How do I use fans effectively?

Why a fan does not cool a room, the window arrangement that actually does, and how to position one so it works.

Difficulty: beginner
Time: 15 min
Read: 4 min
Safety: warning

Short answer

A fan cools you by evaporating sweat — it does not lower the room temperature and its motor adds a little heat. To cool a room, put a fan in a window blowing outwards with another window open elsewhere, which exchanges air. Do that at night when it is cooler outside.

Fans are used in the least effective way in most houses: one in the middle of a closed room, aimed at a person. That is not wrong exactly, but it is doing only half of what a fan can do, and only for one person.

Safety

A fan cools people, not rooms — in extreme heat above about 35°C, blowing hot air at someone can make them worse rather than better, particularly anyone elderly or unwell. Never leave a fan blowing directly at a baby or young child all night. Keep cables out of walkways and never run a fan with a damaged cable or guard.

Step by step

Understand the two different jobs.A fan can cool a person by moving air over skin so sweat evaporates, or it can move air through a building to exchange hot air for cooler. They need completely different positioning and most households only ever do the first.
Exhaust rather than circulate.Put a fan in a window facing outwards, with a window open on the other side of the house. That pushes hot air out and draws cooler air in. It changes the room temperature, which a fan in the middle of the room cannot.
Do it when outside is cooler.Usually from mid-evening. Running an exhaust fan when it is hotter outside draws hot air in. Check the outside temperature before opening anything.
Create a path through the house.Air has to be able to get from the inlet to the outlet. Open internal doors along the route, and put the exhaust fan at the hottest end — usually upstairs, since heat rises.
Use a second fan for the person.Once air is moving through the house, a gentle breeze across the bed or the sofa does the evaporative half. Two fans doing different jobs beats two fans doing the same one.
Aim it at skin, not at the room.For the personal cooling job, the air has to reach you. A fan oscillating across a room at head height with a sofa in the way is doing very little.
Use the lowest speed that works.Higher speeds add more motor heat, more noise and more electricity for a marginal gain. Low speed close by beats high speed across a room.
Add moisture only in dry heat.A damp towel in front of a fan cools the air passing it, which helps in dry conditions. In humid weather it adds humidity and makes the room feel worse.
Use ceiling fans in the right direction.Anticlockwise in summer, pushing air down for a breeze. Most have a small switch to reverse them, and in winter clockwise on low pushes warm air down from the ceiling without a draught.
Clean the blades and guard.Dust on the blades unbalances them, makes them noisy and measurably reduces airflow. Unplug first, and it takes two minutes.
Put the fan on a timer at night.An hour or two gets you to sleep, and it stops before the noise and draught wake you at four. Most fans have one, and a plug-in timer works otherwise.
Do the shading first.Fans are the last step, not the first. Closing curtains on the sunny side during the day prevents far more heat than any fan can move afterwards.

Tips

A cheap indoor-outdoor thermometer tells you when to open up and start exhausting. Guessing usually means opening windows several hours too early.
Point an exhaust fan out of an upstairs window. Heat collects at the top of the house and that is where you want the outlet.
A fan in an empty room is wasted. It only cools people, so turn it off when you leave.

Common mistakes

Running a fan in a closed room — It circulates the same hot air and the motor adds slightly to it. It helps whoever is in the airflow and does nothing to the room.
Exhausting when it is hotter outside — It pulls hot air in. Check the outside temperature and wait until it drops below the inside before opening up.

If it doesn't work

Fan running all night, room still hot

Cause: Circulating rather than exchanging air — Fix: Put it in the window facing out and open another window elsewhere in the house.

Fan noisy or wobbling

Cause: Dust on the blades unbalancing them, or loose fittings — Fix: Unplug, clean the blades and guard, and check the mounting screws.

Room feels more humid with a damp towel arrangement

Cause: Evaporative cooling in already humid air — Fix: It only works in dry heat. Remove it and use plain airflow instead.

Upstairs still much hotter than downstairs

Cause: Heat rising with no outlet at the top — Fix: Exhaust from an upstairs window with a downstairs window open, and check the loft insulation depth.

Questions people ask

Do fans actually cool a room?

Not on their own — they move air and the motor adds a little heat. A fan cools you by evaporating sweat. To cool the room, put one in a window facing out with another window open elsewhere so air is exchanged.

Where should I put a fan to cool a room?

In a window, facing outwards, ideally upstairs, with a window open on the other side of the house — and only once it is cooler outside than in. A second fan can then blow across you.