Does putting ice in front of a fan work?

The ice-in-front-of-a-fan trick is one of the most searched cooling questions there is, and the honest answer has three parts: it works, it barely works, and your freezer probably canceled it.

Give the trick its due. Air passing over ice loses heat to it, so the air arriving at you really is cooler than the air in the room. This is not a myth in the way that some cooling folklore is.

Two mechanisms are at work:

• Sensible cooling. Warm air contacts a cold surface and gives up heat.
• Latent cooling. Melt water evaporates into the airstream, which absorbs a great deal more heat than simple contact. This is the same effect an evaporative cooler is built around.

So people who say they feel a difference are not imagining it. The question is how much difference, and for how long.

Melting ice absorbs about 144 BTU per pound — the latent heat of fusion of water, a physical constant. That number makes the comparison easy.

A five-pound bag of ice is worth about seven minutes of a small air conditioner. Twenty pounds — which is a lot of ice to have standing in your bedroom — is worth under half an hour.

And that assumes every bit of it melts into the airstream. In practice a fan blows past a bowl rather than through it, so a large share of the ice melts into the water at the bottom without ever contacting the moving air.

This is the part nobody mentions, and it is the one that settles the question for home-made ice.

A freezer is a refrigeration cycle, exactly like an air conditioner — and like a dehumidifier, both of its coils are inside your house. The cold one is in the cabinet; the hot one is the grille at the back or underneath.

So freezing a tray of water:

1. removes heat from the water in the cabinet, and
2. dumps that heat plus the compressor’s own energy into your kitchen.

The net effect on the house is warming. Then you carry the ice into the bedroom and get back less cooling than the kitchen gained — because no refrigeration cycle is 100% efficient and the compressor’s electricity all became heat too.

Home-made ice in front of a fan is a net heater for the house. It moves a little cooling from the kitchen to the bedroom and loses some on the way.

Bought ice is different — the freezing happened in someone else’s building. It is still only 144 BTU a pound.

The latent part of the effect — melt water evaporating — is where most of the cooling comes from, and it has a cost.

Your body sheds heat by evaporating sweat, and that needs air with spare capacity to absorb moisture. Adding water vapor to the room reduces that capacity, which makes the same temperature feel warmer.

In a dry climate this trade is worth making — it is the whole basis of evaporative cooling. In a humid one you are giving up sweat evaporation to gain a small amount of air cooling, which is a poor deal and frequently a net loss in comfort.

It also puts water on the floor, which in a bedroom overnight is its own problem.

We are not going to pretend there is never a reason.

• A power outage, where the freezer is off anyway, the ice already exists, and there is no alternative — what else to do.
• A dry climate, where added humidity is a benefit rather than a cost.
• Cooling one person for twenty minutes, sitting directly in front of it, when that is genuinely all you need.
• Bought ice, which at least does not carry the freezer penalty.

Outside those, a fan on its own does most of the work for none of the mess, and anything that needs real cooling needs real cooling.