Hello!
I thought I'd share some random findings on CPU temperature on my Raspberry Pi mini-server.
I have an older Pi1 (B+) in my basement that has been controlling my sprinkler system for several years already, and it seems to always run at a nice lukewarm 31ºC or thereabouts.
It is mounted vertically in a well-ventilated plastic box, plus it never has to do much work because that sprinkler controller is the only software it needs to run. It's always a bit cooler down in the basement too.
My (Pi3 B+) mini-server on my desk, however, seems to always run a fair bit warmer, at around 56ºC at any given time.
That confused me a little bit, as in theory it's not super active.
However, it does have NodeRed running, and InfluxDB, and Mosquitto. And it receives a sensor update message every minute. So that might actually be doing a fair bit of work underneath the surface.
This is the dashboard I'm using it for. I decided to show the CPU temperature on it, because it was easy... that's where this deeeeeep thoouuuuught moment started 
(This wast just a moment ago, so the temperature is with the updated cooling)
But, the biggest detail might actually be the cheap plastic case I have... it has terrible ventilation. Only the end opposite the USB plugs is ventilated. I decided to drill a pattern of holes above the main chip to add some airflow, but I don't remember that doing much at all.
I then thought of my basement Pi, and wondered if the vertical positioning made a difference... so I propped up the mini server with the ventilated end upwards, and it actually does make a difference of a few degrees: 53º give or take. Not too significant though.
I remembered having a tiny heatsink from some old computers, so I plopped that onto the main chip, with some of that heat transfer bird-poop stuff.
Turns out that goopy stuff doesn't hold it on... but lucky for me the case is just the right size to hold it in place without it being stressed about it.
Now it can sit on my desk at about 53ºC, without having to be propped up.
Propping it up on edge reduces it down to about 49ºC.
Interesting, but the few degrees difference really doesn't seem to matter in the grand scheme of things 
Here's a picture of my Raspberry Pi mini-server. It's using a Pi 3B (+, I think?).
I did also buy an aluminum alloy case off Amazon recently, for my Raspberry Pi 4 - it was on sale for only $15CAD, so I thought that might be a good solid option for silent heat dissipation as well as keeping it safe and looking like a good little server rather than some random PCB sitting there 
So I might replicate my current setup onto the Pi4 to see how things look there.
Stay well,
-Nico
Edit: An Update!
Last month I ordered a heatsink case off AliExpress to see if that would make a difference. At about $11 Canadian dollars, it was an inexpensive thing to try out, plus the case looks a bit more professional.
And it seems to help a fair bit: I installed it earlier today, and the temperature now stays steady at around 44ºC, compared to recent temperatures of around 50ºC in the modified plastic case which was standing on edge. And considering the original temperatures were running around 56º, that's a significant improvement.
The metal case feels quite warm to the touch, so it seems to be moving the heat from the board out well enough.
Also now it no longer seems to matter how the case is set up - propping it on edge doesn't seem to make it any cooler. I don't intend to run it hard, so this case will be perfect to keep it at a reasonable temperature.
I suppose if necessary, an additional heatsink could be applied.
Another edit:
yes, placing a big heatsink from an old motherboard on it brought it down another 5 or so degrees C (even without any heatsink paste). But that heatsink with all the fins is at least twice as big as the enclosure itself, so it's not staying.
Of note... an empty pop/beer can also brings it down a few degrees, so if needed for a burst of computing power, I can just put a beer can full of ice water on the case
Cheers,
-Nico
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