Laptop hot and loud? What the fan is telling you, and what to do about it

A laptop does not usually announce that it is overheating. It gets a little louder every month, a little hotter on the lap, a little slower when you ask it to do two things at once, until one day the fan roars at a web page and you notice. By then it has usually been running hot for a long time.

The good news is that heat is one of the most fixable faults a laptop has. The bad news is that it does real damage if you leave it, and one version of it is a safety problem rather than a nuisance. This guide is about telling the versions apart from the outside, without tools, and knowing which one you have before you decide what to do.

Why heat matters more than it seems to

A laptop is a small box with a lot of electronics in it and one route for the heat to leave: air pulled in through the underside, pushed across a set of metal fins, and blown out of a vent at the side or the back. Everything about that route is marginal by design. There is not much air, the fins are small, and the fan is the size of a biscuit.

When the route works, the processor runs at full speed and the fan spins up and down with the work. When it stops working, two things happen. First, the machine protects itself by slowing down, which is why a hot laptop is also a slow laptop. Second, the components run hotter than they were designed to for hours at a time, and that shortens their life quietly, over years. Heat does not break a machine on the day it gets hot. The bill arrives later, as a board fault or a battery that no longer holds a charge.

So a fan that roars at a web page is not a loud fan. It is a fan doing its job against cooling that no longer works.

The four causes

Nearly every hot laptop we see comes down to one of four things, and they are worth keeping apart, because the fix for one does nothing for another.

1. Dust in the cooling

The commonest cause by a long way. The fan pulls air through the machine constantly, and the air carries dust, hair, fibres from clothes and bedding, and whatever else is in the room. Most of it passes through. Some of it lodges on the inside face of the cooling fins, and over a few years it builds into a grey felt mat that the air can no longer get through. The fan spins faster and faster against a blocked exit, which is the roar you hear, and the machine still cannot shed its heat.

From the outside, the signs are a fan that runs loud and almost constantly, warm air that feels weak at the vent, and sometimes a vent that looks furred up when you shine a torch into it. The fix is a clean, not a new part. A proper clean means the base off and the cooler out, because the mat is on the inside of the fins where no can of air can reach it.

2. Tired thermal paste

Between the processor and the metal block that carries its heat away there is a thin layer of paste. Its job is to fill the microscopic gaps between two surfaces that look flat but are not, so heat crosses from the chip to the cooler. Over years the paste dries, shrinks and cracks, and the chip can no longer hand its heat on fast enough. The fan works harder for less, and the machine throttles sooner.

From the outside this looks almost identical to dust, which is why the two are usually dealt with together: once the cooler is out for cleaning, replacing the paste is a few minutes more. If a laptop has been cleaned and still runs hot and slow under load, old paste is the usual reason.

3. Something working the machine harder than you think

Not every hot laptop has a hardware problem. A program running flat out in the background will drive the processor to full speed, the fan to full noise and the case to full heat, with nothing wrong in the cooling at all. The usual culprits are a browser with too many tabs open, a stuck update, a sync service indexing a large folder, a game or a video tool left running, and occasionally something unwanted that has found its way on.

The tell is that the heat comes and goes with what you are doing, and that the machine settles down after a restart. On Windows, Task Manager (right-click the taskbar) shows what is using the processor; on a Mac, Activity Monitor does the same. If one thing sits at the top using most of the processor while you are doing nothing, that is your cause. If the machine is hot and loud with nothing much running, look to the other three.

4. A swollen battery

This is the one that matters. A battery that is ageing badly can start to puff up inside the case. People usually notice the symptoms before the cause: the laptop no longer sits flat on the desk, the trackpad has lifted or has become hard to click, the case has a bulge, or a gap has opened along a seam. A swelling battery also runs warm, so the machine can feel hot in a place that has nothing to do with the vent.

A swollen battery is not a battery job you wait on. Stop using the laptop, do not press on the bulge, do not charge it, and get it looked at. On most models the battery is a straightforward replacement, and the rest of the machine is usually fine. What you do not do is carry on for another month because it still works.

How to tell them apart without tools

None of these checks cost anything and none need the base off.

1. Listen and feel. A fan that roars constantly, with weak warm air at the vent, points at dust. A fan that roars only when you do something heavy, and settles afterwards, points at a program.
2. Look at what is running. Task Manager or Activity Monitor, sorted by processor. One thing at the top while the machine is idle is cause three.
3. Look at the case. Set the laptop on a flat table. If it rocks, if the trackpad has lifted, if there is a bulge, that is cause four, and it is the one you stop for.
4. Shine a torch into the vent. Grey felt across the fins is cause one, and it is a clean.
5. Think about age and use. A laptop a few years old that lives on soft furnishings has almost certainly got dust and ageing paste, whatever else is going on.

What a proper clean actually involves

People sometimes expect a clean to mean a blast of compressed air. It is not that. A proper clean means taking the base off, disconnecting the battery before anything else is touched, removing the cooler, clearing the fins and the fan, replacing the thermal paste, checking the fan spins freely and that the battery is not swelling, and putting it back together. It is unglamorous work and it is some of the best-value work there is, because a machine that was crawling and roaring is often transformed by it, with no new parts at all.

If a part is needed, it is usually a fan whose bearing has worn (you hear it as a grind or a rattle rather than a roar) or a battery. Both are routine on most laptops.

Things that help a little, and one that does not

Keeping the underside clear helps, because most laptops breathe through it. A hard table beats a duvet; a stand or even a book under the back edge beats neither. A cooling pad helps for the same reason and no other: it cannot push air through blocked fins, and it does nothing for paste or a battery.

Compressed air from outside, with the machine off and the fan held still, will shift loose dust from the vent. It will not remove the mat on the inside of the fins, which is the dust that matters, and if you spin the fan with the air you can damage its bearing. Use it as a stopgap, not a fix.

The thing that does not help is ignoring it. A hot laptop is telling you something, and all four of the things it might be telling you are better dealt with early.

When to get it looked at

If the laptop is hot and loud most of the time, if it slows down when it gets warm, if it has cut out under load, or if anything about the case has changed shape, it is time. Tell us what it is doing and when, and whether the fan is loud or silent. We will tell you what it sounds like and what is likely to be involved before anyone travels. If it is a clean and fresh paste, you will have the machine back feeling like a different laptop. If it is a battery, you will have stopped using it at the right moment.