Where to put a fan

A fan does not lower the temperature of a room. It moves air, and moving air does two useful things: it accelerates heat loss from skin, and it mixes air that would otherwise sit in layers.

Which of those you are trying to achieve determines where the fan goes, and the three answers are not compatible with each other.

This is the mechanism most people actually want, and it is the one where placement matters least in principle and most in practice.

Airflow over skin accelerates evaporative and convective heat loss. The effect depends on air speed at the skin, and air speed falls off sharply with distance — a jet spreads and slows as it travels.

• Close beats big. A small fan two feet away delivers more air speed at your skin than a large fan across the room, on a fraction of the power. Nine watts versus fifty is a real difference in both directions.
• Aim at exposed skin. Arms, neck, face, feet. Air moving over a jumper does very little; the evaporation has to happen at the surface.
• At or slightly above body height. Angled gently down at a seated person, not up from the floor at their shins.
• Fixed, not oscillating, if you are the only person in the room. Oscillation shares the same airflow across an arc, so you get it intermittently instead of continuously.

The corollary is the most useful thing on this page: if nobody is in the room, turn the fan off. It is doing nothing except adding its motor heat to the space.

Sometimes the problem is not that you are hot but that the room is uneven — a stuffy corner, a warm layer near the ceiling, an area the air conditioning never reaches.

That calls for the opposite placement: aim the fan away from people, at the far wall.

The air column travels, hits the wall, spreads along it and returns along the floor and ceiling. That circulation pattern turns the whole room over. Aiming at people instead produces a breeze on them and no pattern at all.

Three things this needs:

• A coherent column. A circulator with a deep shroud projects air a long way. A wide diffuse output dissipates in a few feet.
• No oscillation. A sweeping head never sustains flow in one direction long enough to establish the pattern.
• A clear path. Furniture, curtains and open doors in the wrong place absorb the column before it arrives.

This is also how you help an air conditioner reach a room it does not serve well — move the cooled air along rather than adding capacity. Common in a hot upstairs.

A fan in a window is a different machine. It stops moving room air around and starts replacing it, which genuinely changes the temperature.

The rule that decides whether it works is timing.

In most climates that means the window fan runs from a couple of hours after sunset until shortly after dawn, and everything is shut through the afternoon.

Two openings beat one. Air needs an exit as well as an entrance. A fan exhausting from one window with another open across the room moves far more air than a fan pushing into a sealed space, because it is not fighting the pressure it creates.

If you only have one window, a twin window fan with independent motors solves it — intake on one side, exhaust on the other, through the same opening.

If you have two openings and two fans, the most effective arrangement in the whole category costs about 9.1¢ a night at 31 W each.

1. One fan exhausting at the window on the warm side of the building, or the higher one if there is a height difference.
2. One fan intaking at the window on the cool, shaded side — or leave that window simply open if you only have one fan.
3. Interior doors open along the path between them, so the air has a route.

This works with the stack effect rather than against it: warm air already wants to leave high and cool air already wants to enter low, and the fans just increase the rate.