What uses less energy? Fan vs A/C?
May 3, 2010 10:19 AM   Subscribe

What is more efficient: a fan or a window air conditioning unit?

I have a tower fan and a window A/C unit for cooling my bedroom (about 100 sq. feet), but I'd like to keep my power bills down if possible. What would be more energy efficient in hot, humid weather? Open windows and fan or closed windows and A/C (and fan if needed)?
posted by youcancallmeal to Home & Garden (8 answers total) 2 users marked this as a favorite
The fan, by far, but it won't do anything to make it cooler inside.
posted by Threeway Handshake at 10:22 AM on May 3, 2010

The fan uses far less energy, but it also does a much worse job of cooling you off. AC units lower the temperature in the room they are at work in, fans use evaporative cooling to cool off something directly in front of the fan. The room stays the same temperature, the person gets cooler.
posted by OmieWise at 10:23 AM on May 3, 2010

You need define "efficient" for your purposes. The fan uses far less electircity, but won't cool the room. The AC uses a lot more electricity, and will cool the room.

How hot does it get?
Does it cool off significantly in your area at night?
How much of your day do you spend in the room?
Do you work a night shift (and sleep during the day)?
Do you have a hard time sleeping if it's warmer than X degrees?
Does the AC unit have an thermostat that it uses to automatically shut itself off when a certain temp is reached?

In my opinion, it's probably most *cost* efficient to use the AC unit to knock the room temperature down to a temp approaching comfortable, then use the fan with the window closed. This assumes it's warmer outside than inside, there's no significant breeze flowing through your window if it were to be open, and your room won't rapidly heat up again.
posted by terpia at 10:31 AM on May 3, 2010

There's just a trade-off between energy efficiency and a decent indoor climate. Go for the fan if you prioritize the former, and AC if you prioritize the latter.

Tip: if you go the fan route, try to replace or supplement the tower fan with one that can fit in your window. We live in Toronto (godawful summers), and with a powerful circular fan in the bedroom window, we were able to cool off our otherwise stifling bedroom pretty well. Without it, the room acted like an oven for hours after sunset.
posted by Beardman at 10:33 AM on May 3, 2010

The most energy efficient things you could do is ensure that the room/house are properly insulated and shaded.

Hot sun blazing onto a window? Try something like this.
posted by Cool Papa Bell at 10:40 AM on May 3, 2010

As everyone else has said, you have to trade off between coolness and energy use. It seems like you're asking, however, which hits that "sweet spot" best, the sweet spot being the best trade off between energy use and coolness. This will be affected by a few different factors:

- When you want it to be cool: If it's at night, for sleeping, and you have some other factors in your favor, a fan might do. For daytime, a fan is almost always going to do a poor job of cooling.

- Humidity: the more humid your climate, the more heat stays at night. However, if you live in a place with relatively low humidity, then you can do some things to make your house cool without AC. For instance, keeping the shades and windows closed during the day, and opening them at night, making use of fans and cross-breezes to bring the cooler night air into your house. However, if you live someplace with high humidity, this won't work as well.

- The structure of your house/apartment. I used to live in an apartment in Thailand where a fan, in the middle of the hottest part of the year, was all I needed to stay cool at night. Sometimes I would even wake up at night because the fan was too cold! Obviously, Thailand is quite humid, but this worked because the building was designed with heat in mind, with a fairly open structure that allowed for cross-breezes. But in Minneapolis, in the summer, sleeping without an AC was miserable because buildings in Minneapolis are made to maximize heat, not minimize it.

After years of resisting getting an AC, I finally got an energy efficient one with a thermostat. I set it at 72, which is a very comfortable sleeping temperature, and it turns off when the room reaches that temp. I only use it at night, and only in my bedroom, with the door closed. It has a pretty negligible impact on my electricity bill.
posted by wholebroad at 10:52 AM on May 3, 2010

I have found that if you:

1) Open one window in a house as high as possible, and put a big fan in it pointed out
2) Open another window on the opposite side of the house as low as possible
3) Turn that fan on full blast at night and let it exhaust all that hot air and draw cool air in through the other window

You can usually get the house nice and cool. Then, either right before bed or early in the morning, close all your windows and shades and avoid doing anything that generates heat (don't cook/bake, don't leave TVs/monitors on longer than you have to, etc.).

This will usually keep things pretty nice until the afternoon hits, at which time you can either turn on the AC for those few hot hours, try to get out of the house, or just suffer until it cools off again.
posted by Menthol at 11:04 AM on May 3, 2010 [2 favorites]

I'm a big (ahem) fan of window fans. I have one that goes almost year round, so I know a thing or two about them. Personal fans are good too. I've also considered getting a hammock or waterbed to prevent that pocket of heat between the body & the mattress that builds up uncomfortably, even when the rest of your body is at a fine temperature.

You want something powerful like this whole house fan to really get the air moving.

Small plastic window fans don't move much air, so won't do much to change the temperature in your house. More powerful window fans (and I have yet to see a plastic window fan that's got any decent amount of power) move more air, cooling the house better.

To cool off, intake is more important that exhaust. The air will find ways to exhaust, but bringing cooler night air in using a powerful fan will (as long as it's cool outside) cool things off quickly.

You can flip the fan to exhaust when it's cooler and it'll keep the temperature fairly well, while keeping the air in the house "fresh." With a decent seal around the fan (so air doesn't 'leak' in or out) you can leave the fan in the window on exhaust 24/7 without drastically changing the temperature.

Other things you can do to regulate the temperature in your home:

Those "honeycomb" blinds are designed that way to keep pockets of air (the best insulator is pockets of air - think goos down) near the window to help keep the hot air out. They're best if flush against the window since once the heat gets in, it's difficult to send it back out.

You can buy window tint especially designed to limit heat & UV while keeping the visible spectrum fairly intact. So it's not as drastic as car window tints, but does dim light a bit. You can buy them in rolls & stick them on your windows.

Consider painting your roof white - it really works at keeping the house cooler.

Geothermal heating/cooling - if you want to perform the major structural changes need to implement it - is very efficient. Basically the temperature underground is usually the average of the year-round temperature (so if you average 72 degrees, far enough below ground it will almost always been 72 degrees - winter, summer, doesn't matter) so a geothermal pump sends air down to be cooled/warmed and works to keep indoor temperature consistent year round.
posted by MesoFilter at 12:16 PM on May 3, 2010 [3 favorites]

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