iMac 27" runs VERY hot - too hot to touch.

While most newer Apple computer's fans run quiet, maybe your fans are not working. I've felt hot iMacs but nothing thats too hot to touch. Take it to the Apple Store.

These are the temps and fan speeds for mine right now -

Even though my ambient is much higher than yours (and 94F is correct), most of my temps are a lot lower than yours.

You didn't show the fan speeds in your picture... are they reported?

A couple of months ago I installed Diablo II in Snow Leopard. Took about an hour, as I recall. The machine got real hot, hot enough that the fans went to high speed, quite audible. Most of the time I don't hear the fans at all.

If your fans aren't keeping up with the need for cooling, it would appear that a trip to the hospital may be needed for your machine.

This is probably a stupid question, but ya never know - you did remove the clear plastic wrap from around the machine before running it? Just thinking of things that could block the cool air inlet (on the bottom) or the hot air exhaust (on the top).

When I was copying a couple of tens of GB of files from my old mac to my new iMac (21") using target mode, the top edge of the Mac got alarmingly hot (not too hot to touch but hotter than I would expect the case to get). It never recurred and I put it down to the CPU etc. working verey hard. I think, as an electrical engineer, that the top of your machine is far too hot and needs early attention.

BTW, how did you get those temperature figures?

SteveKir wrote:

BTW, how did you get those temperature figures?

I use the iStatPro widget. It displays lots of stuff, probably more than is useful for me -

Apple - Downloads - Dashboard Widgets - iStat Pro

See if an SMC Reset helps. SMC controls the fans.

Shut down, unplug, wait ten minutes, hold in the power button for five-ten seconds, replug and restart.

Here are my (normally cool) iMac:

The image is rather small, and only part of it is captured, but I can't see how to cure those. (I'm using Grab > CMND + SHIFT, then SPACE)

SteveKir wrote:

The image is rather small, and only part of it is captured, but I can't see how to cure those. (I'm using Grab > CMND + SHIFT, then SPACE)

Try using Command-Shift-4 to grab the part of the screen you want.

When you press that key combo, the cursor changes to a crosshair - drag that to enclose the part of the screen you want to capture. When you release the mouse, a screenshot of that portion of the screen will be deposited on the desktop. Double-clicking the screenshot will automatically open it in Preview, where it can be annotated if needed.

I tried that but it captured only part of the area dragged. I discovered Screenshot Plus, available from:

http://www.apple.com/downloads/dashboard/

which offers 3,908 widgets! It copies the whole widget without needing to drag an area (like SnapzPro can do with whole windows).

I've got a recent 27" iMac that has a burning smell coming from it. A first repair replaced the logic board, because the technician saw a piece of scotch tape touching the processor. But it started smelling again, after I got it back, when I was doing a video chat with Skype, which uses a lot of processor power. At the time, the temp on the back of the iMac was 85 C (185 F). An Apple level 2 support technician thinks this is way too hot, and suspects a problem with one or more heat sensors, as the fans were close to the lowest speed at the time. I'm awaiting a second repair.

I too recently picked up 27" iMac, 3.4Ghz i7, 16Gb, SSD + 2Tb running Lion.

It had to be replaced as it had a bad iSight camera but I'd already moved over about a 300Gb of data before I looked at the camera.

I secure erased (3 pass) all my data from the 2Tb drive. The back of the iMac was warm on top right (facing the screen) all through the erase. I had ripped one of my DVDs as a test using Handbrake, watched iTunes tv shows while surfing, using iPhoto and it barely broke a sweat.

Replacement arrived a few days ago but I only got to run it up properly this weekend and it was up and running for 60 mins with barely any activity. General surfing and moving my Home area to the disk drive from Flash drive. Nothing major - not even copying my data yet.

I went to adjust the screen so naturally grabbed the top left and right corners - they were hot! All the way across the top panel of the iMac it was very hot. The top 3rd at the back - all across the back, was extremely hot. I understand the PSU is on the left side and GPU on the right (facing the screen) so this is why these areas get warm/hot.

I understand that the casing is designed to dissipate heat but this was really hot!

I have iStat Pro running and all the temps (Celsius) appear to be ok even when the iMac is busy. Here is an example of stats during a DVD rip as a test. I've also had both GPU stats up to 60+ but remain under 70 so far. Fans are running fine and haven't gone much above 1200rpm.

These internal stats mirror most others that are posted in various discussion topics. But what about the external temperature surface temp??

Well, I have one those fever strips. The kind you place on your forehead. So I placed it on top of the iMac and across the back when it was at its hottest. Result - higher 40 C / 104 F which is all the thermometer goes up to. Basically the colour indicated the temperature was greater than the last reading available. But from Kirk's post above I wouldn't be surprised the back of mine was reaching 80 C. I'm pretty sure I could cook an egg on it.

