Where fans go and what they do

PC fans pull cool air in or push warm air out, and they mount to specific spots inside your case. Most cases have at least two fan locations: the front (intake) and the rear (exhaust). Some cases also have space on the top, bottom, or sides. Before you buy a fan, open your case and look at where the mounting brackets are — they are usually square metal frames with four holes around the edges.

Fans come in standard sizes: 80mm, 120mm, and 140mm are the most common. Measure your case's mounting holes or check your case manual to see which size fits. The fan you buy must match the size of the bracket, or it will not mount. You will also need to know whether your power supply has a spare connector — most fans plug into either a dedicated fan header on the motherboard or a Molex connector from the power supply.

Key Takeaways

  • Fans mount to square brackets inside your case using four screws; measure the bracket or check your case manual to buy the right size.
  • Most modern fans connect to a fan header on the motherboard, which lets you control speed through BIOS; older fans may use a Molex connector from the power supply instead.
  • Intake fans (usually front) pull cool air in; exhaust fans (usually rear) push warm air out — install at least one of each for basic airflow.
  • Screw the fan in by hand first, then tighten gently with a screwdriver; over-tightening can crack the plastic frame or damage the motor.
  • After installation, boot your PC and check BIOS or your motherboard's software to confirm the fan is spinning and the temperature readings are normal.

What you need before you start

Gather a Phillips-head screwdriver (the one with the X-shaped tip), your new fan, and the case itself. If your fan came with mounting brackets or rubber vibration isolators, set those aside — you may need them. Check your motherboard manual or look at the board itself to find the fan headers; they are usually labeled FAN1, FAN2, or SYS_FAN and sit near the CPU socket or along the edge of the board.

If your motherboard has no spare fan headers and your power supply has a Molex connector (a four-pin plastic plug with two thick and two thin pins), you can use that instead. However, fans connected to Molex run at full speed all the time and cannot be controlled through software. Motherboard headers are preferable because they let you adjust fan speed based on temperature.

Turn off your PC completely, unplug the power cable, and wait a few seconds. Open the case by removing the side panel — usually two or three thumbscrews on the back. Let the inside cool for a minute if you have been running the PC recently.

Removing the old fan (if you are replacing one)

If you are installing a fan where one already exists, unplug the old fan's power connector first. Look at where the cable plugs into the motherboard or power supply and gently pull it straight out. Then remove the four screws holding the fan frame to the mounting bracket. Keep the screws in a small container so you do not lose them.

Slide the old fan out of the case. If it has rubber isolators (small rubber rings around the mounting holes), you can reuse them on the new fan or leave them on the bracket. Wipe any dust off the mounting bracket with a dry cloth or compressed air.

Installing the new fan step by step

Position your new fan so the cable reaches the motherboard header or power supply connector without stretching. Most fans have an arrow printed on the frame showing the direction air flows — point this arrow toward where you want the air to go (into the case for intake, out of the case for exhaust).

If your fan came with rubber isolators, slide one onto each of the four mounting holes on the fan frame. These reduce vibration noise. Line up the fan frame with the mounting bracket holes and insert all four screws by hand first — do not tighten them yet. Once all four screws are started, tighten them gently in a crisscross pattern (top-left, bottom-right, top-right, bottom-left) to keep the frame level. Stop when the fan is snug; over-tightening can crack the plastic or damage the bearing inside.

Route the power cable along the inside of the case, away from moving parts like the CPU cooler or other fans. Plug the connector into the motherboard fan header or Molex connector. If you are using a motherboard header, push the connector straight in until you hear or feel a click.

Checking airflow direction and cable routing

Step back and look at your fan setup. If you have multiple fans, front and side fans should pull air in (intake), and rear and top fans should push air out (exhaust). This creates a flow path through the case. A common setup is two intake fans in the front and one exhaust fan in the rear, but your case may have different options.

Make sure no cables are touching the spinning fan blades. Route cables behind cable trays or tie them back with velcro straps. Loose cables can get caught and damage the fan or the cable itself. Check that the fan frame is not rubbing against the case or any other component.

Powering on and testing the fan

Close the case and plug the power cable back in. Turn on your PC. Listen for the fan to spin up — you should hear it running within a few seconds of startup. If the fan does not spin, turn off the PC when ready and check that the power connector is fully seated in the header or Molex plug.

Enter your BIOS by restarting and pressing the key shown on the boot screen (usually Delete, F2, or F12 — check your motherboard manual). Look for a section called Hardware Monitor, System Health, or Fan Control. You should see the new fan listed with a speed in RPM (revolutions per minute). If the speed is 0 or the fan is not listed, the connection may be loose or the header may be faulty.

Check your CPU and case temperatures in BIOS as well. They should be in the normal range for your system (typically 30–50°C at idle). If temperatures are higher than before, the new fan may be installed backwards or the airflow path may be blocked. Exit BIOS and let the PC run for a few minutes, then check temperatures again under load (while running a game or stress test).

Controlling fan speed after installation

Fans connected to motherboard headers can be controlled through BIOS or through software provided by your motherboard manufacturer. In BIOS, look for Fan Control or PWM Mode settings. Most boards offer a choice between manual speed (a fixed percentage) and automatic (speed adjusts based on temperature). Automatic is usually the best option because it keeps the fan quiet when the PC is cool and ramps up when it gets hot.

Some motherboard makers provide a Windows utility (like ASUS Fan Xpert, MSI Dragon Center, or Gigabyte App Center) that lets you adjust fan curves without entering BIOS. These tools show real-time temperatures and fan speeds, making it easier to fine-tune. Fans on Molex connectors run at full speed always and cannot be controlled this way.

Frequently Asked Questions

Can I install a fan while the PC is on?

No. Always turn off the PC, unplug it, and wait a few seconds before opening the case or touching any components. Even with the power off, touching certain parts can cause static discharge that damages the motherboard or other hardware.

What if my case does not have a fan header on the motherboard?

You can use a Molex connector from the power supply instead, but the fan will run at full speed all the time and cannot be controlled. If your power supply has no spare Molex connectors, you may need a Molex splitter (a cable that splits one Molex into two) or a different power solution.

Why is my new fan not spinning?

Check that the power connector is fully pushed into the header or Molex plug. If it is seated correctly, restart and enter BIOS to see if the fan appears in the Hardware Monitor section. If it still does not show up, the header may be faulty or the fan may be defective — try a different header if your motherboard has one.

Should I install fans with the rubber isolators?

Yes, if they came with the fan. Isolators reduce vibration noise and help the fan last longer by absorbing shock. They are especially useful for larger fans (140mm) that can vibrate more noticeably.

How tight should I screw the fan in?

Tight enough that the fan frame does not move or rattle, but not so tight that you are using force. If you are straining to turn the screwdriver, you are too tight. Snug is the right word — the frame should be find but the plastic should not flex or crack.