A ceiling fan can heat up to about 100°F when running, adding heat to a house. If no one is sitting or standing near the fan, leaving it on is counterproductive.
Many times, I have walked past my neighbors’ homes and have seen their porch ceiling fans running—with no one there to appreciate them. All the fans are doing is wasting electricity and contributing a little heat to the outdoor air. I am tempted to turn the fans off, or leave the neighbors a note.
Leaving porch fans on is bad, but nowhere near as bad as doing it indoors, especially in the summer. Few people understand the basic concept of fans—that they make you feel cool due to the movement of air across your skin. This air evaporates moisture, which wicks away some heat. The same way a breeze cools you off, a ceiling fan can make you feel cooler—but only if you are close enough to feel the air blowing on you.
When I tell people about this concept, many say that it helps circulate the air around the room and keeps the house more comfortable. However, in the summer, there are very few situations where moving air around a house improves comfort. In the winter, if there happens to be a big temperature difference between the floor and ceiling, it might be useful to run a fan clockwise (in “reverse”) to circulate the air, but that’s not what most people do.
A 1996 study by the Florida Solar Energy Center determined that using ceiling fans appropriately for cooling could allow homeowners to raise the air-conditioner’s thermostat setting by 2°F, resulting in about a 14% annual cooling energy savings. However, the same study found that most people do not adjust their thermostats up when using ceiling fans, so using fans actually increased their overall energy use.
Obviously, running a fan when no one is in the room wastes electricity, but the dirty little secret is how much heat fans can produce when running. I always knew this, but was shown the proof when I was inspecting some well-built LEED homes. One of my associates had an infrared (IR) camera with him. Scanning walls and ceilings showed that the homes were very well-insulated and air-sealed, but when the camera caught the running ceiling fan, the camera image revealed a hot motor.
The temperature of the motor was far higher than anything else in the room, including windows exposed to direct sunlight. Not only was the fan not cooling the people who weren’t in the room, but it was also working as a little space heater.
I am not suggesting that we should stop using ceiling fans—just that they shouldn’t be on if no one is in the room. If people only use them when necessary in the summer and set their thermostats a bit higher, then the extra heat is a small price to pay for the comfort and overall energy savings.
When selecting a ceiling fan, look at the fan’s efficiency, usually expressed in cubic feet per minute (of airflow) per watt. The Energy Star website provides a list of all labeled ceiling fans in a downloadable Excel spreadsheet, which can be sorted by efficiency (bit.ly/EnergyStarFans).
The most efficient fan on the list, and among the most expensive, is the Haiku S3150, Looking at an IR image of one of the fans showed the temperature of the fan’s motor at high speed is only about 81°F. If (like me) you keep your house in the high 70s to low 80s, a fan motor like this won’t impact your home’s temperature much.
You don’t need to spend a fortune on a ceiling fan. There are many efficient models that are reasonably priced, and there is no reason to get rid of those that are working fine. You should, however, only use them when they will keep you cool, and raise your thermostat when you do.
This article was adapted from Fine Homebuilding’s GreenBuildingAdvisor.com.