Should I keep fans on all the time in hot weather?

My mother and I got into a big argument today over our beliefs. We have conflicting beliefs on a subject.

I believe that having fans on all the time wastes electricity. My mother believes that you absolutely have to have fans on all the time in every room to keep the air as cool as possible. I claim that it makes no difference if nobody’s in the room at the time the fan’s on, and that if you just turn the fan on when you’re in the room, it’ll be okay.

What’s the straight dope?

The fans are really only able to do a few things: move air from a cool place to a warm one, circulate the air in a room (or throughout the house) so it’s all a more uniform temperature, and create a breeze which causes sweat to evaporate from people faster so they get cooled by evaporation (and, to a lesser degree, assuming it’s cooler than body temperature, convection).

If the house is all about the same temperature, and you’re not blowing air from a room which is cooler than the rest of the house (such as the basement), then the only thing a fan can do is cool people off if it’s blowing on people.

A fan will keep the air ‘mixed’. That is, it’ll move the cool air up off the floor, but it can do that pretty quickly. The main way a fan cools is that it moves air across your body and makes you feel cooler (and actually does cool you a bit). When no one is in the room it’s wasting electricity. Not only that, a fan actually raises the temperature of the room. Air friction against the blades and the heat that the motor generates are working against you. In anything bigger then a closet, you’re not going to notice it, but it’s still there and it’s one more thing to add to your argument.

Without introducing something cool into the room (evaporating refrigerant, cool water etc), a room won’t get cooler by simply spinning some pieces of plastic around*.

IMO, if you’re going to be leaving the room for move then 10 or 15 minutes, it’s worth the extra effort to turn the fan off.

*Unless you’re using the fan to move air from a cooler part of the house or exhaust hot air out of the house and pull cool air in, but I get the feeling you’re just talking about circulating air in a single room.

Fans have hot little electric motors, in a closed system they will increase the temperature.

I have not turned on my A/C at all this summer, one of the hottest on record
for where I live (Greensboro NC) including two days in the 100s.

I don’t know about what a fan might do to affect general room temp, but having
a fan blowing directly on me has been a tremendous help. Without the fan I would
be tempted to turn on the A/C. With the fan I am not tempted at all.

Does the OP have air conditioning? If so, then that’s a different answer than one if she doesn’t.

My grandfather never had air conditioning in his house, and he had a complicated system of windows open/closed depending on the time of day, and fans on/off or blowing in/out depending on the temperature inside and out. He did a surprisingly good job except for the most brutally hot days of the year.

Whatever you do, don’t leave a fan running overnight in a closed room. Every Korean in the room will die!

OP here. We don’t have air conditioning.

And yes, TriPolar, I am aware of the dangers of fan death. :stuck_out_tongue:

A fan is not a major electrical draw. The difference in your electric bill is negligible whether you leave them on or turn them off when you leave the room.

Just so everyone really understands this, the cost of running a typical fan for 24 hours is about seven cents (24 watts @$0.12 k/w). Turning the fan off for an hour saves $0.0029 cents.

I point this out, not to deflate the OP’s claim that the fans are wasting energy, but to help folks illustrate how inexpensive electricity is, compared to the life-saving effects of air-conditioning during a heatwave. There was a story on the news last week about an elderly woman who had died, and whose son said the mom kept turning the AC off when he left because “it cost too much.”

A small window unit that draws 500 watts costs six cents an hour to run, or $1.44 to run it all day long. It’s probably wasteful to run it when it’s 60 degrees outside, but the idea that you’re saving money while you’re sweltering is poppycock.

As for the OP’s question, she is right. The fans work by creating a wind chill effect, not by actually cooling the room. No benefit is created when the room is empty. However, that’s not the case if we’re talking about an exhaust fan which is pulling hot air out of an attic, for example. it’s also not true if the fans are used in conjunction with A/C units, and thus assisting in spreading cooled air to other rooms.

the ability to use a window air conditioner to cool a room is an all depends.

it could cost $50 to $100 a month to run depending on the age of the unit. some locations may not have the electrical service to run one no matter what the cost of electricity; i.e. it will blow the fuse.

Thanks, all!

I’ve heard this before, and with ceiling fans as well, but do they really raise the temperature to any noticeable degree? The box fan in my room is always on, but doesn’t feel warm at all when I touch it; unlike my TV, computer, and especially my wireless router which is so hot I could probably fry an egg on it. :eek:

temperature, by feel, might not be a good indication of heat put into the room. a fan cools the motor by its functioning as an appliance.

Also, it’s probably pretty negligible in a full sized room, but heat is heat and why add it to the room if you don’t have to. I usually don’t give people grief over it unless they tell me how much money they’re saving by turning the AC off and leaving their oscillating/ceiling fans on all day. At that point I think it warrants and explanation. Turning off (or raising the temp) is one thing, but leaving the fans on won’t keep the place cool when you’re not home. You’re better off closing the blinds/curtains and keeping the sun out.

I’ll bet if you put a fan in a small closet and let it run for a few hours it would get pretty warm in there after a while. It’s the same in a large room, but if you do it like this you’ll concentrate the heat so that you notice it.

Under the right (wrong) set of circumstances, in a commercial cooler the compressor can stop working but leave the blowers in the cooler running. If that goes on long enough, the temp can rise to well over room temperature. If this happens at night (when the display lights are often) this is just from the heat generated by the motors* and the air friction. It looks like I have about 220 watts worth of motors in this cooler, but I was still surprised at how warm it got. In fact, one thing most stores will do when a compressor dies is to turn off the lights and fans.

I live in Florida and I have a two story house. I put a window fan in the upper floor and I turn the window fan on at night and turn the AC off. In the morning, I will wait until the outside temperature gets over 80 and close the windows. If it gets 84 indoors, then I will turn the AC on. It just started raining, so I turned the AC off and turned the fan back on. I always have a fan sitting next to me. My electric bill is much smaller than any of my friends.