M3 iMac thermal design

[John Malkovich]

This is more a rant than a question. I bought the new M3 iMac 8GB/256GB, 8/10 cores, four USB ports, a couple of weeks ago. Already dislike how applications start up faster on my Mac mini M1 and how my external USB4 drive is twice faster than the internal SSD. But the thermal design (if there is any) is bad.
This machine has two fans and when I turn them to maximum via Macs Fan Control, I do not feel any air streaming down through the bottom edge. What is also unclear is where is the air intake? If it is through the same holes in the bottom edge, then the air makes a rather short trip through the bottom part and probably taking in a part of hot exhausted air. Moreover, fans at maximum have very little to no effect on the temperature of the internals. Looking at the hottest part, which is power supply proximity, it will go up to 60+ Celsius and sometimes even touch 70 degrees. The whole computer is more than warm, top to stand, and we are talking moderate load here. Admittedly, the weather has been really hot here lately, but this cooling is so weak and nowhere near the one in Mac mini, that I don't think this is good in the long term.

 

[arw]

Moreover, fans at maximum have very little to no effect on the temperature of the internals. Looking at the hottest part, which is power supply proximity, it will go up to 60+ Celsius and sometimes even touch 70 degrees

Is this sensor by any chance located at the actual PSU, which is the big, white external power brick?
If so, fan speed can in no way affect said temperature.
Also, if that is the hottest part and the CPU doesn't even reach 70°C, that sounds like (more than) adequate thermal management as there is +30°C (K) headroom until throttling even begins.

edit: But you're right. From looking at teardowns, the warm air from the heat sink fins is just blown inside the case instead of being actively directed out through vent holes. Apparently Apple doesn't deem it necessary given the iMacs large surface area. But the enclosure getting warm is no surprise as it acts like sort of a heatsink.

 

[John Malkovich]

Hello and thanks for your response. I wouldn't know, but I guess the temperature sensor is based inside the computer, where voltage stabilisation section is. You mention absolute maximum temperature ratings, when emergency cooling kicks in. This is Apple's traditional approach, to delay fans until really, really necessary. But the temperatures I am seeing are not good for electronic components in the long term. I also would like to see how this affects the accuracy and longevity of the screen, too.

 

[arw]

I also would like to see how this affects the accuracy and longevity of the screen, too.

A valid concern I have no insight into. I suspect, this is one reason why the SoC is located in the "chin" and not behind the display.
I am with you, the main goal of the AS iMac seems to be design and thinness and not sustained higher temperatures of power users.
But in the two weeks I set up an M3 iMac 24GB/1TB, 8/10 cores, the case didn't even get close to as warm as my Intel MacBooks regularly do even during basic tasks. So I wouldn't worry. Also, it's the second generation of this iMac design and if it were problematic, I suspect Apple would have addressed it.

 

[Chuckeee]

Just an observation from looking at the teardown videos, Those two iMac fans are small, Even compared to the fans in the MBP.

 

[John Malkovich]

I cannot tell if they are even smaller than those in laptops, but last night I was encoding a music album from AIFF to mp3 and the power supply proximity immediately jumped to 107 C and stayed there as long as the encoding was running. So, even under moderate loads, the internals will go up to temperatures which are not safe anymore.

 

[arw]

(...) even under moderate loads (...)

Did you check Activity Monitor during the encode? If the app is optimized for multicore, it may not be a 'moderate' load but instead maxing out the CPU. Which is a good thing.

the power supply proximity immediately jumped to 107 C

I don't have the iMac anymore, so I can't compare my PSU temperature. But from other tests, stressing the CPU results in almost exactly your 107°C. And this is with thermal throttling.

source: digitec.ch

the internals will go up to temperatures which are not safe anymore

Apple apparently does deem them safe. Or at least safe enough for their envisioned lifespan.

Too bad you are past your return period. Because I feel you, I'd also be stressed out about +100°C temperatures on a regular basis (regardless if that feeling is actually justified or not).

The AS iMac never had the thermal capacity of a Mac mini, it has however a different target audience.
In the end, you can give Apple a call for your peace of mind but I doubt they will tell you anything different.

edit:

I cannot tell if they [the fans] are even smaller than those in laptops

I tried my best in comparing two iFixit teardowns to scale (although from the M1 version).
At least the MacBook blows the hot air OUT of the case 😅

 

[kagharaht]

I cannot tell if they are even smaller than those in laptops, but last night I was encoding a music album from AIFF to mp3 and the power supply proximity immediately jumped to 107 C and stayed there as long as the encoding was running. So, even under moderate loads, the internals will go up to temperatures which are not safe anymore.

I decided to try this on my iMac M3, 24gb/2TB/4port/8-10 core. I took video file MKV and ran Handbrake Pre-set to 1080p for Apple TV 4K. 25% into the encode process I measure the temperature on the same spot as you did. I'm reading 105 degrees or 40.5 celsius. I opened up a bunch of apps and checked to make sure cpu load is full with activity monitor and the temperature is still around 105-106 degrees 40-41 Celsius. I don't see a heat issue with mine and I have all CPU utilized with Handbrake as activity monitor is showing. I hear the fan running and measure the heat. It comes out to 95 degrees Fahrenheit at the hottest point. Oh if you're getting 107 Celsius that's 224 degrees Farenheit. I mean water boils at 212 Fahrenheit!