The CR Box: a ~$70 DIY Air Purifier That Actually Works

Why this odd cube exists

During the 2020 smoke seasons, engineer Richard Corsi and filter-maker Jim Rosenthal popularized a simple idea: a box fan pulling air through furnace filters cleans a lot of air for very little money. The U.S. EPA later put DIY builds in a smoke chamber and measured it — the multi-filter designs performed comparably to commercial purifiers for smoke-sized particles, with no fan-motor fire concern in their testing.

That’s the whole pitch: not a hack, a measured result.

The shopping list (~$60–90)

• One 20-inch box fan — any reputable brand with a UL/ETL mark (~$25–35).
• Four 20x20x2 MERV-13 furnace filters (~$60–80 per 4-pack). MERV-13 is the spec that matters; 1-inch filters work, 2-inch flow better and last longer.
• Duct tape or painter’s tape, and the cardboard from the fan box.

Build it (20 minutes)

1. Check the arrows. Every filter has an airflow arrow on the frame. All four arrows must point inward — toward the center of the cube. The fan pulls air through the filters and blows clean air out the top.
2. Tape the four filters into a square tube, arrows in, edges flush.
3. Cap the bottom with the cardboard square from the fan’s box, taped to seal.
4. Set the fan on top, blowing up. Tape it to the filter edges so air can’t sneak in around the corners — leaks are the main performance thief.
5. Optional but worthwhile: cut a cardboard ring (“fan shroud”) that covers the flat corners of the fan face, leaving only the round blade opening. This blocks backflow at the corners and measurably improves output.

Run it on the highest speed the room tolerates; medium overnight. In a smoke event, park it in the room where people actually spend time and keep the door closed for a clean room (the broader plan is here).

Care and feeding

• Filter life: during heavy smoke, expect weeks, not months. When the filter faces turn visibly gray-brown, or airflow noticeably drops, swap them. Under occasional use, 3–6 months is typical.
• Between smoke events, run it periodically or store it; it also moonlights as a dust/pollen cleaner.
• Safety: modern (2012+) UL/ETL-marked box fans include a thermal fuse, and both EPA and UL testing found no concerning temperature rise with filters attached. Standard fan rules still apply — hard surface, clear of fabric.

CR box vs. buying a purifier

	CR box	Commercial HEPA
Cost up front	~$60–90	$150–300
Clean air delivered	Comparable (chamber-tested)	Rated CADR
Noise	Loud-ish, fan-like	Engineered quiet modes
Looks	A cube of furnace filters	Furniture-adjacent
Auto/sensor modes	You	Often built in

If the budget allows and the room is a bedroom, a quiet sensor-driven unit is a real quality-of-life upgrade — here’s how to size one honestly. If you need a lot of clean air this week for not much money, build the box. Check whether smoke is actually coming with our free AirAware tool first.