How often do you take your own body temperature? Unless you’re a woman keeping a watch on their ovulation, I suspect the only time you might do that is when you suspect you may have a fever. That’s because, by and large, we leave our body to get on with regulating its temperature, a task it does from birth to death. Macs have a similar system in CoreDuet, handled by the System Management Controller (SMC), and active whenever that Mac is running. This article explains how to work with CoreDuet to ensure your Mac keeps cool.

Signs of thermal strain

Like your body, when your Mac is subjected to thermal stress in high ambient temperatures and by working hard, it may show signs of thermal strain:

• The outer case of a notebook becomes noticeably hot to touch, particularly on the underside. If you’re resting it on bare thighs, that becomes uncomfortable.
• Models that are fitted with fans run them up, a sound that’s normally easy to hear.
• kernel_task starts throttling the CPU by taking a high percentage, as shown in Activity Monitor’s CPU view.
• Eventually, if stress continues, the CPU and other systems are throttled more obviously, and your Mac becomes sluggish to respond.
• Finally, it may shut down altogether.

There’s one important situation where none of those happen, but your Mac is at grave risk of thermal damage: that’s when it’s passively warmed, when shut down or asleep in a very hot environment such as inside a parked car, or in sunlight. Before even attempting to use that Mac again, ensure it’s thoroughly cooled down, and remind yourself never to repeat that. Macs are designed to be used in ambient air temperatures of 10˚-35˚C (50˚-95˚F), and temperatures higher than that can damage their logic board, battery and other expensive components.

What to do

If your Mac is showing early signs of thermal strain, and still running, encourage heat dissipation by immediately:

• opening its lid wide if it’s in clamshell mode
• ensuring all ventilation openings in its case are free of obstruction
• clearing space around its entire case, including the underside, to aid convective cooling
• if the Mac is in sunlight, move it to the shade
• if you have an external fan, turn that on to cool its case by forced convection
• if air conditioning is available, move the Mac to a cooler area.

Reduce its heat production by immediately:

• quitting all open apps, if necessary force-quitting any that won’t cooperate
• killing all non-essential processes that are taking more than 5-10% CPU in Activity Monitor except kernel_task, which is a sign of thermal strain and not a cause of it
• disconnecting any mains charging cable, provided there is sufficient charge in its battery.

Apple silicon Macs generate less than 5 W of heat when only idling, and are therefore best left running in idle if their fans are running. Unless an Intel Mac has become so hot that it’s at risk of shutting itself down, it’s usually best to avoid shutting it down if its fans are running, although it’s likely to continue generating more heat.

Some like to put cold packs against the case, which can cool the outside down more quickly than an external cooling fan. If you wish to try that, ensure that condensation doesn’t enter the Mac, particularly through its keyboard or ventilation openings, and don’t obstruct either the keyboard or those cooling ducts.

False alarms

Rarely, a Mac may show signs of thermal strain when it’s not hot at all. This most commonly occurs when the fans run at full blast at all times, even when starting it up from cold. This can be fixed in an Intel Mac by resetting its SMC, but that’s not an option for Apple silicon Macs, where Apple recommends closing the lid and opening it again. If that doesn’t work, then try restarting it.

If your Mac persist in this behaviour, try running hardware Diagnostics to see if that detects a fault, and contact Apple Support, as it may need repair.

Manual control

Some users prefer to manage cooling systems including the fans manually. Although this makes as much sense as manually controlling your own body temperature or the cooling system in your car, if you really feel you must, and are prepared to accept full responsibility for the consequences of any errors you may make, then it can be done in relative safety.

The golden rule is to activate its cooling system sooner and more aggressively than CoreDuet would. That will result in lower temperatures, higher energy consumption and shorter battery endurance. In the longer term, because cooling fans are electro-mechanical devices that will eventually wear out, you may find yourself having to get them replaced sooner.

References

Keeping cool: physics
Apple’s advice