Corsi-Rosenthal Box Guide

A Corsi-Rosenthal box is a useful DIY option for filtering smoke particles when a commercial purifier is unavailable or unaffordable, provided you can accommodate the build and maintain its filters. Our choice is the documented four-filter layout: MERV 13 side walls, a solid cardboard base, an upward-blowing box fan and an outlet shroud. Choose the Blueair Blue Pure 211i Max instead if you want Auto and Night modes and a documented filter-change procedure without rebuilding taped joints. EPA's DIY guidance explains the evidence and operating requirements.

Quick picks and parts list

These are component choices for the documented layout, not a claim that this exact combination has a measured CADR.

Compare specifications and all conditions

Fan component

Our choice

Air King 9723, US 20-inch box fan

Quantity

One

Check before buying

ETL marking, newer unit, matching manual and grounded outlet

Filter components

Our choice

Nordic Pure 20x20x1, standard MERV 13, four-pack, SKU 20x20x1M13-4

Quantity

Four identical filters

Check before buying

Select MERV 13 and four per box; measure actual frames

Base and shroud

Our choice

Cardboard, including suitable fan packaging

Quantity

Base plus outlet shroud

Check before buying

Trace the assembled parts before cutting

Seals and tools

Our choice

Duct tape; scissors or utility knife

Quantity

Enough to seal every joint

Check before buying

Include fan-to-box gaps; optional weather-stripping can help seal connections

Ready-made alternative

Our choice

Blueair Blue Pure 211i Max

Quantity

One appliance

Check before buying

Choose the purifier rather than a replacement-filter listing

Full comparison table ↔

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Job	Our choice	Quantity	Check before buying
Fan component	Air King 9723, US 20-inch box fan	One	ETL marking, newer unit, matching manual and grounded outlet
Filter components	Nordic Pure 20x20x1, standard MERV 13, four-pack, SKU 20x20x1M13-4	Four identical filters	Select MERV 13 and four per box; measure actual frames
Base and shroud	Cardboard, including suitable fan packaging	Base plus outlet shroud	Trace the assembled parts before cutting
Seals and tools	Duct tape; scissors or utility knife	Enough to seal every joint	Include fan-to-box gaps; optional weather-stripping can help seal connections
Ready-made alternative	Blueair Blue Pure 211i Max	One appliance	Choose the purifier rather than a replacement-filter listing

Job	Our choice	Quantity	Check before buying
Fan component	Air King 9723, US 20-inch box fan	One	ETL marking, newer unit, matching manual and grounded outlet
Filter components	Nordic Pure 20x20x1, standard MERV 13, four-pack, SKU 20x20x1M13-4	Four identical filters	Select MERV 13 and four per box; measure actual frames
Base and shroud	Cardboard, including suitable fan packaging	Base plus outlet shroud	Trace the assembled parts before cutting
Seals and tools	Duct tape; scissors or utility knife	Enough to seal every joint	Include fan-to-box gaps; optional weather-stripping can help seal connections
Ready-made alternative	Blueair Blue Pure 211i Max	One appliance	Choose the purifier rather than a replacement-filter listing

The materials and airflow layout follow NOAA's four-filter instructions, supplemented by EPA's shroud guidance and Harvard's sealing advice. Keep replacement filters available before smoke arrives, as EPA recommends.

Choose the four-filter version before ordering

The selected design uses four nominal 20 × 20 × 1-inch MERV 13 filters as vertical walls, with cardboard underneath and the fan above. NOAA documents the solid-base layout; EPA's research page includes a four-filter, 1-inch MERV 13 configuration with a cardboard shroud. This combines documented construction guidance with current component choices; it does not reproduce EPA's tested hardware.

Dimensions need a dry fit. Nordic Pure's separate size FAQ gives its nominal 20x20x1 frame as 19½ × 19½ × ¾ inches. The current product selector leaves its actual-size field blank, and the maker's measuring guidance says to verify dimensions. Measure the delivered filters before cutting cardboard.

With those stated frame dimensions, arranging each panel end against the next panel's face makes a calculated 20¼-inch square filter enclosure, with walls 19½ inches high. The Air King specification sheet lists the fan as 21.38 × 6.38 × 22.5 inches. Turned face-up, its published overall dimensions give a planning footprint of 22.5 × 21.38 inches, and a conservative height allowance of 19½ + 6.38 inches, plus cardboard and tape. These are planning calculations, not measured finished dimensions; check projections, support and stability on the actual fan.

Fan component: Air King 9723

The 9723 has a 20-inch blade, three-speed rotary control and ETL listing, according to its official specifications. Its linked operating manual describes a fused Blue Plug and a three-prong grounded plug. Buy a new unit and verify its marking and date information: EPA's guidance calls for a 2012-or-later fan with UL or ETL marking. An old fan with the same model name does not establish that requirement.

There are two important documentation limits. Air King's product page markets the fan for commercial spaces, while its linked manual says it is not intended for commercial, industrial or agricultural settings. Follow the manual's narrower use; this guide covers a home build. The manual also says mounting other than specified voids the manufacturer's warranty, so consider the DIY mounting a material tradeoff.

The 9701 is a separately sold stand, not the fan. Order the 9723 itself. Neither the fan's airflow specification nor its ETL listing supplies a CADR or certification for the assembled cleaner.

Filter components: Nordic Pure MERV 13 four-pack

Choose 20x20x1M13-4, the standard MERV 13 pleated four-pack. Nordic Pure's official variant data confirms that SKU and pack quantity. Its selector also offers other ratings and media: “MERV 13 + Carbon” and “MERV 13 Tru Mini Pleat” are different selections. Keep this parts list consistent rather than assuming all variants reproduce the same airflow.

