanted it to be. The router crashed once every few months and the way I set up the flash drive and operating system made updating inconvenient. I decided it was time to build another better router. Again the new Intel D2500CC mainboard shipped with integrated Atom CPU but a single fanless heat sink for both CPU and chipset. The new case has a ventilation grid right above the mainboard so I hoped that would be enough for running it fanless. But just like before the temperatures were a tad too high for a device running non-stop. This time though the case didn’t have a stand and there was not enough space between the heat sink and case for putting a chimney directly on the heat sink. Wondering what I could do about that I just played around putting some paper tubes on top of the case. It seemed to work. To improve the draft of the stack I increased the height by stacking several rolled up A4 sheets of paper on top of each other. If the sheets are rolled up unevenly they become stackable by putting the narrow end of the upper tube into the wider end of the tube below. Because the effective height of the chimney is the distance between the highest entry at the bottom and the lowest exit above I put another sheet of paper on the upper ventilation grid of the case to close it. This way I hope the case becomes part of the chimney causing draft through the whole case and not just from below the paper tube.

Of course this is not what I intend to be the final version but as provisional solutions usually hold the longest I will most likely go with “I will properly fix that later” until I build the next router.

You probably want to see some data now that all this actually has some measurable effect on system temperature. Before writing this post I ran sensord on the router to collect temperature data over a week and removed the paper tube and closing sheet for some time. Here is the graph of the CPU temperature over 2 days. Try to spot when I removed the chimney and when I put it back on. (Hint: it’s obvious)



As you can see using chimneys to create some additional draft for passively cooled systems does work and without any of the drawbacks a fan has. All it takes is some thinking where and how to place the chimney and some space in or on the computer case. Obviously the effect is not huge but given the exceptionally low cost of a few sheets of paper and tape I can’t think of a reason why it shouldn’t be at least considered when building a fanless system.


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