Air cooler or fan: what each one actually does

A fan does not bring a room's temperature down. It creates a draught, and moving air speeds up the evaporation of sweat: it is your skin that cools, not the room.

A water cooler genuinely lowers the temperature of the air it puts out, by two to four degrees, by evaporating water through a filter.

That evaporation needs dry air. In a closed room humidity rises and the effect dies within an hour: you have to leave a window ajar.

In the shop: USB air cooler — mist and seven colours and Water air cooler — for one room, not a corridor.

The experiment is easy to run at home: put a thermometer in a room, switch a fan on, wait twenty minutes. The needle does not move. It even creeps up very slightly, because the motor dumps its own heat into the room.

What you feel is real all the same. Skin sheds heat by evaporation, and a still film of air clinging to the skin slows that evaporation down. The fan sweeps that film away: evaporation restarts, and the sensation gains three to four degrees.

The practical consequence is direct: a fan running in an empty room is useless. It only cools what sits in its stream. That is also what makes it frugal — around thirty watts against eight hundred to a thousand for a portable air conditioner.

The principle is called evaporative cooling, and it is as old as porous water jars. Incoming air passes through a water-soaked filter. To go from liquid to vapour, water needs energy, and it takes that energy from the air passing through. The air comes out colder and damper.

The measured gain at the outlet is around three degrees in ordinary conditions, more if the air is very dry. Over a modest volume — a bedroom, an office, a corner of a living room — it is unambiguously noticeable.

The trade-off is written into the principle: the humidity released stays in the room. If nothing carries it away, the air saturates, evaporation slows, and the cooler ends up producing nothing but lukewarm wind. A window ajar is enough, and it is counter-intuitive enough that many disappointed buyers never tried it.

A second consequence of the same mechanism: in an already humid region, by the sea or in a tropical climate, the gain is small from the start. That is not a fault in the device, it is physics.

Neither is air conditioning. Air conditioning extracts heat from a volume and dumps it elsewhere — which is why it needs a duct to the outside or a unit on the wall. A cooler dumps nothing: it turns sensible heat into humidity, indoors.

In a poorly insulated top-floor flat, neither will fix the underlying problem, which is heat coming in through the roof. A shutter closed on the sunny side and night-time ventilation often win more degrees than any appliance.

And for one person sitting at a desk, the cheapest answer is still to cool the body rather than the room: a neck ring or a small wearable fan costs fifteen to thirty euros and follows the person from room to room.

Dry room, top floor, a window that opens: the cooler gives the best result per euro spent, and it requires neither building work nor a freeholder's agreement.

Desk, bed, sofa — a single occupied spot: a fan is plenty, it draws three times less, and aiming it well matters more than its power rating.

Humid climate or a room that cannot be aired: neither will keep the promise. Better to spend the budget on blocking the windows and on wearable cooling.

One last criterion decides more purchases than people admit: noise. A two-speed device whose lower speed is already loud will never be used at night, whatever its stated performance.