Where to place a fan in a single-sided or cross-ventilated home

A fan does not cool the air. It moves it. The motor even adds a small amount of heat to the room. Everything useful a fan does comes from either evaporating sweat off your skin or shifting air between inside and outside, and the second one depends completely on where you put it.

First, work out your layout

Look at where your windows are, not how many you have.

Single-sided: every window is on the same façade. Air has no natural path through the flat, so an open window mostly exchanges air with itself. Common in small flats, studios, and anything on the corridor side of a block.

Cross-ventilation: you have openable windows on two different façades, ideally opposite ones. Air can enter one side and leave the other. This is dramatically more effective, and it is worth opening an interior door to unlock it.

Should the fan face in or out?

There is no single direction that works for every room.

• Face it inward when one window is your only useful opening and cooler outdoor air needs help entering
• Face it outward when another window or door can supply cooler air and the fan can exhaust warm air
• Do not use either direction to exchange air while outdoors is warmer than indoors; close the window and use the fan on people instead

Start by choosing the intake and exhaust, then point the fan in the direction that supports that path.

Single-sided homes

With one façade, you have to create the pressure difference yourself.

• After sunset, once the outdoor air is cooler, place a floor fan on the windowsill facing into the room, angled slightly upward
• Angling upward matters: it throws the incoming cool air across the ceiling rather than into a wall, and it mixes the room instead of stirring one corner
• If you have two windows on the same façade and they are reasonably far apart, use one as intake and put the fan in the other facing outward
• Open interior doors so the moving air has somewhere to spread

Cross-ventilation homes

Here you are steering a flow that already wants to happen.

• Open the intake wide, choosing the shady or greener side, a courtyard rather than a sun-baked street where you have the choice
• Open the exhaust, typically the street side, just a crack
• Place the fan 1 to 2 metres back from the exhaust window, facing outside

The crack is the counter-intuitive part. Narrowing the outlet speeds the air leaving through it, which pulls harder on the intake side and increases the total exchange. Opening everything equally gives you a gentle drift instead of a flush.

One fan or two?

With one fan, put it where it solves the weakest part of the path: bringing air into a single-sided room or exhausting it from a cross-ventilated home.

With two fans, avoid pointing both at the same room. Use one to support the cooler intake and the other to exhaust on the warmer side, with interior doors open between them. If the fans fight each other or simply recirculate one corner, turn one off and simplify the path.

Fans and people

For cooling a person rather than a room, the rules are different. Use oscillating mode rather than a jet aimed at your face, which dries eyes and airways over a long evening. And in an empty room, switch the fan off: it is heating the air very slightly and cooling nobody. More on this in how to cool your body down.

Timing still rules

None of this helps if the air you are pulling in is hotter than the air you already have. Fan placement is a Phase 2 activity: it belongs to the hours when outdoor air is genuinely cooler than indoor air. During the day, the fan cools you, and the windows stay shut. See the three phases of heat.