The Corsi-Rosenthal box is a DIY air purifier invented in 2020: four or five furnace filters taped into a cube with a box fan on top. It moves a huge volume of air through cheap MERV-13 media, and independent testing repeatedly shows it rivalling purifiers costing many times more. For a London home fighting traffic PM2.5, it's the best value in clean air — full stop.
You can build one yourself, or buy the same physics pre-engineered, balanced and quieter as a Luggables kit.
Ready to clean up your London air?
Luggable MERV-13 kits deliver up to 681 m³/h CADR — built on the Corsi-Rosenthal principle, from £229. Ships from Oxford in 2–3 days.
Shop the UK store → See all kitsWhat you need to build one
- 4–5 × MERV-13 furnace filters (a taller five-filter cube moves more air)
- 1 × 20-inch box fan (or a PC-fan array for lower noise)
- Cardboard base + a cardboard "shroud" to reduce dead space over the fan
- Foil tape
Total DIY cost is roughly £60–£120 depending on filters. It takes about 20 minutes.
CADR and cost, compared
| DIY CR box | Luggables kit | £500 sealed unit | |
|---|---|---|---|
| CADR | ~350–600 m³/h | 369–681 m³/h | ~200–400 m³/h |
| Noise | Box-fan hum | Under 40 dB | Low |
| Filter type | Standard MERV-13 | Standard MERV-13 | Often proprietary |
| Upfront | £60–£120 | £229–£279 | £400–£700 |
Why the design works
The Corsi-Rosenthal box looks almost too simple to be effective, and that is exactly why it is worth explaining. A box fan on its own pushes a lot of air but filters nothing. Tape four or five MERV-13 furnace filters into a cube beneath it and you turn that airflow into clean-air delivery: the fan draws room air in through a large filter surface on every side, so resistance stays low and throughput stays high. Because the filter area is so much bigger than the slim cartridge in a typical purifier, the media loads slowly and airflow holds up well. The result is a clean-air delivery rate that repeatedly rivals machines costing several times more. It is not magic — it is surface area and airflow doing the work.
MERV-13 is the right media grade for this. It captures around 93% of fine particles, which covers the PM2.5 that drives most health harm, while staying open enough for a box fan to move serious air through it. Step up to true HEPA and the single-pass capture rises, but the resistance chokes a box fan's airflow so badly that overall clean-air delivery falls. That trade-off is the whole point, and we cover it in detail on MERV-13 vs HEPA.
Building one in the UK
Most of the original build guides are American, and a couple of details differ here. UK mains is 230V, so use a fan sold for the UK market rather than importing a US 120V box fan. Twenty-inch box fans are also less of a high-street staple in Britain than in the States, so you may need to order one online, and the common filter sizes on sale differ too — measure before you buy and be prepared to pad a slightly undersized filter with foil tape. The build itself is forgiving: a cardboard base, the filters arranged as a cube with the airflow arrows pointing inwards, a cardboard shroud to cut dead space over the fan, and foil tape to seal the gaps. Twenty minutes and roughly £60–£120 in parts.
When a pre-built kit makes more sense
DIY wins on upfront cost and is genuinely satisfying to make. The honest downsides are that it is a bulky cardboard cube, the box fan hums, and it will never look like living-room furniture. A pre-built Corsi-Rosenthal box kit keeps the same physics but arrives quiet, balanced and tidy, using standard replaceable MERV-13 filters rather than proprietary cartridges. If you would rather not build, or you want something that suits a shared space, that is the trade to make — and you can compare both against the wider market in our best air purifiers for 2026.
Frequently asked questions
Is a Corsi-Rosenthal box as good as a real air purifier?
On clean-air delivery it often outperforms purifiers costing far more, because it moves so much air. The trade-offs are size, box-fan noise and looks — which pre-built kits solve.
What MERV rating should the filters be?
MERV-13. It captures fine PM2.5 while keeping airflow high. Lower MERV lets particles through; true HEPA chokes the airflow of a box fan.