Corsi-Rosenthal Box UK: Complete Build Guide

The Corsi-Rosenthal Box is a DIY air purifier that costs £50-100 and performs comparably to £300+ commercial HEPA units. Over 250,000 have been built worldwide. This UK-specific guide shows you how to build one with locally available parts.

What is a Corsi-Rosenthal Box?

Created by Dr. Richard Corsi (UC Davis Dean of Engineering) and Jim Rosenthal (Tex-Air Filters), the CR Box combines a box fan with 4-5 MERV-13 filters to create powerful air filtration. It's been validated by EPA, CDC, and 3M scientists.

UK Build Options

Option 1: Vent Axia 14" Build (Recommended)

Vent Axia 14" box fan: £30-40
4× 3M Filtrete MPR1900 filters (20×25×1"): £60-80
Duct tape: £5
Cardboard for base and shroud: Free
Total: £95-125

Option 2: Cooltone 12" Slimline (London)

Cooltone 12" box fan (available London): £25-35
4× Filtrete 1900/2200 (14×24×1" or 14×25×1"): £50-70
Assembly materials: £5
Total: £80-110

Assembly Instructions (15-45 minutes)

Step 1: Check Filter Direction

All filter arrows must point INWARD. The airflow arrows show which direction air should flow through the filter.

Step 2: Build the Filter Shell

Stand 4 filters vertically in a square formation
Tape edges together with duct tape (create airtight seals)
Ensure no gaps between filters

Step 3: Add Cardboard Base

Cut cardboard from fan box to fit bottom opening. Tape securely to close the cube.

Step 4: Mount Fan on Top

Place fan on top with airflow direction pointing UP (pulling air through filters). Tape all edges to create airtight seal.

Step 5: Add Shroud (Optional - Increases Efficiency 20-30%)

Cut cardboard to create a funnel/shroud directing airflow. This reduces turbulence and improves performance.

Performance Specifications

CADR: 600-850 CFM (1,020-1,445 m³/h)
Particle Removal: 60% of 1μm particles, 90% of 10μm particles
Room Coverage: Up to 100m² (depending on configuration)
Noise Level: ~50dB (standard box fan)
Power Consumption: 50-120W
Filter Lifespan: 3-6 months (replace when visibly dirty)

Where to Buy UK Parts

Vent Axia fans: Screwfix, Toolstation, Amazon UK
3M Filtrete MPR1900: Amazon UK, specialist HVAC suppliers
ePM1 55% filters: ARM Environments (UK delivery)
CleanAirKits: Pre-assembled PC fan versions (quieter, free shipping UK >£100)

Testing Your Build

Check for air leaks by holding tissue paper near seams while fan runs. Tissue should be pulled toward filter box. Re-tape any gaps.

Safety Notes

Never leave unattended (standard box fan safety)
Place on stable, flat surface away from curtains
Turn off when not in room for extended periods
Keep away from water and moisture

Frequently Asked Questions

How much does a Corsi-Rosenthal box cost to build in the UK?

A UK Corsi-Rosenthal box costs about £50-100 to build: a 20-inch box fan (£25-40), four MERV-13 filters (£15-30 total) and tape. That's a fraction of the £200-800 for a commercial HEPA purifier delivering similar clean-air output.

How effective is a Corsi-Rosenthal box?

Very. A 5-fan Corsi-Rosenthal box produces a CADR of roughly 800-1,200 m³/h — more clean air than most commercial purifiers — and removes 90%+ of PM2.5, pollen, smoke and mould spores. In a typical UK room it delivers 5-6+ air changes per hour.

What filters do I need for a Corsi-Rosenthal box in the UK?

Use four MERV-13 (or ISO ePM1 50%+/F7) filters sized to your fan — commonly 20x20 inch or the nearest metric equivalent. MERV-13 is the sweet spot: high capture of fine particles without over-straining the box fan.

Is a Corsi-Rosenthal box better than a HEPA purifier?

For value and total clean-air output, yes — it delivers far more CADR per pound. A HEPA unit wins on per-pass efficiency (99.97%) and quiet, compact design. For most homes the CR box removes airborne particles just as effectively over time at a fraction of the cost.

How loud is a Corsi-Rosenthal box?

On its lowest setting a box-fan CR build runs around 45-55 dB — similar to a quiet fan and fine for daytime or a distant bedroom. Reduce noise by running a lower fan speed (the large filter area keeps airflow high even when slowed).