Help & documentation

The idea

Standard furnace or HVAC filters are inexpensive, easy to find, and surprisingly effective. In countries where they are not standard, you can use whatever locally available filters offer the best value. Mount several of them in a box. Pull air through them all at once using a bank of computer fans. The result is a low-cost air cleaner that is extremely powerful yet very quiet. As the filter area is large, air passes easily through the media, eliminating the need for a large, powerful fan. This maintains a high capture rate and low noise level while ensuring a high total airflow.

Airflow beats peak rating: why MERV 13 can out-clean HEPA

How quickly a room is cleaned is its Clean Air Delivery Rate, or CADR. It is defined by the AHAM AC-1 standard. CADR is the capture rate multiplied by the airflow. So airflow matters just as much as the percentage the filter traps. True HEPA media captures 99.97% at 0.3 µm. But it is dense, so it restricts airflow and the fan moves less air. A MERV 13 filter lets far more air through for the same fan. In fact, denser, higher-rated filters do not always raise CADR, because the lost airflow can cancel the gain.
So, while a HEPA filter may capture more particles in a single pass, a MERV 13 filter driven by the same power fans can make many more passes in the same amount of time and therefore often achieve a cleaner result.

Why PC fans: quiet, controllable, and easy to maintain

The box has a large filter area, so each fan only has to overcome a little resistance. That is exactly where PC case fans excel. Good 120 or 140 mm PWM fans move 45 to 70 CFM at roughly 12 to 25 dBA. That is far quieter than a box fan at full speed, so more tolerable and less likely to be turned off or down by those sensitive to noise. 140 mm fans deliver a given airflow at a lower speed, so they are even quieter. Other advantages:

• Speed control: a 4-pin PWM fan lets you trade noise for airflow, or run it quietly overnight.
• Safe, low voltage: 12 V DC fans are simple to wire and stay cool.
• Cheap and repairable: fans are standardized and easy to swap. The filters are common furnace filters you replace on a schedule.
• Redundancy: if one fan fails, the rest keep running until you can replace it.

Replace the filters approximately every 6 to 12 months during standard use. Very heavy smoke, like from wildfires, can clog a MERV 13 within weeks. A clogged filter restricts airflow and lowers CADR, so timely changes keep the box performing.

Free, open, and yours to make

The FilterBoxBuilder lets anyone, or any group with access to a laser cutter or 3D printer make several different types of DIY air cleaners to suit nearly every situation. You use cheap, common parts. No account, purchase, or sign-up is required. We believe clean indoor air is a public good. The more people who can make a good purifier from local materials, the better. There is no catch: the builder produces complete, ready-to-make files for free.

If the builder saves you money or helps you breathe easier, you can support our work. Tell others about our products, or buy our kit below.

When a finished kit makes more sense: the Nukit Tempest Pro

A homemade box is great if you enjoy making things and have the tools. But maybe you would rather not source parts, cut, print, and adjust the fit. Or you simply want something that lasts for decades. Then consider our flagship product, the Nukit Tempest Pro. It uses the same PC-fan, MERV 13 principle, engineered into a finished, all-metal unit.

Choose a method at the top: 3D print or Laser cut. Then set your filter size, fan size, and fan placement. The 3D view updates as you work. The export button produces the files for your method: a 3MF print kit, or an SVG cut sheet.

You set the numbers, and the box is generated to match. So any filter and fan combination produces a box you can build. The sections below explain each control. Most are in the main panel, and a few are under the Advanced accordion that opens when you click it.

Filter layout

One side places a single filter on one face of the box, ideal for wall or ceiling mounting. Both sides puts a filter on two faces, for roughly twice the filter area and airflow in the same footprint. If you leave one side without fans or filters, you can put it against the wall to safe space. 3D Print also offers Four sides: a tower with a filter on each wall and fans on top. This design should have at least 150mm clearance from walls and furniture on all sides.

More filter area slows the air through the media. That raises the capture rate and lowers noise (see Why a PC-fan filter box).

Back fans (one-side box)

After choosing a one-side box for your filter layout, if you select Back for the fan placement with no wall fans, you can create a panel-style air purifier. This is a slim box that takes air in through the front filter panel and pushes it out through the array of fans on the back plate.

This design is the most portable and offers the most CADR for size, but may be a little louder than other designs due to its single filter and large number of fans. An open backed portable folding music stand is an ideal way to mount and use this style.

Box depth

On the one-side back-fan panel, Box depth sets how deep the box is. It is the gap between the filter and the back plate, used instead of the fan-diameter chamber. The default is 70 mm. A shallow box is more compact. A deeper box gives the air a larger plenum, which can even out the flow and prevent uneven filter loading. This setting only applies when Back fans are on and no wall fans are selected.

Filter size (width, length, thickness)

Choose a stock size from the menu, or enter your own. Filters are sold by a rounded nominal size, such as 20"×20"×1". But the actual size can be very different depending on the brand. So measure your filter, and enter the true width, length, and thickness in millimetres so the housing fits. Thicker filters with deeper pleats hold more media, so they capture more and clog more slowly. A 2" (about 50 mm) MERV 13 outperforms and outlasts a 1" filter at the same airflow. Use the swap button to exchange width and length, changing the box from portrait to landscape.

Fan size

This is the diameter of the PC fans you will use, commonly 120 or 140 mm. It sets each fan opening and screw pattern, and sizes the housing around them. 140 mm fans move the same air at a lower speed, so they tend to be quieter. 120 mm fans are easier to source and pack more densely. Match this to fans you can actually buy.

