DIY Box-Fan Air Filter: HEPA + Carbon Build

If you have priced up a decent air purifier lately, you already know the problem. Covering two or three rooms with plug-in units climbs past a few hundred pounds fast, and replacement filters turn into a subscription you never signed up for. A DIY box-fan filter takes the opposite approach, pairing a cheap but powerful fan with proper furnace filters to shift loads of air through real filter media for under eighty pounds.

The design is widely called a Corsi-Rosenthal box after the air-quality researchers behind it, and independent testing reported by air-cleaner reviewers found these homemade cubes delivering clean air at rates in the same ballpark as mid-size plug-in purifiers. This guide adapts the build for UK homes with parts you can actually order this week, prices in pounds, and an honest account of what a cardboard cube can and cannot do for your air.

Most guides assume American hardware stores and dollar prices, so here is why you should read ours instead: every part below is matched to UK sizes, ratings, and shops.

How the build works, in plain English

A box fan on its own just pushes dusty air around the room. Four pleated panel filters taped into a cube change the equation completely, because every cubic metre the fan moves must first pass through dense filter media. The fan sits on top blowing upward, room air enters through all four sides at once, and four filters worth of area keeps resistance low enough for even a budget fan.

The two filter types split the work sensibly. Pleated particle filters rated MERV-13 in American money, or ePM1 70 percent and above in the European ratings you will see on UK listings, catch the fine stuff: smoke particles, pollen, pet dander, and the invisible fraction of DIY dust. A thin activated-carbon sheet over the intake faces adsorbs smells and light fumes from cooking, paint, and varnish.

None of this replaces ventilation or a respirator when you are generating dust at the source. Think of the cube as background cleanup for the whole room while extraction and masks handle the sharp end, exactly the layered approach we recommend in our broader rundown of workshop air quality and lung protection.

What you need: the UK parts list

Gather everything before cutting, because a missing shroud makes for a frustrating Sunday:

• A 20-inch (about 50 cm) box fan with a flat, square front, which typically costs between thirty-five and forty-five pounds from high-street electrical retailers, and any sturdy model works provided all four sides take tape cleanly.
• Four 20 x 20 x 1 inch pleated furnace filters rated MERV-13 or ePM1 70 percent minimum, which UK filter specialists and marketplace sellers stock for roughly eight to twelve pounds each, and matching the 20-inch size exactly matters far more than the brand name.
• One activated-carbon pre-filter sheet or cut-to-fit roll, usually ten to fifteen pounds, which you will trim into four panels for the intake faces.
• Two large sheets of sturdy cardboard plus strong gaffer tape or foil HVAC tape for the base cap, the fan shroud ring, and sealing every seam against leaks.
• Two bungee cords or one ratchet strap to hold the cube rigid, plus a craft knife, marker pen, and steel ruler.

Look for MERV-13 on US-branded filters or ePM1 70 percent and above on European-rated stock, and skip washable electrostatic mesh panels for this build because their open weave lets fine smoke straight through.

The build, step by step

Work methodically, because airtight seals are the entire secret here, and a leaky cube just recirculates dirty air while looking busy.

1. Clear a table and put on respiratory protection before cutting anything, because cardboard and fresh filter media throw up dust and fibres, so a proper JSP M632 FFP3 valved mask is the right call for ten minutes of cutting.
2. Check the airflow arrow printed on each filter frame and arrange all four filters so every arrow points toward the centre of the cube, which keeps air travelling through the media the right way.
3. Tape the four filters edge to edge into a square tube, taping the full height of each corner, then cap the bottom with double cardboard taped airtight on all sides.
4. Seat the fan on top of the cube with the grille facing upward so it blows toward the ceiling, then tape the fan base to the frames all the way around until sealed.
5. Cut a cardboard ring that covers the gap between the round fan housing and the square filter opening, which stops air looping back down the corners, and tape this shroud flat.
6. Trim the carbon sheet into four panels, one per filter face, and tape them lightly around the edges on the outside of the cube so incoming air meets carbon first and the pleats stay cleaner.
7. Strap the whole cube with bungees or a ratchet strap, stand it a foot clear of walls, plug it in, and hold a tissue near each face to confirm every side pulls air inward.
8. Run medium for everyday filtering and save high for smoky evenings or the hour after dusty jobs, since noise climbs fast on top speed.

