Every watt consumed by a processor or graphics card turns into heat, and that heat has to leave the case. When it can't, modern components don't break immediately: they protect themselves by lowering their clock speed. The result is a computer that is slower, noisier and, in the long run, more prone to failure. Understanding how cooling works helps you solve many of these problems with a few simple steps.
The path of heat
Heat follows a chain:
- from the processor die to its metal lid (IHS);
- from the lid to the cooler base, through the thermal paste;
- from the base to the fins, often via heat pipes;
- from the fins to the air, pushed by the fans;
- from the air inside the case to the outside, thanks to the case fans.
Each link in the chain can become the bottleneck. An excellent cooler is of little use if hot air stays trapped in the case.
Thermal paste
The surfaces of the processor and cooler look smooth, but at a microscopic level they are uneven. Thermal paste fills the microscopic gaps full of air, which is a poor conductor. It isn't meant to form a thick layer: the thinner and more even it is, the better it works.
Practical tips:
- an amount the size of a grain of rice or a small pea in the centre is enough for most desktop processors; for very large ones you can use a line or an X;
- the cooler's pressure spreads the paste by itself;
- before reapplying, remove the old paste with isopropyl alcohol and a lint-free cloth;
- many new coolers come with paste pre-applied: in that case don't add more.
Liquid metal compounds conduct much better, but they are electrically conductive and corrode aluminium. They aren't suitable for first-time builders.
Air and liquid coolers
| Type | Strengths | Limits |
|---|---|---|
| Tower air cooler | reliable, cheap, almost nothing can fail except the fan | bulky, may cover RAM slots |
| Low-profile air cooler | suits compact cases | limited capacity |
| AIO liquid (closed loop) | handles high loads well, frees space around the socket | pump can fail, rare risk of leaks |
For most processors a good tower air cooler is enough and easier to live with. With an AIO, the radiator should be mounted so that the pump isn't the highest point in the loop, otherwise air collects there and the pump becomes noisy.
Airflow in the case
Cool air should come in from the front or bottom and leave from the rear and top, also following hot air's natural tendency to rise.
- A common and sound configuration is two or three front fans as intake and one rear fan as exhaust, plus possibly one or two top exhaust fans.
- Slight positive pressure (more air in than out) means dust enters mainly through filtered openings.
- Fans have an arrow on the frame showing the airflow direction: check it when mounting them.
- A closed front panel, glass or solid, with few side vents, can greatly restrict airflow: it's one of the most important criteria when choosing a case.
Temperatures: what's normal
There are no universal values, because they depend on the model. As a guide:
- at idle, a desktop processor often sits between 30 and 50 °C;
- under sustained load, many modern processors deliberately run close to their limit and reduce clock speed when they reach it;
- graphics cards and NVMe SSDs have different thresholds, given by their manufacturers.
Behaviour matters more than absolute numbers: if performance drops after a few minutes of load and temperatures stay at the limit, the cooling isn't sufficient.
Fan curves
In the BIOS or with the motherboard software you can set curves linking fan speed to temperature. A gentle curve, with fans slow up to medium temperatures and a gradual increase, greatly reduces noise without compromising cooling. If the motherboard allows it, it's worth tying the case fans to the graphics card temperature too.
When the PC runs too hot
In my work, before replacing components, I always check in this order:
- dust on filters, cooler fins and fans;
- fans that are stopped, mounted backwards or wrongly connected;
- a cooler mounted crooked or with the protective film still on the base;
- dried-out thermal paste on computers several years old;
- cables obstructing airflow.
In short
- Heat follows a chain: every link can limit cooling.
- A little thermal paste, evenly spread, works better than a lot.
- A good air cooler is enough for most processors.
- Cool air in from the front and bottom, exhaust from the rear and top.
- Against overheating, start with dust, fans and mounting.
