I was in one of the big box home improvement stores yesterday and thought I’d take a look at ceiling fans while I was there. Ten years ago when I was building a house and buying a bunch of ceiling fans, it wasn’t so easy to figure out which fans were energy efficient and which weren’t. That’s not the case anymore because every ceiling fan I looked at had a label on the box that tells you how much air movement you can expect for each watt of electricity you put into the fan.
In this blog, I’ve talked about air flow in duct systems and bath fan air flow, but I’ve only written one previous article about ceiling fans. That one was about what a waste of energy it is to leave fans running when no one’s in the room. Today I ameliorate that deficiency as we take a quick look at the energy efficiency labels on ceiling fans.
Air flow, whether you’re talking about duct systems, bath fans, heat recovery ventilators, or ceiling fans, is measured in cubic feet per minute, usually called simply cfm. The higher the number, the more air is moving. Energy usage can be a bit confusing because of the whole rate versus cumulative quantity issue. The watt is the unit for the rate of energy consumption. The kilowatt-hour is the unit for the quantity of energy consumed.
Efficiency of any type is generally an output divided by an input. We can go with either rates or quantities when we look at energy efficiency of ceiling fans. For example, we could give the efficiency in terms of the rate of air flow per unit time (cfm) divided by the energy use per unit time (watts), which in fact is what we do. The labels on the boxes I looked at all showed a box with three numbers, all specified for the fan running at high speed: the air flow (cfm), the rate of electricity use (watts), and the air flow efficiency (cfm/watt). You can see this in the label shown above.
The range of ceiling fan efficiencies that I saw in the store ran from 35 cfm/watt on the low end to 106 cfm/watt at the high end. Most of the fans were in the 60 to 75 cfm/watt range. One factor that affects this number is how big the fan is. That 35 cfm/watt fan was a tiny 24″, whereas the 106 cfm/watt fan was 52″. The range of efficiencies for the larger fans, according to the label, is 51 to 176 cfm per watt for fans that are 49″ to 60″.
A few of the boxes had more energy data than the simple box shown above. They also happened to be the fans that had the ENERGY STAR label. In addition to showing the numbers for high speed, they also showed the air flow, energy consumption, and efficiency for low and medium speeds.
If you look carefully at the two labels above, you’ll see one other thing there that I saw on nearly every label: a money saving tip. Here it is:
That’s right. Fans don’t cool the space. They cool people. They actually heat the space because the motor gives off heat, and even the air movement turns to heat.
Now you’re ready to head off to the store and buy yourself an energy efficient ceiling fan. If you’re looking for fans at the really high end, you’ll probably need to look somewhere other than the big box stores. The label says you can get a large fan that gets up to 176 cfm per watt, but 106 cfm per watt was the highest efficiency at the store I visited.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a popular book on building science. He also writes the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.
Using Ceiling Fans To Keep Cool Without AC by Martin Holladay at Green Building Advisor
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