Pi 4 metal top of processor

It's a heat spreader, not connected to the internals electrically (of course?). What you plan to do is what all the alloy cases do that act as heatsinks; you'll be safe. Hardest part will be to get the distance just right: not too close or you'll bend the board and the connections, not too far or it won't transfer heat.

Ref.:

Thanks for the confirmation. I was pretty sure that was the case. My intention is to mount the Pi upside down with long bolts and springs pushing it down (it will be fully enclosed) so that should keep everything safe.

smear of heatsink compound will help as well

There are various flexible thermally conductive materials, such as:

Make sure to have a close look at all the Pi 4 thermal images you can find. The SoC is a good contact point but there are a few others that get hot, too. If you don't connect to those and keep the board upside down, you might trap the heat.

Also, I'd be wary of using springs because they will probably bend the board. That might damage the traces and all those minuscule solder points. Maybe not a problem if they're very slack.

Good point! Thanks.

Ventilation is important: I once designed a small audio equaliser in a sealed 19" rack case - it was only about 15 watts total draw, but after

WE drilled a few holes in the case and that fixed it.

Airflow is better than transmission through even metal cases. The problem is that with no air movement the air inside becomes an effective thermal insulator.

Furthermore *forced* air cooling can multiply heat loss by up to a 5 times or more.

Would the RPi-400 be relevant in your usage?

Indeed. I been reading up on all the reports various people have made, and come to the conclusion I was heading down the wrong path - note past tense :)

It now looks I'll get best results with normal mounting, a finned heatsink stuck on top with a 30mm fan placed alongside so it blows through the fins.

I want to avoid external ventilation as there is mains inside, so there wou ld need to be all sorts of additional protection barriers to deal with small h ands and/or brains!

No. I looked at it and immediately rejected it - it's just completely wrong.

Have you any comparable figures for passive 'chimney effect' cooling, i.e. use a metal contact heat sink on the chip(s) but fit, say, a 50mm x

Chimney heat pipes do work, I know from my TV servicing days. However, the unit can be used upside down, or even on one edge so that's not really an option!

I'll get a fan, and try it first with no heatsink, and blowing across the top of the Pi. If that's cools it enough, then I won't bother to go further.

As it stands, the absolute hottest I've managed to get it when fully loaded up was 74C, so even that is usable, though obviously it's not a good idea!

Not very, or we wouldn't use fans ;-)

may be in a server room where there is natural air flow caused by ventilation - but the fan multiplies the cooling factor for sure

You can always move more air with a fan, but sometimes a chimney moves enough and when it does it is quieter, more reliable and larger than a fan.

My Pi4 is on 24/7 because I use it as a PVR for recording TV programmes. The recordings are written to an external USB HDD (ie not to the SD card "system disc"). The Pi reports a CPU temperature of around 60 deg C which I understand is within safe limits. The Pi is in a standard plastic case (as sold by Raspberry Pi Foundation) with no fan; the case feels warm but not hot to the touch. The Pi (plus a Pi 3 for logging data from a weather station) and the external HDD are kept in a small cupboard of a TV cabinet, which only has a hole cut in the back to take cables. The temperature inside the TV cabinet is about 27 deg C - so a little warmer than normal room temperature.

One thing I noticed about the Pi4 compared with the Pi3 is that its CPU temperature remains a lot more constant; the Pi3's CPU is normally about 40 deg C but rises to about 80 deg C as soon as I run Firefox (so I don't run FF on it - partly because of the temperature rise and partly because it runs like a wounded snail on the Pi3).

Easily!

Still, if you want it lower than 60, get just about *any* other case. Those "official" cases are the worst. Do not get the fan for that case because reviews say it whines loudly. These are good & fairly cheap

To be clear: at idle.

Yep, pretty much what I've understood, but I was hoping that somebody would have numbers to quantify the difference between fan and chimney cooling.

FWIW the only Pi I'm currently running continuously is an old 512MB Pi