Box/Exhaust

On the four-side filter tower, Box/Exhaust (under Advanced) swaps the top panel's grid of PC-fan openings for a single central fan hole ringed with mounting screw holes. This is the classic single-fan "filter cube" arrangement: instead of many case fans, you fit one larger box or exhaust fan over the opening.

The fan hole diameter and up to two rings of screw holes are adjustable, so you can match a specific fan, blower, or duct flange.

When using a box fan, the fan hole diameter should be the same as the fan blade to form a shroud. Leave the fan hole diameter at 0 to let the builder automatically pick a sensible default from the filter width.

Place two holes in each corner using the first and second Ring settings, then pull nylon zip ties through the fan grill to secure a box fan to the top of the filter cube.

Cord hole diameter

This is the diameter of the DC power-jack body, cord grommet, or power-cord pass-through. Set it to fit whichever you decide to use. Set it to 0 for no hole at all. The builder keeps the bore clear of the fan openings. If it would land under a fan, the fan bank re-packs to make room, rather than moving the cord.

Cord corner offset

This is the distance from the corner to the cord hole. It is used when the cord is positioned toward one end of the wall and you want to give it a little extra space.

Print volume

This is your 3D printer's usable bed size. Check the manufacturer's website to be sure. Pick the volume that matches your machine. Any part that is too big for the bed is automatically split into chunks that fit, so a large box still prints on a small printer. The chunks glue together using short lengths of 3D printer filament as alignment pins.

Engrave part labels

This etches each part's name lightly onto the cut sheet, so you can tell the pieces apart while assembling. Turn it off for a clean cut. Engraving adds laser time, so place labels on the inside face if you want them hidden.

Reference scale

This draws a ruler of a known length on the sheet. You can then confirm your cut came out at 1:1 before committing material. After importing the SVG or DXF, measure the line. If it is off, fix your document units or printer scaling.

A. Laser-cut plywood (or acrylic)

Cutting

• Use flat, void-free plywood or cast acrylic. 3 mm is ideal, because thinner stock may warp and fight the joints. There are purpose-made materials (laserply, MDF) made specifically for laser cutting that should be used whenever possible.
• Measure the thickness with calipers and set the material thickness to the real value.
• Cut one test finger joint first. Tune the kerf / fit allowance until it taps together snugly without forcing.
• Keep Engrave labels on for your first build. Sand off any scorch with a light pass of fine sandpaper.

Dry-fit, then glue

• Assemble the whole box without glue first, to confirm every joint and the filter slot work.
• Glue plywood with wood PVA (such as Titebond), or acrylic with acrylic cement. Apply a thin bead inside the slots, not on the faces.
• Square the box against a flat surface, and clamp or tape it lightly. Wipe off any squeeze-out before it cures.
• If there is room, you can seal small gaps around the filter with self-adhesive foam weatherstrip. Then all the air passes through the media, not around it.
• If you choose to paint or seal the box, use low- or no-VOC products and allow substantial curing time.

B. 3D-printed with alignment pins

Printing

• Print in PLA or PETG. PETG tolerates heat and sunlight better for a unit left running, and tends to last longer.
• Walls of 2 to 3 mm (3 to 4 perimeters) are plenty, at the maximum layer height for your nozzle diameter. 10% infill is fine, since the box carries little load.
• Large parts are split into bed-sized chunks.
• The exporter auto-orients chunks to minimize supports. If you print hexagonal fan grills vertically, cleaning out the supports will be difficult.

Alignment pins & gluing

• Split faces meet on printed alignment pins and matching holes. Dry-fit them with pins inserted to confirm the chunks seat flush before gluing.
• If the alignment pin holes are too tight, you can clean them out with a hand-held 1.8 mm drill bit, or slightly increase the hole size in the Advanced section.
• Glue chunk seams with cyanoacrylate (super glue) for PLA and PETG. The pins hold registration while the glue sets.
• Test-fit the filter and fans before final gluing.
• Hint: Glue two pieces at a time, wait for that to cure, then glue those assemblies two at a time. Three or more seams in a row can be difficult to clamp without the whole thing buckling.

Mounting the fans

• Position fans to pull air through the filter and exhaust out the fan openings. Check the arrow on the fan hub or hold a small piece of tissue paper in front of the fan to see which way it it blowing
• Use the four fan-screw holes per fan. Self-tapping fan screws bite directly into the plastic body of the fan. Alternatively, you can use bolts, or pull-through rubber fasteners.
• Wire PWM fans to a 12 V supply or hub sized for the total current. Route the cable through the cord pass-through.
• Add grills to any exposed fan if you did not include the optional honeycomb feature.

What the license covers

This applies to the builder itself, the documentation on this page, and the output you generate with it: the cut sheets, DXF and SVG files, 3D models, and the boxes you make from them. You are free to use all of it, including for commercial purposes, with no account, fee, or permission needed.

What you can do

You can build boxes for yourself or for others, sell what you make, share the files, and remix or adapt the designs into something new. We encourage it. The whole point is to put clean air within reach of as many people as possible.

The two conditions

Attribution. Give credit to Nukit, link back to this builder, and note if you changed anything. A simple line such as "Based on the Nukit FilterBoxBuilder, licensed CC BY-SA 4.0" is enough.

ShareAlike. If you remix or build on the designs and share the result, release your version under the same CC BY-SA 4.0 license so others keep the same freedoms.