Building a PC-fan Corsi-Rosenthal box

A quiet, low-power clean-air box you can build with simple hand tools. This guide covers two build tiers — a fast, tool-light volunteer build and a more durable multi-season build — plus the construction details that make the fans secure, sealed, and reliable.

What it is

Four MERV-13 furnace filters stand vertically to form the four walls of a box. A solid bottom seals the base. A top panel carries five quiet computer fans — one in the center and four toward the corners — that pull room air in through all four filters and push clean air out the top. Because air passes slowly through a large filter area, it cleans effectively while staying quiet, and computer fans use a fraction of a box fan's power.

Parts (this build)

Also: utility knife, straightedge, marker, a ~115 mm circle template, and — for the durable tier — M4 screws/washers and a drill or hole saw. The fans' airflow direction is marked by small arrows on the frame.

Choose your build tier

The construction details that matter

Size the holes to the air opening, not the fan body

Cut each fan hole to the fan's open aperture (~115 mm), not the full 120 mm frame. That leaves a few millimeters of panel all around as a landing surface for the fan to seat and seal against. A full-size hole gives you nothing to seal to. Use a template so every hole is identical — it's the biggest speed-and-consistency win for group builds.

Put the gasket on the underside, where the air force seats it

Each fan throws air upward, so by reaction the fan is pushed down onto the panel whenever it runs. Put your foam gasket on that bottom interface (fan frame against the panel, around the hole) — normal operation keeps it compressed and sealed. Mount fans with the airflow arrow pointing up/out.

Sealing here isn't cosmetic: the lowest-pressure point in the box is right under each fan, so any gap lets room air get pulled straight down past the fan, bypassing the filters. A good fan-to-panel seal forces the fan to draw only air that came through the four MERV-13 walls.

Use double-sided closed-cell foam ring gaskets

Cut square-ring gaskets ~120 mm outer / ~115 mm inner from closed-cell foam gasket sheet with adhesive on both faces (open-cell sponge leaks air). Buy pre-made or punch your own at scale with a die. A peel-and-stick gasket both seals and tacks each fan in position during assembly — the mounting (below) does the structural holding, so the adhesive never has to survive transport on its own. Tip: pre-apply gaskets to the bottom sheet's holes as a kitting step so build-day is just peel-place-fan-cap.

Hold the fans down — two ways

Tier A — the gasketed two-sheet sandwich. Cut two sheets. Both get the five ~115 mm holes. Fans sit on the bottom sheet (on their gaskets, arrows up); the top sheet's hole rims (smaller than the 120 mm bodies) trap the fan frames so they can't lift, drop, or slide. Bond the two sheets together at the perimeter AND in the alleys between the fans — not just the edges. That cross-bonding turns the two thin sheets into a stiff sandwich panel and keeps both surfaces flat (it's what prevents the center from bowing when only the edges are joined). Zip ties between fans are optional extra clamping.

Tier B — bolt to a rigid panel. With 3 mm HDF or plywood you can skip the sandwich: seat each fan on its gasket and drive four M4 screws (with fender washers) through the panel into the fan's corner holes. Most rugged, fully serviceable, ideal for boxes that travel and get reused.

Wire it so the connections are protected

1. Daisy-chain the five fans with the PST connectors / Y-splitter. Lay the cabling flat against the inside face so it can't sag into the airflow or touch the blades.
2. Connect the chain to the adapter's +12 V and ground; leave the PWM/control wire unconnected (the fans then run at a speed set by the supply voltage). Set the adapter to 12 V to start; you can lower it (down to ~5 V) to run quieter/cheaper.
3. Bring the single power lead out through a small notch at one corner with a strain relief (a zip tie or knot anchored to the sheet) so tugging the cord never stresses the fan connectors. Insulate the connection (tape or heat-shrink).

Make it reusable: the registration lip

For boxes you'll refurbish across seasons, add a locating lip so the top and bottom register precisely to the filter cube — this holds the filters square during taping and makes re-assembly and re-sealing repeatable.

• Cut the bottom sheet (and the top panel) to the outer footprint of the four joined filters, so edges align for taping.
• On the inner face of each, add a raised lip — a laminated square (or four strips) sized to the filters' inner opening minus ~2 mm clearance — that the filter cube drops around. (Measure your actual joined-filter inner opening; corner joinery varies.)
• The filters slide over the lips and hold square; you tape the outer perimeter seams to seal. At a filter change, untape, lift the top, swap filters, re-register on the lips, and re-tape.

Assemble, power on, place

1. Build the top (and base) per your tier, with gaskets, fans arrows-up, wiring strain-relieved, all penetrations sealed.
2. Stand the four filters on the base (over the lip if used), arrows pointing IN; tape the vertical corners.
3. Set the top on (over its lip), fans blowing up; tape the top and bottom perimeters.
4. Seal every seam. The only air paths should be in through the filters and out through the fans.
5. Plug in. You should feel clean air out the top and hear very little. Place it where it can pull from the room; run it continuously during smoke events; dial the voltage down for quiet at night.

Running cost & upkeep

Running 24/7, a unit like this costs on the order of pennies a day at Maine electricity rates — far less than an old box-fan build. Replace filters when gray/loaded or each season (electret charge fades); a quick vacuum of the filter faces between changes helps. Keep seams sealed; with the durable tier, the fan-top and base are reusable across many filter changes.

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