Without air conditioning, days above 30°C quickly become hard work in British homes, buildings designed for a century and a half of cold winters, meant to trap heat rather than shed it. Fewer than one home in twenty here has any form of aircon, so when a heatwave settles in, improvisation takes over. A viral video offers a low-cost answer: build a «DIY aircon» for about £7, using a cool box, a fan and a few frozen water bottles. Here is the method, step by step, and the energy sums that tell you what it is really worth.

Why everyone is talking about it

As heatwaves pile up, many households start eyeing a real air conditioner. But a portable unit is expensive to buy (rarely under £250 for anything decent) and, once plugged in, draws between 1,000 and 2,500 watts. At the current price cap rate of 26.11p per kWh, that is around 26p an hour and over £6 a day if it runs around the clock: enough to add noticeably to a summer bill. Renters face an extra hurdle: a fixed unit is out of the question, and even a portable one needs a window that accepts an exhaust hose.

Hence the success of the DIY alternative, popularised by the American channel HouseholdHacker and revived on social media every time the temperature climbs. The recipe needs nothing exotic: a polystyrene cool box, one or two tumble-dryer vent fittings from any DIY shop, and the fan you already own.

The £7 method, step by step

1. Cut the openings. Place the vent fittings and your fan on the lid of the cool box, trace their outline, then cut with a sharp knife so the holes fit snugly. A tight fit matters: every gap leaks cold air sideways instead of pushing it where you want it.

2. Load the ice. Fill the box with frozen water bottles or freezer blocks, then close the lid. Bottles beat loose ice: nothing to mop up, and they go straight back in the freezer for the next round.

3. Start the fan. Sitting on its opening, the fan pushes air into the box; the air comes out of the vents cooled. The video's author measures air blown out at 10 to 15°C, for about ten minutes of assembly and roughly £7 of materials.

The method step by step, in video, watch it on YouTube.

What this home-made «aircon» is really worth

The contraption works, but its limits are real. It delivers localised, temporary cooling: the cold air only reaches the area in front of the vents, and the ice melts within an hour or two. It is, at heart, a boxed-up version of the old frozen-bottle-in-front-of-a-fan trick, more directional and a touch colder, but it does not lower the temperature of the room, and it will not fight a 34°C afternoon on its own.

Refreezing the bottles also carries a hidden cost: your freezer works harder to form the ice again: roughly 0.2 kWh, or about 5p, per two-litre bottle. So the device is not quite free to run. And a practical caution: as the ice melts, condensation forms; keep the box on a tray and the fan's cable well clear of any drips.

Its direct electricity use, though, is just the fan's: 15 to 75 watts, which means a few pence for a whole night at the price cap. Set against the £6-a-day appetite of a portable air conditioner, the arithmetic is not even close.

Worth remembering: air at 10–15°C for an hour or two, aimed at one person, for the running cost of a simple fan plus a few pence of refreezing. Useful for falling asleep on a tropical night; pointless if you hope it will cool the living room.

Who it makes sense for (and who it doesn't)

The £7 build is an honest solution for short, targeted use: a desk during the hottest hours, the bedside at lights-out, the room of someone who struggles most in the heat, and it suits renters precisely because there is nothing to install. If what you want is hours of whole-room comfort, physics gets in the way: genuinely cooling air requires a refrigeration cycle, and that cannot be built from a cool box. In that case, think about a real unit, chosen by energy rating, sized to the room, and used with the thermostat at 25–26°C, because every extra degree you demand of it shows up, month after month, on your bill.