Hot iMac and higher fan speeds.

[ggulliver]

I've recently noticed that my 2008 24" iMac is noisier than before.

iStat shows the CPU fan at 2000-2500, and some seemingly hot temperatures - measured with minimal activity.

----------
Ambient Air 26°C
CPU A Heatsink 51°C
CPU Core 1 56°C
CPU Core 2 55°C
Display Screen Proximity 51°C
Graphics Processor Chip 1 68°C
Graphics Processor Heatsink 1 68°C
Graphics Processor Temperature Diode 74°C
Hard Drive Bay 1 54°C
Main Logic Board 55°C
Optical Drive 48°C
Power Supply Position 1 82°C
Wireless Module 56°C
----------
I know they can often run hot, but don't know enough to decide if there's a potential problem here.

I installed smcfancontrol and bumped the speeds to successfully lower the temperatures. But that caused an unwanted problem... on trying to return the speeds to default they remain high - and only by a reboot can I get them down.

Having reset the SMC, I now have speeds of 700/1200/1800 (OD/HD/CPU).

 

[GGJstudios]

The Intel processors used in Macs are designed to automatically shut down to prevent damage if they truly overheat. CPU Tjmax = 105C (221F), GPU Tjmax = 100C (212F) on i3, i5, i7 processors. (Source: Intel)

If you're not already using it, iStat Pro (free) or iStat Menus ($16) will give you accurate readings of your temps and fan speeds, among other things.

Unless there is a rare defect in a Mac, most temps are well within the normal operating range, considering the workload being put on it. Websites with Flash content, games and other multimedia apps will put higher demand on the CPU/GPU, generating more heat. This is normal. If you're constantly putting high demands on your system, such as gaming or other multimedia tasks, expect temps to rise and fans to spin up accordingly. It's just your Mac doing its job to maintain temps within the normal range.

It is also quite normal for your Mac to become extremely hot to the touch during intensive operations. The aluminum body transfers heat more effectively than other materials used in computer casings, so you will feel the heat more. This doesn't indicate that it's overheating and will not harm the computer to be hot to the touch.

Your fans are always on when your Mac is on, spinning at a minimum of 2000 rpm (for MBPs) or 1800 rpm (for MBAs, MBs and minis). iMacs have 3 fans with minimum speeds in the 800-1200 range. They will spin faster as needed to keep temps at a safe level.

If your fans are spinning up without increased heat, try resetting the SMC.
(PRAM/NVRAM has nothing to do with these issues, so resetting it will not help.)

The intake and exhaust vents are in the back of the computer near the hinge on all Mac notebooks (except the new MBP with retina display, which has intake vents along the sides at the bottom). The iMac vent is a slot on the back near the top of the computer. Make sure the vents remain unblocked to allow your computer to perform at its best.

Learn about the fans in your Mac
Apple Portables: Operating temperature

For Flash-related issues:

• Find your Flash version and make sure it's the latest version available.
• Install ClickToFlash (Safari), Flashblock (Firefox) or FlashBlock (Chrome) to control which Flash content plays on websites.
• Use the YouTube HTML5 Video Player to watch YouTube videos, when available. (May impact fullscreen viewing. See link for details.)

 

[ggulliver]

Thanks for the comprehensive answer.

UPDATE: A day later, with most of the above suggestions already done, I'm still concerned that the CPU fan is often spinning up to 1800-2500 with very low activity. And that PSU is usually 80-85 - which seems unhealthily high.

At least SMCfancontrol seems to work better... fans revert to default after I boosted them, albeit taking maybe 5-10 minutes to do so.