I read somewhere that you should use ceiling fans to cut down on your cooling bills. I just bought a house with fans in every room. Should I turn the fans on and leave them on all the time or should I only have the fans on when I’m in the room?
The fans only make you feel cooler when you’re exposed to them-they don’t drop the temp.of the space.Turn 'em off when you’re not in the room.
Agreed. It’s an issue of the air moving across your skin that helps cool you. They do you no good if you’re not there.
They actually can make things worse in most situations. Take the room I’m in now. Hot attic right above me. Heat from the attic flows all day thru the insulation and ceiling into this room The rate of flow depends on the temperature difference between the attic and the air next to the ceiling. The cooler the air at the top of the room, the faster the heat flow. More total calories in the air. Fan off, the air at the top is a couple of degrees warmer than with the fan on. Ergo, less heat flow, cooler room (slightly).
So the fan should only be on in this case if there’s a person near it. (Or other object that enjoys evaporative cooling.)
Since this involves logic and Science, 99% of the people on the planet will assume it goes the other way.
Actually, ftg, my assumption would be that you need more or better ventilation of your attic. If you have heat “flow[ing] all day thru the insulation” between you and your attic, something is wrong with the way your house is built. 
Turning on a ceiling fan will actually increase the air temperature in a room for two reasons:
- Creating turbulence in a fluid (in this case air) will increase the fluid’s temperature due to molecular firction.
- The fan motor creates heat.
So the air temperature will slightly increase when you run a ceiling fan, regardless of whether or not anyone is in the room.
But…
Though the air temperature has slightly increased, the flow of air across a heat-producing body will create a wind chill effect. This has the (very real) effect of decreasing the temperature of any heat producing object due to convection. This includes you, of course. (And in addition to regular ‘ol convection, your body has another cooling mechanism: evaporation of water. The air flow helps with this, also.)
Since you are a heat producing body, you will cool off when the fan is on. But when you’re not in the room, you’re just wasting energy.
No, no, no! It’s small portable fans that do the things described above. Or a ceiling fan rotating in the wrong direction.
If used properly, ceiling fans will cut both heating and cooling costs. You must use them to exchange air, by running them the proper direction for each season. Think of all the ceiling fans you see in old movies, before AC was prevalent. They wouldn’t have used them if they didn’t work!
You’d think in hot weather you’d want it blowing downwards, right? But nope. In the winter, heat tends to rise to the ceiling. The higher the ceiling the worse it is. Then you turn the fan on so it blows downward.
In the summer, when the cooler air sinks to the floor, you turn it on blowing upward. In both situations a leisurely speed is sufficient (and uses less energy).
If you have fans upstairs and downstairs both, Labboy, use the upstairs ones in the summer to draw cool air up, and the lower ones in winter to draw warm air down. And run 'em slow. We only have one downstairs now, but we’ve figured it makes at least a five degree difference with both heat and AC. Not earthshaking, but it adds up.
This line has to be the worst bit of reasoning I’ve seen today.
Bleeding people to balance bodily humors … they wouldn’t have done it if it didn’t work!
It’s true that in the summer you get a vertical temperature gradient, and that the coolest air is near the floor. But when you turn a ceiling fan on, it tends to increase the air temperature near the floor, no matter what direction it’s turning.
Let me back up… you seem to think that if a fan is blowing in the upward direction in the summer time, ¡t will “keep the hot air toward the ceiling.” This is not true. A ceiling fan circulates air, and for most rooms it basically eliminates the vertical temperature gradient. For example, let’s say the air temperature near the floor is 80 F, and the temperature near the ceiling is 90 F. When you turn on the ceiling fan, the temperature will come to steady state at around 85 F everywhere in the room. This is true no matter what direction the fan is turning. (This is just a first-order approximation; I’m leaving lots of other effects out.) In fact, the air temperature will slightly increase (to say 86 F) due to reasons listed in my previous post.
Here ¡s what happens: Let’s say you turn on the ceiling fan on a hot day, with the fan blowing “up.” Air in the center of the room will be sucked up into the fan. The air will then hit the ceiling (which is quite toasty BTW), move horizontally to the walls, then flow downward. The air will then flow toward the center of the room where it will get “sucked back up” into the fan. The effect of this is that the air temperature actually increases near the floor, while the temperature near the ceiling decreases. In an ideal case, the fan will eliminate temperature gradients (floor temperature = ceiling temperature).
Again, I’m leaving quite a few subtle effects out. But this is basically what happens. (Most noteworthy is what happens when there’s lots of distance between the ceiling and fan.)