Every heatwave, the same trick does the rounds online. Cut the necks off a few plastic bottles, push them through a sheet of card and hang the panel in an open window. The claim is bold: your room gets five degrees cooler, with no electricity and only a few pounds of kit. The gadget even has a name, the eco-cooler. A team of researchers has now put that promise through a laboratory rather than taking it on trust, and the verdict is unforgiving.
Where the idea came from
The design is credited to Ashis Paul, a creative director at an agency in Dhaka, who is said to have worked it out while revising physics with his daughter. It was built for families in Bangladesh who live under tin roofs, where summer temperatures pass 40C and mains power is often absent.
The theory sounds neat. Air squeezed through the narrow neck of a bottle should expand on the way out and cool down, the same chill you feel when you blow on your hand with pursed lips. The promise of cooling a home for almost nothing did the rest, and the world's press ran with it for the best part of a decade.
What the study actually measured
The research appeared in the journal npj Climate Action in November 2024, with authors from Heidelberg's global health institute, Lund University, BRAC University in Dhaka and Stanford. They built seven versions of the eco-cooler and ran each one inside a wind tunnel and a climate chamber, 92 experiments in all.
The settings copied a real Bangladeshi summer: air at 30C, 35C and 40C, humidity from 40 to 70 per cent, and wind from 0.2 to 4 metres per second.
Across every run the temperature barely moved. The best the panel managed was a drop of about 0.2C, and only at the lowest wind speeds. In some set-ups the air coming through was actually a touch warmer.
Why the five degrees never arrive
The problem is not effort. It is physics. Even in the most generous theoretical case the study could build, the maximum drop was 0.85C, and that sliver of cooler air never spread past the mouth of the bottle. It never reached into the room.
To hit the headline five degrees you would need a colossal pressure gap, with air leaving the bottles at around 100 metres per second, roughly 360 kilometres per hour. No sheet of card in a window can create anything close to that. The authors put it plainly: there is no evidence the eco-cooler works as a passive cooling device.
Good to know: shading works best on the outside. An external shutter or awning stops the sun before it reaches the glass, while an internal blind only catches the heat once it is already in the room.
Myth versus reality: what to do, what to skip
Once the physics is stripped out, the split is clear. These are the moves that genuinely take the edge off a hot flat.
- Close up early: shut blinds, curtains and windows the moment the sun is on the glass, before the room warms.
- Open up at night: once it is cooler outside than in, throw the windows open and let the air move through.
- Point the fan at people: the breeze only helps skin that can sweat, so aim it where someone is sitting.
- Keep the air con at about 26C: it still feels comfortable and every degree lower adds to the bill.
And these are the habits worth dropping, starting with the trick that set all this off.
- Do not expect a bottle panel to cool the room: the lab found no effect you could feel.
- Do not seal your only window with the panel: blocking the opening cuts the fresh air you actually need.
- Do not leave a fan running in an empty room: with no skin to cool, it just uses electricity.
- Do not crank the air con down to 18C: it will not cool any faster and the running cost jumps.
What a fan and air con really do
It is worth clearing up the fan, because most homes own one. A fan does not lower the temperature of the air; it moves it around. The relief you feel comes from sweat evaporating off your skin, not from a cooler room. Above 30C the benefit fades, because the fan is only pushing warm air about, though it still helps while your body can sweat and costs very little to run.
Air conditioning is the only one of the three that genuinely lowers the room temperature, and it is also the only one with a real bill attached. Here are the three lined up side by side.
Before spending anything, use what is free. Closing up in the morning and airing the flat at night does more than any window gadget, and it costs nothing at all. If you do reach for the air con, keep it sensible and let it run at a steady setting rather than chasing an icy room. Our guides on air conditioning running costs and staying cool in a heatwave go further, alongside our energy efficiency tips.