With more heatwaves expected across Europe and North America, this summer is already proving a challenge for those who can’t use their Macs in rooms that have effective air conditioning. This article explains how you can improve your Mac’s cooling to cope better with extreme summer temperatures. It accompanies my previous account of what to do with a hot Mac.

The problem

Computers and peripherals are powered by electricity, and almost all the power they use is generated as heat. If that heat can’t be dissipated as fast as it’s produced, then the temperature of critical components rises, and ultimately will cause them to cease functioning. Before that, some materials such as plastics inside the computer or device may melt, resulting in damage to the logic board and high replacement cost. Unchecked temperature rise can even cause fires, although those are thankfully exceedingly rare.

Heat transfer

Heat generated by electronic components is normally dissipated by three means:

• conduction from a chip or component direct to another solid in immediate contact with it;
• convection of a fluid, almost always air, which moves when it’s heated by the component; this can be free convection, or forced by a fan;
• radiation, invisible exchange between two surfaces in direct line of sight, even when there’s a vacuum separating them, by infra-red radiation.

Humans and other mammals rely most on a fourth form, by evaporating sweat or another liquid from their surface into air, something you can’t do safely with a Mac.

Conduction

Hot electronic components surrounded by still air don’t dissipate heat well, as still air is a good insulator. An obvious illustration of that is in the duvet jacket you wear in the cold, which traps layers of still air in its down or fibre filling, so reducing heat loss from your body.

In many circumstances, electronic components make physical contact with a metal heat sink, to conduct heat away from the component into the heat sink, and dissipate it from there. But even with precision machining, it’s common for small air gaps to impair conduction, so a paste is applied between the component and the heat sink. Provided that thermal paste is more conductive than air, it will increase the rate of heat conduction away from the component.

CPUs and other chips inside Macs are the most likely to be equipped with heat sinks, and to have thermal paste applied between them to improve heat transfer. This is also often seen with separate GPUs.

Thermal paste

In some cases, thermal paste may not have been well applied to begin with; in others, it migrates or is otherwise lost over time. In those cases, careful application of thermal paste between the chip and its heat sink can significantly improve cooling by conduction.

In some older models of Mac, an experienced and careful user may be able to disassemble the Mac to the point where its heat sinks can be removed for application of thermal paste. That may be delicate surgery: before even contemplating trying it, watch iFixit’s account of how to disassemble your Mac down to its logic board. Bear in mind that any damage you do won’t be covered by any warranty or AppleCare, so you’d have to bear the cost of any replacement logic board yourself.

Most recent Macs require special tools to perform this, and some need special replacement components too. The best way to obtain these, together with full instructions, is through the Apple Self Service Repair programme, if that’s available in your location.

For almost everyone, the only practical way to check and replace thermal paste in a Mac is to have this performed by an Apple Authorised Service Provider, who will be delighted to quote a cost that might convince you that, without good evidence that your Mac needs it, it may be worth leaving for the moment.

External SSDs

Some external SSDs also rely on conduction to a heat sink for their cooling. That isn’t normally accomplished using thermal paste, though, but a thermal pad made of specially conductive silicon rubber. If your SSD can benefit from better thermal contact with the aid of a thermal pad, they’re cheap and readily available. The common failing here is not ensuring good contact between the upper surface of the pad and its heat sink, requiring a pad of just the right thickness.

Free convection

Unless the air around a heat sink or bare component is blown by a fan, its heat is then dissipated by free convection, occurring when the air in immediate contact with the hot surface is heated and rises because warmer air is less dense. This isn’t an efficient way to lose heat, and is normally aided by increasing the surface area of the hot surface using cooling fins. The larger the fins, the greater their surface area, and the more heat they’re able to dissipate, provided that the air surrounding them is able to move away to let cooler air come into contact with the surface of the heat sink.

Here, heat sink design becomes most important, together with ensuring that air can flow freely. Blocked cooling slots on the case must be cleaned out, and the warm case of a laptop like a MacBook Air must have good clearance to allow the air to move around it. Some laptop stands encourage this, but putting a laptop onto a soft surface can prevent it.

Forced convection

Even a small low-powered fan is much better at moving cooling air than relying on free convection, as you experience on your own skin when it’s hot. Cooling fans inside a computer case are carefully designed to draw or blow cooler ambient air over hot components and heat sinks. They rely on being kept clean at all times, and their intakes and outlets remaining free of all obstructions. Removing dust and dirt from them is thus a vital step in ensuring they work efficiently.

