Corsi-Rosenthal Box (DIY: box fan + 4 MERV-13 filters)

How it works

A 20 inch box fan taped to a cube of four MERV-13 furnace filters, blowing outward. The large filter area means low resistance, so a cheap fan moves a great deal of air through a great deal of media. It was designed in 2020 by Richard Corsi and Jim Rosenthal as an open, unpatented answer to expensive purifiers, and independent testing has repeatedly found it competitive with commercial units several times its price.

What the market looks like

There is no market, which is the point — it is a public-domain design assembled from commodity parts. What exists commercially is kits at CAD $80–$180 that bundle the filters and a shroud, and a few boutique versions in tidy cases at CAD $250–$400. Buying the parts separately is cheapest: a box fan is CAD $35–$70 and MERV-13 filters are CAD $15–$30 each. Filter quality is the only real variable; buy from a known furnace-filter brand rather than the cheapest listing.

Features that actually matter

Very high CADR for the money

Typically 600–900 cfm, which exceeds most CAD $500 consumer purifiers. Four filters' worth of surface area is why.

MERV-13 media

Captures PM2.5 effectively. Not HEPA-grade per pass, but it processes so much more air per hour that total particle removal is comparable or better.

Cardboard shroud

A simple cardboard ring around the fan outlet improves airflow measurably — by roughly 20–50% in published testing. Do not skip it.

Cheap consumables

Replacement filters are ordinary furnace filters at CAD $15–$30, available everywhere, with no proprietary lock-in.

Repairable and scalable

Build a second one for another bedroom in twenty minutes.

What it really costs

Up front

CAD $100–$180 for fan, four filters and tape. A pre-packaged kit runs CAD $120–$200.

Consumables

Four filters per change, CAD $60–$120. Change every 6–12 months, or every few weeks in dense smoke.

What else you'll need

Duct tape or foil tape, and a piece of cardboard.

Over five years

CAD $400–$700 including filter changes — well under half the cost of a comparable commercial purifier.

Figures are typical street prices in Canadian and US dollars as of September 2026, gathered from public retail listings. They move with the season and with the exchange rate — treat them as a planning band, not a quote.

How it differs from the alternatives

Versus a commercial HEPA purifier

More airflow per dollar and much cheaper filters. Louder, physically large, and it looks like a taped box fan in your bedroom. That aesthetic objection is the honest reason most people buy the commercial unit instead, and it is a legitimate reason.

Noise

A box fan on high is 55–65 dB. Many people cannot sleep next to it. Running on medium sacrifices some CADR and is usually the practical compromise.

Safety

Use a fan with a fused plug and a thermal cut-out — fans made after roughly 2012 generally have both. Do not leave any box fan running unattended for days in a room with children, and do not obstruct the intake.

The community angle

Schools, community centres and band offices across BC and Alberta have built these by the dozen during smoke seasons. If you want to help a neighbourhood breathe, this is the cheapest way to do it at scale.

If not this, then what

• A commercial HEPA purifierQuieter, tidier, more expensive to buy and to feed.
• A single filter taped to a fanWorks, but a quarter of the filter area and a much noisier, less effective result. Build the cube.