A Corsi-Rosenthal box is a do-it-yourself air cleaner: a cube of furnace filters with a box fan on top, taped together so air is pulled through all four sides. It was popularised during the pandemic because it moves a lot of air through decent filtration for very little money. This guide walks through a genuine UK build — the parts you can actually source here, how to assemble it properly, the safety points that matter, and an honest account of what it is like to live with. We will also be upfront about where a purpose-built kit makes more sense, because sourcing a 20-inch box fan in Britain is the single hardest part of this project.
681 m³/h CADR, MERV-13, under 35 dB — powerful whole-home clean air — no proprietary lock-in, ships from the UK.
Check price →Use code CORSI10 for 10% off at checkout.What a Corsi-Rosenthal box actually does
The idea is simple. A box fan on its own moves air but cleans nothing. Strap four filters around it in a cube and seal the joints, and every cubic metre the fan pulls has to pass through a filter first. Because the combined filter area is large, air moves through it slowly, which means low resistance for the fan and high total airflow for the room. A well-built box can deliver a clean-air delivery rate comparable to portable purifiers costing several hundred pounds.
It captures the same things a good filter captures: fine particulate (PM2.5), pollen, dust, smoke particles and a large share of airborne respiratory particles. It does not remove gases, odours or carbon dioxide unless you add activated carbon, and it is not a substitute for ventilation. Think of it as a high-volume particle filter, not a magic box.
UK parts list
Sourcing in Britain is a little different from the American tutorials you will find online. Here is what you need and what to watch for:
- One 20-inch box fan. This is the awkward part. The classic flat, square box fan is common in the US and comparatively rare in the UK. You may need to search seasonal ranges, online marketplaces or import one. A standard round pedestal or desk fan will not seal against a filter cube, so it is not a straight swap.
- Four to five furnace or HVAC filters rated MERV-13, or the European equivalent F7 to F9 (ePM1 grades under ISO 16890). MERV-13 removes roughly 90 per cent-plus of fine particles. UK filter sizes are quoted in millimetres, so match the filter footprint to your fan; a five-filter build (four sides plus a base) seals better and moves more air.
- Strong tape. Cloth or foil HVAC tape works well. Avoid weak parcel tape, which peels as the fan warms.
- A large sheet of cardboard to cut a shroud (more on that below) and to cap any open base.
- A knife or scissors, and a marker pen.
Check the arrow printed on the edge of each filter before you buy — it shows the direction of airflow, and every filter must point inward towards the fan.
Step-by-step assembly
- Lay the filters out and find the airflow arrow on each frame. Orient every filter so the arrow points inward, towards the centre of the cube. Air should flow from the room, through the filter, into the box.
- Stand four filters on their edges to form the four walls of a cube, arrows facing in. Tape the vertical seams where the filters meet, sealing the full height of each corner so no air can sneak past the filters.
- Cap the bottom. Either use a fifth filter as the base (arrow up, into the box) or cut a piece of cardboard to size and tape it over the base so it is fully sealed. A sealed base forces all air through the four filter walls.
- Place the box fan on top of the cube facing upward, so it pulls air up out of the box and blows it towards the ceiling. Confirm the direction before taping — you want it drawing air through the filters, not pushing against them.
- Tape the fan to the top of the cube all the way around, sealing the gap between the square fan housing and the filter frames. Gaps here are the most common cause of poor performance.
- Add a shroud. Cut a piece of cardboard to cover the corners of the fan grille, leaving only the central circle (the actual fan blade area) open. This reduces air recirculating around the dead corners of the fan and can noticeably increase clean-air output for a few minutes of extra work.
- Do a final seal check. Run your hand around every joint with the fan running; if you feel air escaping from a seam rather than the top, add more tape.
Label the build with the date so you know when the filters went in.
Safety
This is mains electrical equipment, so treat it accordingly.
- Never block the fan intake or outlet. The filters must stay clear and the top must blow freely. Restricting airflow makes the motor work harder and run hotter.
- Keep it dry and stable. Place it on a flat, hard surface where it will not be knocked over, away from water, curtains and bedding.
- Do not run it unattended for long periods or overnight if the fan is old, damaged or overheating. Cheap box fan motors are not always designed for continuous duty. If it smells hot or the motor labours, switch it off.
- Use a UK-standard plug and a fan rated for UK mains (230V). If you import a US 110V fan, you need a proper voltage converter, not just a plug adapter — do not skip this.
- Replace the filters every few months, or sooner in dusty or smoky conditions. A clogged, grey filter restricts airflow and strains the motor. Vacuuming a filter is a short-term stopgap at best.
- Keep it away from children and pets who might poke fingers or paws through the grille.
Realistic expectations: it works, but…
Let us be honest, because most build guides are not. A Corsi-Rosenthal box genuinely works. For the cost of some filters and a fan it will clean a room's air impressively fast. But there are real trade-offs:
- It is loud. A 20-inch box fan on full is not something you want running next to your desk or bed for hours. Turn it down and the airflow drops with it.
- It is bulky and unattractive. A taped cube of furnace filters roughly half a metre on each side is not a piece of furniture. It takes up floor space and looks like exactly what it is.
- The fan is the weak link. Box fan motors are cheap, and running one continuously shortens its life. It is also the hardest part to buy in the UK.
- Sealing takes patience. A sloppy build with air gaps performs far worse than the numbers suggest.
None of this means you should not build one. It means you should go in knowing what you are getting: excellent airflow, at the cost of noise, size and a bit of fiddling.
Want the same airflow without the build? Two honest alternatives
If the appeal is clean air rather than the project itself, there are purpose-built options that solve the two biggest weaknesses of the DIY box — noise and the box fan — while keeping the same MERV-13 filtration and cube geometry.
Which route is right for you?
Choose the classic DIY Corsi-Rosenthal build if you want the absolute lowest cost, you already have or can source a 20-inch box fan, and noise and appearance do not bother you — for a garage, workshop or short-term wildfire-smoke event, it is hard to beat on price.