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  • dumb question regarding fan for desktop computer

    Noticed that the existing fan on my computer takes a little time to 'get up to speed' when the computer is starting up, so decided to replace the fan. Easy to do, four screws and the power, no problems.

    Dumb question... which way is the fan supposed to face? Is it supposed to suck air in through the back (and I'd feel the air on the inside of the tower) or is it supposed to blow air out the back (and I'd feel the air on the outside of the tower)?

    And no, I didn't see any little arrows on the fan itself, and there were no instructions with the replacement fan, and I didn't bother to check when I took the old one out because that would have been clever and I thought it would have only installed one way...
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  • #2
    First of all, there should be 2 arrows moulded into the fan housing (they're small, and sometimes hard to see). One will show the direction the fan rotates, the other will show the direction the air flows. If you can't find these arrows, look at the fan blades - they're curved. The hollow side of the blade is on the side the air blows out of.

    As to the direction of airflow on the old fan, take a close look at the opening where it was mounted. Is more dust built up on the fan side, or on the other side? Dust tends to build up on the "upstream" side - if the fan is blowing air out of the case, dust will build up on the fan side of the opening (assuming the fan is mounted on the inside of the opening, where there's more space). A lot of times, a fan will be installed using self-tapping screws into the plastic of the fan housing - look at the screw holes in the old fan to see which side the screws cut their threads into.

    Even with this information, the original fan may have been mounted the wrong way. Fans don't cool the computer directly, they cause air to flow through the computer, with the air entering being cooler than the air leaving. There will be at least one other fan (in the power supply) in the case. Does this fan suck air into the case or blow it out of the case? Are there any major heat generators near where the fan in question was mounted? Based on the airflow of other fans in the case, would air flow be better if the fan in question were to draw air into the case, or blow it out of the case?

    Also, do you have the right kind of fan? Not only are there different sizes, but there are 2-wire fans (generally come with a connector that plugs into one of the disk drive power plugs from the power supply) and 3-wire fans (3rd wire is a rotation speed sense) that plug onto a stake header on the motherboard.
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    • #3
      I've found that the location of the fan will tell you how it should be mounted. If it's on the back, it should blow out. If it's on the front by the hard drives, it will normally be pulling air in. This is not always the case, but just something to start with.
      I once had a Dell that would pull the cool air in over the CPU and then push the hot air into the rest of the computer. This model got a reputation of burning out fast because of it. Dell put out a tech update that said that the fans should be reversed to stop the heat from causing breakdowns.

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      • #4
        Fans are meant force airflow through the case so a good general rule to follow is to keep the fans all facing the same direction. So if a front fan is forcing air to the back then the fan at the back of the case should be facing the same direction to keep the airflow going in the same direction.

        Now as for figuring out which way they point, I have noticed that the arrows being on the fan a hit or miss scenario. In a worse case scenario you can mount it, turn on the computer and hold a piece of tissue paper up to the vent so it hangs loosely. If it bends away you're good. If it bends toward, shut down the system and turn it around.

        It's not like a CPU heatsink where it will critical alert almost instantly. You can run your computer for a few minutes to figure out if it's right or not.
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        • #5
          I'll add my two cents to this.

          If you still have the old fan, and can remember how it was mounted, compare the new fan with the old and duplicate the installation. As has been mentioned before in the thread, the 'concave' side of the fan blades is the 'exhaust' side of the fan. In most instances, the 'motor' side of the fan is on the 'exhaust' side.

          Failing that, these are the general rules:

          Fans located on the back of the power supply unit should blow outward. Fans located on the bottom of the PSU should draw air from the system case INTO the PSU. Fans at the front or side of the system enclosure should draw air INWARD. Fans directly cooling a CPU or GPU chip should blow air ONTO the chip's heat sink. If your case has a top-mounted fan, it should blow air 'upward'.

          In most 'tower' cases, the preferred air path front-to-back, and bottom-to-top.

          You can use a two-wire fan to replace a 3-wire fan, though it means the fan will turn at full speed all the time if you do. In some instances you will have to cut the connector off and replace it with the connector from the old fan; you just ignore the yellow wire if the replacement fan doesn't have one.

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          • #6
            What they said -- But perhaps simplified a little

            If there's only one fan, put it on the back and it should blow OUT. If you can't ID this based on the arrows or the blades, the side with the label is "outflow" on most fans. If unsure, plug it in but don't actually attach it to the case. Figure out which way from the actual airflow, shut down the computer, and finish the install. If there are multiple locations, higher is better -- you're getting rid of heat, after all

            Multiple fans -- Front fans should blow IN, others (top/bottom/back) generally out. NOTE that the total amount of air blowing should be as close to equal as possible -- e.g., 2500 in, 2500 out if you can swing it.
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            • #7
              This is all good basic advice.

              There is however a school of thought that having more air blown in than out is a good thing, for reasons of dust exclusion. This is known as "positive pressure" airflow. This way, all the incoming air is concentrated at the intake fans, which can be fitted with dust filters. Any leaks in the case will tend to leak air outward, rather than sucking unfiltered air in.

              In "positive pressure" airflow, it remains important to keep the air moving in a consistent direction. So that means intake fans at the front and/or side, and exhaust fans at the back and/or top. The only difference is in the relative number of each, and the provision of dust filters.

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              • #8
                Mounted at back of computer, blowing air out successfully.

                Thanks to everyone for all the information!
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