A four-pack supplies the walls, with no spare set. Other pack quantities are offered, but count filters before ordering. Nordic Pure's FAQ explains that individual frames may lack a printed MERV rating; check the box's rating sticker and order details. Locate each airflow arrow before assembly. For this box, arrows point toward the empty center, not toward the room.

Assembly: arrows, base, seals, fan and shroud

Work with the fan unplugged. NOAA specifies inward filter arrows, upward fan discharge, a cardboard bottom and sealed fan-to-box gaps. Harvard explains why tight joints matter: air should pass through the filters instead of bypassing them.

1. 1

   Dry-fit the walls

   Stand the matching filters upright in a square. Point every airflow arrow inward. Check the fan's fit before committing to tape.

2. 2

   Seal the enclosure

   Tape each vertical corner without crushing the frames. Trace and cut the cardboard bottom, then tape its entire perimeter.

3. 3

   Fit the top fan

   Confirm it blows upward, then unplug it before mounting. Follow NOAA's cord-routing illustration and protect the cord from pinching. Tape the fan to the box and bridge gaps with cardboard.

4. 4

   Add the outlet shroud

   Cut cardboard to fit the discharge face, with an opening matching the blade sweep as EPA specifies. Keep the existing grille in place and keep cardboard clear of moving parts.

5. 5

   Check before running

   Inspect the corners, base and fan connection for gaps. Confirm the assembly is stable, the outlet is clear and the switch and plug remain accessible.

The shroud covers the outlet's corners around the circular blade area. Harvard describes it as preventing backward airflow around the fan's corners. Trace this fan's blade area through its grille; a hole size from instructions for another fan is not automatically the right cut.

Plan replacement access now. Our practical suggestion is to leave accessible tape ends at the fan joint and label the filter specification and assembly date on the base. A taped box needs a way to open again; do not bury the filter edges beneath decorative coverings. Recheck arrows and reseal the enclosure whenever filters are changed.

Operation and replacement during smoke events

Plug the fan directly into the appropriate wall outlet. EPA says not to use an extension cord; the Air King manual also cautions against power strips and other cord-connected devices. Keep the cord away from traffic, do not use a damaged or malfunctioning fan, do not leave children unattended around it, and maintain working smoke detectors. Keep all filter faces and the upward outlet clear of obstructions.

Check the filters daily during wildfire smoke. EPA says to replace them when dark brown or grey, dirty, or smelling of smoke, and at the end of a smoke event. Its research found dirty filters could make DIY cleaners almost ineffective. Have replacements ready rather than relying on a calendar reminder alone.

Nordic Pure's ordinary HVAC interval is 30–90 days for 1-inch and 2-inch filters; that is not a DIY smoke-event service promise. Unplug the fan before opening the box, replace loaded filters, then rebuild the seals and confirm the arrows. EPA also identifies increased sound and heat as possible DIY drawbacks. If the build's sound makes it impractical where you need it, consider the appliance option below or our quiet bedroom purifier guide.

What this build establishes about smoke, CADR and HEPA

CADR means clean air delivery rate: EPA describes it as the volume of filtered air a cleaner delivers. EPA's chamber research used simulated wildfire smoke; a result for that particular fan, filter and assembly does not become the rating of another build. We assign this parts list no measured CADR, room-size rating or sound level.

MERV describes filter particle-capture performance. EPA lists HEPA separately and says its HEPA efficiency specification is not based on the MERV system. Call this a MERV 13 DIY particle cleaner. Adding filters does not establish a HEPA label for the finished box.

Harvard's DIY guidance says these particle-filter devices do not remove gaseous pollutants. Smoke particles, smoke gases and lingering odors are different buying problems; this standard filter selection does not establish gas or odor removal. EPA also says portable cleaners do not replace ventilation. A box is not evidence of whole-home performance.

• Yes: Documented design for filtering airborne particles
• No: No established gas-removal capability for this particle-filter build
• Partly: Room results depend on the actual components, assembly, filter condition and space

Ready-made alternative: Blueair Blue Pure 211i Max

Choose the 211i Max if your priority is an appliance with automatic controls and a defined replacement procedure. Its official manual documents Auto mode, which responds to particle readings, and Night mode, which uses the lowest speed and dimmed lights. Manual fan-speed controls work on the unit; the app adds remote control and filter monitoring.

The manual shows a washable fabric pre-filter and a replaceable particle-and-carbon main filter. It explicitly says not to wash or reuse the main filter. That documented maintenance process is the reason for this alternative pick. It is not a claim of equal performance at the same sound level as the DIY box. Use the maker's smoke CADR in the product card when sizing a commercial unit; see our air purifier buying guide for the broader shortlist.

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How we choose

This guide draws on documents, without physical product testing: EPA's research, NOAA's instructions, maker documentation and Harvard's sealing guidance. Dimensions calculated from specifications are labeled. New component ASINs remain unverified, so their buying links go to the makers.

Verdict

Build the four-filter, solid-base box with an outlet shroud if you can accommodate its footprint, inspect the filters and reopen the seals for replacement. Buy the Blue Pure 211i Max if Auto/Night controls and a documented appliance maintenance routine better fit your room. For either choice, prioritize the operating instructions and replacement-filter supply over an unsupported whole-room performance promise.

Official sources

Start with EPA's research and operating guidance and NOAA's assembly sheet. Component manuals, the exact filter variant and supplementary construction guidance are linked in their sections above.