The Real Spin on Keeping Cool With Ceiling Fans

Fan mail from some experts who explain how fans make us feel cooler.

Before air conditioning became common, fans were often used to keep the air moving, which cools people by evaporating sweat. I thought that was all they did, which is why I have quoted Green Curmudgeon Carl Seville, who says Ceiling fans are evil. He wrote in Green Building Advisor:

I am surprised at how few people understand the basic concept of fans — that they make you feel cool due to the movement of air across your skin. The same way a breeze cools you off, a ceiling fan can make you feel cooler, but only if you are close enough to it to feel the air blowing on you. If you can’t feel it, it isn’t doing any good.

That’s why it is pointless to have a fan on when nobody is in the room; then it is just generating heat from the motor, which is why they are evil -- they are heating when you want cooling.

© Batman Fan
FromCatchy headline of the day: "Ceiling fans are evil"

Those are two points from Carl, but TreeHugger is also a big fan of Energy Vanguard’s Allison Bailes, who makes 7 points you may not know about ceiling fans. He points out that beside evaporative cooling, fans also aid in “convective cooling.” I had to ask what this means:

Lloyd, convective cooling is moving warmer air out and cooler air in. When a breeze aids in evaporative cooling, it's moving humid air away and replacing it with drier air. The former is sensible cooling since it lowers dry bulb temperature. The latter is a form of latent cooling since it lowers the vapor pressure of the air near the skin, thereby allowing more water to evaporate from the skin.

I wanted to argue this point, because heat rises, so I thought that the air moved by a ceiling fan was likely warmer than the air down where the person is, but Alison is a PhD in Physics and I’m just an architect.

© Comfort zone chart Victor Olyay

I just rely on Victor Olgyay’s wonderful drawing from his 1963 book Design with Climate, which shows that comfort is a mix of temperature, humidity and air movement. If the air is too humid, then a fan is not going to cool you because there is less evaporation.

© Raffaele Iannello

Allison makes some other important points:

• Follow the efficiency ratings that tell you how much air is moved per watt of power
• Bigger is better. “That's why the company Big Ass Fans makes big ass fans.” (I once wrote that they had a stupid name, that no architect would ever specify a Big Ass fan. I was wrong.)
• Slower is more efficient.
• And the all-important one everyone worries about: No, it won’t decapitate you.

Another point that Allison picks up from Martin Holladay of Green Building Advisor is that if you have air conditioning, a fan won’t save you any money. “The hypothesis is that people will raise the AC thermostat setting if they're feeling the breeze of the ceiling fan, but the data don't support it.”

Carey Smith of Big Ass Fans/Promo image

Allison turns the thermostat up, but most people don’t. However, in one of the few examples of smart home tech that I think is actually useful, the Nest thermostat can talk to the Big Ass Haiku fan and adjust the thermostat accordingly.

According to the press release, each degree you raise your Nest Thermostat saves you up to 5 percent on energy costs. A fan can make you feel quite a few degrees cooler, and Big Ass notes that "If each home thermostat was raised 6 degrees, we would reduce carbon emissions by 78 billion pounds, the equivalent of taking 3.2 million homes off the grid for a year."

That is probably an exaggeration since fans might work in the dry southwest but not in the humid southeast. But if you have a smart fan and a smart thermostat and live in the right place, it is possible that you might actually save a few bucks.

Read it all at Energy Vanguard.