Introduction: $3 Computer CPU Intake Fan Duct

About: If there's one thing I've learned about being an adult, it is this: there's always another project. Over the years, I've tackled a ton of projects and built some cool stuff, and now I'd like to help people wh…

Having a intake duct straight from the side of your computer case into the CPU fan can give you much better cooling than any other (air) cooling option. Instead of using air taken in from a front port, which has time to warm up from other components, the duct sucks in fresh air from the outside environment. Having an opening in the case in line with the CPU fan helps, but the air from this method is at best a mixture of some fresh air and some already from the case. There are some commercial solutions that aren't too expensive (usually $10-$20), but what I've made is definitely the right price - only $3.

Having used a collapsible fan duct for a few months, I finally decided that it was too much of a pain in the neck. First I had to modify it out of the box to fit on the fan for my CPU heatsink, then I had to duct tape it in place, and it would sometimes grind against the fan when it ran - it was a pain. I took it out and finally built something I'd been knocking around my head for a while.

This project is designed to fit an 80mm side vent, which seems to be standard on most mid-range cases nowadays. It will work with larger (or smaller) sided vents, but obviously you'll have to modify your materials, calculations, and assembly for that.

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Step 1: Materials and Tools

Just a friendly reminder: Always know the proper use of any of your equipment - read any manuals and take all safety precautions. Safety glasses are recommended in just about any endeavor including tools, so if you think you need them, WEAR THEM. I am not responsible if you hurt yourself attempting this mod and I hope you don't. I managed to give myself a nice burn with my soldering iron doing another mod not long ago, which just goes to show you what can happen if you're not careful.

I used the following tools and materials in this build (there are other ways to do this, but this is what I had available to me):

  • tape measure
  • calculator with trig functions
  • Miter saw (can be replaced by a regular hand saw)
  • Drill
  • 5/32" drill bit
  • 3" Hole saw (can be replaced with a scroll saw or a square saw)
  • drill press (makes using the hole saw easier)
  • calipers (not necessary, but helps)
  • computer case with side intake vent close to CPU fan (if you're handy enough you can make yourself a vent)
  • 80mm by 80mm (3.15" x 3.15") piece of 1/4" plywood
  • 5" piece of 3" PVC piping
  • 4 regular case fan screws (or just two if that's all you can scrounge)
  • cyanoacrylate (superglue)
I say that this project only costs $3 because that's about what it cost me. All I had to buy was the PVC and the plywood. The PVC I had cut at my local hardware store (most, if not all, will cut to length for you) and the plywood was from a much larger sheet I used in a different project. Everything else I had on hand, so your costs may vary a bit. (Especially if you have to buy any of those tools ;-)

Step 2: Measurements

If you're lucky, your side intake vent lines up exactly with your CPU fan. If that's the case, you can skip the majority of the steps below. However, if you're unlucky like me, you'll have to put that trigonometry you learned in high school to the test (who would have thought you'd use that again?).

Begin by measuring the distance from the top of the case to the top part of the vent hole (measuring to the screw hole is good). Do the same for the side and write these measurements down. My original prototype was for example 4-5/8" from the top and 3-1/2" from the side.

Now remove the case door and measure the position of the CPU fan. Use the same method as above, except you'll have to eyeball it a little. It doesn't need to be incredibly accurate, just within an eighth of an inch or so. Mine were 5-7/8" (top) and 3-7/8" (side).

After this, determine how deep into your case the CPU fan rests. If you have a calipers, you can span a straightedge across the case opening and use the depth gauge to find it. If not, using the measuring tape and the straightedge works fine too. Mine was 3-1/2" deep.

Step 3: Calculations

Enter the mighty trigonometry functions.

First we need to figure out the displacement of the CPU fan when looking at it from above. In my example, the fan was 1-1/4" lower and 3/8" to the right of the vent hole. Plug this into the Pythagorean formula to get a displacement of 1.305" (about 1-5/16").

The number we just found is crucial for the next step. We need to figure out how long to cut the PVC and at what angle. We know the CPU fan sits 3-1/2" into the case, but we can subtract 1/4" because of the plywood piece that mounts the PVC. That leaves us with 3-1/4" deep and skewed 1-5/16" laterally. The Pythagorean formula gives us a total length of 3-1/2" for the PVC.

The angle can be figured out using tangent. With the opposite and adjacent sides known, one can easily figure out the angle theta in the third diagram. However, the saw must be set to cut at an angle of 22 degrees, not the 68 we found (90 - 68 = 22).

Step 4: Cut the Mounting Plate

If you haven't already, cut your 1/4" plywood into a 80mm by 80mm section (that's a little more than 3-1/8" on a side). Now you can use your case panel to place the screw holes. Simply put the mounting plate on a flat surface (that you don't mind getting drilled into) and put the panel on top of it, lining up the mounting plate with the vent opening. Hold it down tight to keep it from moving and use the drill and 5/32" bit to drill the screw holes.

After this it is time to cut the hole for the air to flow through. It just so happens that the hole for the fan in a standard 80mm fan is exactly 3" diameter, so a 3" hole sawing bit is just the thing. It's easiest if you mount the bit on a drill press and clamp the mounting plate to the bench. Or, if you can't find your hole saw like me, use a scroll saw to cut the hole.

Step 5: Cut the PVC

Now the tricky cutting. PVC, being round, will be somewhat problematic to keep steady while using the miter saw. The best idea is to take a piece of sacrificial lumber about 2" high and use that behind the PVC to keep it flush to the guide. Set your saw to the right angle (22 degrees in my case) and cut the first end.

Alas, here's the tricky part. You not only have to keep the PVC from rolling (and thus messing up your angles), but you also have to get it right length. I measured straight down the length of the PVC 3-1/2" and lined up the blade before I cut. It never hurts to double check the measurements before you cut. But if you do screw up, at least the PVC was (hopefully) not even $2 for the 5" length.

Step 6: Dry Fit

Now you have the two pieces cut and it's time to get them lined up correctly. There is no real *easy* way to do this. What I did is temporarily mount the plate on the case panel and then get the PVC situated right. Just place the PVC on top of the CPU fan, close the panel, and use a skinny screwdriver or something to get it positioned right.

Once you're sure the PVC is right, carefully remove the panel (as not to move the PVC) and take the mounting plate off. Now place the plate on top of the PVC and eyeball it square. Carefully replace the case panel and get the mounting plate as close as you can. Remove the panel carefully again once you're satisfied.

With the plywood still on the PVC, take a permanent marker and make some registration marks on the piece. You'll need these to line it up again after gluing.

Step 7: Assembly & Mounting

With the permanent marker marks on, it's time to get your favorite superglue out and put it together. Put a line of glue around the rim of the PVC where it contacts the plywood and press the two together. Hold it for about 30 seconds and it'll be bonded. If you want, add some more glue to seal up anywhere on the seam where air might leak.

Give the glue some time to cure (30 minutes should be plenty, just to be safe). You're now ready to put it on the case. When screwing the fan screws into the plywood, you just want to get them snug; don't overtighten the screws or you'll strip the plywood and lose all the grip from the screws.

Put the panel back on and fire the computer up. Put your hand to the vent hole and you should feel a good air flow into the case. Enjoy a better cooled CPU. Next step (when we all get the money for it) is water cooling :-)