eli5: If hot air rises and cool air falls, why are we told to have our ceiling fans blow up in the winter and down in the summer? Wouldn’t it make more sense to pull the air in the opposite direction it naturally goes to help it circulate?


eli5: If hot air rises and cool air falls, why are we told to have our ceiling fans blow up in the winter and down in the summer? Wouldn’t it make more sense to pull the air in the opposite direction it naturally goes to help it circulate?

In: 6539

When you’re hot you sweat, even a little bit. THe air blowing on your skin feel cool. In the winter you want the cool air lifted off the ground and the hot air pushed down to the ground wo having a breeze directly on you.

I didn’t even know fan blades were supposed to be swapped seasonally so I’ve already learned something new.

The air coming down blowing on you on you feels cooler in summer, and reversing in winter draws cool air up which pushes warm air out and down as it’s displaced by cooler air being forced up.

It’s the difference between being in a room where a fan is circulating the air and sitting directly in front of the fan. Sitting in front of it will have a cooling effect you don’t want in winter.

In the summer, it’s not the circulation you want, it’s the breeze, you want the air blowing down on your skin to evaporate your sweat and cool your body. A fan literally does nothing to cool a room (actually it heats it up) if you’re not under the fan getting directly blowed on.

In the winter you *don’t* want the breeze (because you’re already cold) you want that hot air to come down off the ceiling back to the level you’re walking around in. So the fans blow up, so there is no breeze, but that upwards flowing air *pushes away* the warm air at the ceiling. The air moves until it hits a wall then flows down the wall to the floor. So in the winter the upwards blowing fan circulates the air to bring the warm air down to you.

EDIT – since this is blowing up a bit, if you look at your ceiling fan you’ll see what is typically a black two-position toggle switch. That’s the switch you flip that reverses the blade spin from summer to winter back to summer modes.

EDIT 2 – to be triple clear, if you leave a fan in “winter mode” and use the fan in the summer, you are literally making the room hotter and getting none of the breeze benefit of the fan. You are literally making the situation worse by running a fan in winter mode in the summer.

EDIT 3 – because I worded Edited 2 Poorly – the fan motor itself produces negligible heat. I’m didn’t mean to imply that the winter mode-in summer thing made the room hotter via the fan motor. I meant to state that winter mode in summer will just circulate down the hottest summer air down from the room’s ceiling and bring it down to human level. Considering you’re running the fan because you already feel too hot, bringing down the hottest air in the room to you, but also doing it slowly enough that you don’t get any nice breeze from it, it’s a poor choice.

EDIT 4 – Oh, ok, so final edit. A good number of people have challenged my 2nd and 3rd edits (in a surprisingly constructive & polite way, yay!). So, *I think I’m still right*, but whatever, who the hell am I? I could be totally wrong! I’m in construction, not a doctor of thermodynamics. That being said there are some great “You’re wrong because ….” below that are worth reviewing for debate’s sake.

In the winter, the warm air in the room “pools” around the ceiling. The air dragged up from a ceiling fan in winter mode pushes the warm air away from the ceiling and gets it to circulate back into the rest of the room where you can feel it.

I like the people that mention that a fan only cools you if it is blowing air at you, and technically warms the room (slightly). Throughout my life I’ve stood aghast at people who insist they need to leave their fan running while they are gone, cuz the room will be cooler (somehow?) when they return.

In the winter, the ceiling fan blows warm air down from the ceiling (where it would tend to stratify.)

In the summer, it doesn’t matter which direction the air blows, you just need air movement on your skin to help cool down by evaporating sweat.

Fans don’t cool by moving cool air down to you. They cool by moving air across you body. The difference in air temp between you and the fan is going to be pretty small.

Wait, there are ceiling fans that have the option to blow in both directions?… Today I learned a new thing

Air that’s close to the same temperature mixes readily — it makes no difference if you make the fan blow up or down.

Or rather, the only difference is whether you want to feel the breeze directly on you or not



It also depends on whether you have your vents coming from the floor (as I have noticed up north) or the ceiling (like I often see down south).

Pro tip, clean your fan before you flip the switch. U don’t want all that accumulated dust to go everywhere

Ever had a fan blowing cold air directly on to you in winter? That’s why.

I would submit that running the fan in “winter mode” is actually an effective way to cool you in the summer. As stated in previous posts, it forces the air up, across the ceiling and down the walls. Most people put their furniture against the walls, so you get the breeze you want throughout the room without having to be directly under the fan. Run it at higher speeds in summer to get a breeze, and lower speeds in winter to circulate the warmer air from the ceiling. You can also benefit from the Bernoulli effect at any time of year, making you HVAC more efficient.

Amazed that people
A: didn’t know fans could change direction, and

B: were so wrapped up in the idea that the density of air changes at different temperatures, someone needed to explain that a breeze feels good when it’s hot but not when it’s cold.

Fans are very good at pushing air, but very bad a pulling air (unless there is some kind of enclosure). Put a balloon one meter in front of a fan and one meter behind it and you’ll see what i mean.

So you turn your fan on blowing up in the winter to push the hot air up there around the rest of the room.

In the summer, you want the fan blowing on you to help with evaporative cooling, aka sweat.

Also, within a minute or so of turning the fan on, you’re moving enough air to effectively eliminate the variation between the ceiling and the floor. The higher the fan speed, the more effective this mixing effect becomes. Ceilings typically have few obstacles to air flow than floors, so “winter mode” achieves a more effective circulation effect.