So - is it ok or not?

Yes - if it is designed to remove heat. But should there be so much in such a short space of time? Streaming a 50 min show from BBC iPlayer push up external temp. Should it be too hot to touch?

No - if there is so much to dissipate, is it actually cool enough on the inside as per iStat? Or will it fry the insides sooner rather than later.

The forum discussions aren't helping as some feedback says ok, others say take it back. What is 'normal' or acceptable??

I'll call Tech Support on Monday but right now feeling I should get this one replaced also because my first one, remained cool/warm on the outside. I know this for sure as I kept checking for some reason knowing how hot my MBP would get when busy. But it all confuses me - what causes such heat build up in one machine and not another? Heat sensors (as per Kirk) or other internal hardware?

I guess I am not answering anything here but just sharing experience and looking for answers and reassurance myself. It may help though, in other posts if folk could also supply external body temperatures as all the inside stuff looks to be as ok as should be. Maybe then we (and Apple) could get trend.

Those temperatures aren't excessively hot. However, if you use iStat Menus, you see other sensors, and the one in question for the back of the iMac - inside the back, that is, according to an Apple technician I spoke with - is called the Secondary Heatsink.

I was told that the problem might actually be a sensor on the inside of the display - in my case, when the Secondary Heatsink was 85 C, the LCD Proximity (behind the display) was around 66 C. The technician hypothesizes that this sensor is not working correctly, leading the overally ventilation system to not work correctly either, leading to heat building up. Also, my power supply was at 75 C, which seems high.

I'm still waiting for a technician who will most likely change my display and see if that fixes the problem.

Thank you or quick response. Here's my current stats. The LCD temps don't appear to be excessive but I can't place my hand on the top or back of the iMac for long - it's like a radiator! Secondary Heatsink maxed about 86 C.

Any thoughts?

Those are similar to what I was seeing, though my power supply was hotter. My fan speeds were a bit lower.

I'd follow up with Apple. I'll post again when I get some more info - I should be having an on-site repair next week.

Contacted Apple Tech Support this morning and here is the outcome:

1. Engineer reports that all internal temperatures are normal
2. Anything under 90 C for example for the Heatsink, GPU is ok (I did not cover LCD as my temps were lower than yours)
3. His own iMac was hot on top/right (facing the screen) and he could not place his hand there for more than a couple of seconds
4. Yes, the casing does dissipate heat, is supposed to be hot but naturally, he could not validate any other possible problem without an engineer getting hands-on
5. To my question about heating up so quickly across the entire top and back top 3rd - around 60 mins of what I would not consider too excessive load for an i7 Quad Core - he of course could not validate over the phone
6. The temperature of heat and impact can also be very subjective - again this requires a hands-on look.

All in all, it sounds very normal behaviour for an iMac 27" but in my mind still a little inconclusive.

I'm afraid I don't have an Apple Retail Store near by to quickly pop in and out to check others and ask questions. So my options were to just stick with it and follow up later if needed with Apple Care or as I am still just inside my 14 day grace period, replace this one for peace of mind.

I have reluctantly chosen the option to replace. I say reluctantly, as I *think* that the heat behaviour is normal but without a solid yes/no, I may be returning a good machine. Sorry Apple - but it will just constantly niggle at me otherwise. If the replacement exhibits same behavior, then that is how the iMac runs and my first (with faulty iSight) was just running cooler for some reason.

I will post back once replacement is in hand.

Did you get your replacement? I'm still trying to figure out my problem. It came back from repair - for the second time - and the technician not only did not smell anything burning, as I did, but didn't get temperatures anywhere near as hot as me. (My secondary heatsink got up to 89 C yesterday in a test.)

In my case, it's not a question of a heat as much as the burning smell. It seems that above a certain temperature something starts cooking, but the technician couldn't get those temperatures. Working with an AppleCare technician, I've been trying to figure out why.

Yesterday, I ran tests using the exact same parameters as the repair technician: same user account, runinng Handbrake, with no peripherals attached. And my temps were around 20 degrees higher than his. I'm pretty flummoxed here - could it have something to do with electricity? I've checked my voltage, and it's around 238, which shouldn't be a problem (standard is 230, but you can get +/- 10% according to the electric company). I can't see what colud be different between my office and the technicians workshop (other than the fact that they're in a larger room, but the ambient temps in both cases are similar).

If the next round of tests doesn't find a solution, I'm going to sell this iMac. The technician said that, for him, it's in perfect condition, so I have no qualms about selling it. I'll get a tricked-out Mac mini and 27" cinema display to replace it.