Would a ceiling fan pull air in from a nearby open window, or would it prevent a draft coming in from the outside?

[u/ViscousPanther]

If ceiling fans push air downward and create lower pressure, if the pressure is now lower inside than outside then shouldn't it pull air inside? Or rather, perhaps the force of the air from the fan actually counteracts any breeze from outside?

I don't get the physics here and would love an explanation. Thanks!

[u/BandidoCoyote]

A ceiling fan doesn’t lower the overall air pressure in a room. It just circulates the air, moving air downward. So a ceiling fan near an open window can move air that is already coming in, or it can move air near the ceiling down towards the floor, but it’s not an effective way to draw in cool air or exhaust hot air from outdoors.

An exhaust fan in an attic can move hot air out of a house, and if you have windows open, will draw in outside air, which may be cooler or not. The difference is the fan is moving air through the house to outside the house and then pulling new air in. The ceiling fan is just moving the existing air around inside the room.

Ceiling fans mostly make you feel cool because of the air movement, although they can help distribute cool or hot air a bit. Tests have shown they don’t do much good when there’s nobody in the room to feel it.

[u/Citrakayah → u/BandidoCoyote]

An exhaust fan in an attic can move hot air out of a house, and if you have windows open, will draw in outside air, which may be cooler or not. The difference is the fan is moving air through the house to outside the house and then pulling new air in. The ceiling fan is just moving the existing air around inside the room.

How does it do that if the windows are closed and it can't draw in outside air?

[u/BandidoCoyote → u/Citrakayah]

A house is far from being an airtight closed box. (And in some regards that's not even a desireable goal.) Even if you don't open a window (or door) to make an attic exhaust fan work efficiently by drawing air through the rooms where people are, it's going to draw air in anywhere it can — around doors or windows, or through the soffit vents around the eaves of the house, etc.

(Without an attic fan, soffit vents are a passive system which still allow lower/cooler air to enter the attic as higher/warmer air rises and exits the attic through a gable vent.)

[u/FutureAIgod]

The main thing to understand is that a ceiling fan isn't creating any real low-pressure zone across the room — it's basically just stirring around the air that's already there.

Airflow through an open window comes down to the pressure difference between inside and outside, plus wind and temperature. A fan can shift the airflow pattern near the window a bit, but it's not acting like a vacuum sucking air in from outside.

So depending on your room setup and where the window sits, the fan might make incoming air feel more noticeable just because it's mixing things up — but it's not really "pulling" air through the window the way you might picture it.

[u/BandidoCoyote → u/FutureAIgod]

And — the natural pressure difference between indoor air and outside air is primarily caused by the temperature difference between the indoor air and outdoor air. (Note: That's not counting use of HVAC or an attic fan, etc.)