What it costs to build and run

The build lands around seventy-five to eighty-five pounds all in at current UK prices, with the fan taking half the budget and filters plus carbon covering most of the rest. Tape, cardboard, and straps usually come from the shed odds box.

Running costs stay modest because a box fan on medium draws about the same power as an old-style bright bulb, so an evening of filtering adds a small line to the bill rather than a scary one. Check your fan wattage against your tariff for an exact figure, but background filtering typically costs less over a year than replacement filters for a premium plug-in.

What it can and cannot do

Set expectations correctly and this cube earns its floor space. It steadily drags down background levels of dust, pollen, pet dander, and bonfire smoke in a normal living room or home office, with the carbon layer softening cooking smells and fresh paint whiffs.

It cannot do everything, and anyone telling you otherwise is selling something. It does not remove carbon monoxide, so keep a proper CO alarm wherever fuel burns, and it does not cure damp or mould, which need the moisture source fixed plus genuine ventilation. It is loud on full speed, so run high while out and low overnight, and it never replaces a respirator or extraction during dusty work, as our essential workshop safety guide explains.

Products mentioned

• JSP M632 FFP3 valved dust masks (10-pack) - for safe building now and every dusty job afterwards.
• Karcher WD3 wet and dry vacuum - for the workshop cleanup this build deserves, plus dust extraction at the tool.
• Levoit Core 300S smart air purifier - the quiet ready-made option for bedrooms and anyone skipping the DIY route.

Frequently Asked Questions

How long do the filters last in normal home use?

In a typical living room the pleated filters last three to six months before airflow audibly drops, while thin carbon saturates faster and wants replacing every month or two in hard-working homes. Write the install date on the cap so you judge on evidence, not memory.

Can I run the box overnight in a bedroom?

Yes, since on low speed most people find the hum fades into the background within a night or two, and running low all night beats blasting high for an hour before bed. Light sleepers can run high over dinner, then switch to low at bedtime.

Will it clear cooking smells completely?

It takes the edge off lingering frying and curry smells noticeably, but thin carbon holds far less than a cooker hood, so run your extractor first and let the cube handle the rest.

Is it actually cheaper than buying a purifier?

For background filtering across one room the answer is yes by a wide margin, because the parts cost less than a single mid-range plug-in and replacement filter sets run about half the price of branded purifier refills. The trade-off is noise, size, and looks.

Can I use different filter sizes or fewer filters?

The design only works well when the filter area roughly matches the fan face, so four 20-inch filters paired with a 20-inch fan is the recipe you should follow. Smaller filters strangle airflow, which defeats the whole point.

Does a DIY box protect against airborne viruses?

Moving plenty of room air through good particle filters lowers the concentration of airborne particles generally, which researchers consider helpful, but no homemade box replaces ventilation or staying home while ill.

Final verdict

For well under eighty pounds and one afternoon with a craft knife, this build delivers genuinely cleaner background air in the rooms where you spend your evenings. It looks homemade and it hums, and neither fact stops it working through bonfire season.

Build one for the living room this autumn and judge with your own nose and dusting cloth. If the looks bother you then the quiet plug-in alternative is the Levoit Core 300S, although you will pay two or three times more for the same clean air.

A

Alex Chen

Alex writes about tools, tech and DIY projects for MakerUK. He has a fully equipped workshop and has been building and repairing things for over 12 years.

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Last reviewed: September 4, 2026 · Evidence-based content · Contains affiliate links