Some small external SSD enclosures now offer cooling fans, although they’re not certain to prove superior to a good heat sink. Designs that try to incorporate both a fan and heat sink may not be optimal either, as the fan works with forced air movement, while the heat sink relies on good surface contact without air.

Cooling fans should be provided inside all cases of sufficient size, including those in a NAS system. Provided that ambient air temperature remains within the comfort zone of the user, cooling fans should provide sufficient heat dissipation for SSDs and similar components, as has been demonstrated by vendors such as Asustor. The only remaining question is how those fans are controlled.

Fans in computer cases only move air inside, and can’t move it around the case or the room. An external room fan is thus a valuable addition to provide forced convective cooling, for both you and your Mac.

Radiation

This is a catch for those vendors who don’t know what they’re talking about: any company making claims to cool your Mac or devices using radiated heat is almost certainly trying to fool you. While changing material and case colour may alter its infra-red reflectance, almost all materials lose heat by infra-red emission at similar rates, as they behave close to a theoretical ‘black body’, and are near-perfect emitters. Thus there’s little that you can do to enhance any radiant heat loss.

When radiant heat does become important is with anything left in sunlight, even if that light has passed through glass. Always ensure the whole of your Mac, its display, and all peripherals, are kept out of sunlight, to minimise radiant heating.

Evaporation

Some Macs of the past were liquid cooled, but none is anything like a human and cooled by evaporation. If you’re warm enough to even start sweating, that’s a good indication that your Mac may well be hotter than you, and you should ensure that it’s cooled effectively.

Working through the heat

As room temperature rises above 23˚C (73˚F) the use of some Macs may become limited because of the heat they produce. This is more likely with Intel models, those with more powerful graphics hardware, laptops, and when running apps that use high CPU and GPU %. You may find that using your normal apps results in steadily rising temperatures, as shown by increasing CPU % being given to kernel_task.

In extreme heat, if you can’t use air conditioning, you may still be able to use your Mac, but will need to reduce the load put on its CPU and GPU. One way to do that is to run a minimum of apps concurrently and keep an eye on kernel_task in Activity Monitor’s CPU view. If that rises above 20% or so, close apps to let it fall again. Although that will slow your work, it may still allow you to carry on using your Mac. In models that support it, switching to High Power in the Energy Mode in Energy settings can be advantageous, as that makes its use of fans more aggressive.

Outside air

When trying to keep a room cool it’s valuable to know both the air temperature inside the room, and that outside, as those determine whether opening windows will make the room cooler or hotter. Although forecast temperatures shown by Weather and other apps give a rough guide to outdoor temperatures, they’re not as good as a local thermometer kept in the shade, and may differ by several degrees. Best practice during hot periods is to open windows wide during the cooler hours of night (also a good time to attract moths and other insects!), and close them during the heat of the day, when it’s usually hotter outside than in.

This may need to be modified to take into account humidity. With all the windows closed during the day, relative humidity inside a room can climb towards 100%, making it much harder for humans to keep cool, although it has little effect on our computers provided water in the air doesn’t condense onto surfaces.

Your own comfort

Although we may do a great deal to keep our Macs and their peripherals as cool as possible, never forget your own thermal comfort. When you get hot it significantly impairs your performance. While some become sleepy, it’s more common for hot people to become irritable and short-tempered, and to make bad decisions. Keep your water intake up to ensure you can sweat to stay cool, and rest more in the heat of the day. What’s known as a tropical routine makes good sense: start work as early as possible in the day, by 07:00, stop well before the heat of midday, then take a long siesta before returning to work as the heat eases in the early evening.

Practical measures

• Thermal mats can help ensure good conduction between electronic components such as SSDs and their heat sink or enclosure.
• Put a laptop on a stand to help its case cool.
• Keep all cooling slots in a case clean, free of dust and dirt, and unobstructed.
• Keep any fans clean and free of dust and dirt.
• Force air movement in a room using ventilation and fans.
• Keep all electronics out of sunlight, even if that has passed through glass.
• Connect an external display to a laptop via HDMI when available, rather than Thunderbolt, as HDMI doesn’t charge the laptop and make it hotter.
• If you’re warm or hot, your Mac may well be even hotter.

Apple’s advice, mainly for laptops.