In a nutshell? To provide the same flow (CFM or cubic feet per minute), a small fan (like a box fan) has to move air a lot faster than a larger fan (like a ceiling fan). And to move air fast, it takes disproportionately more pressure — and hence energy — than it takes to move it slowly.
The rest is just arithmetic.
I’m not talking slightly more efficient. It’s well-established that ceiling fans are the most efficient type of home fan you can buy (reference).
I’m talking on-order-of five times as efficient as a box fan. That, comparing the elderly ceiling fans in my house, against the most efficient modern box fan currently sold at Home Depot.
My main point is that the efficiency advantage of ceiling fans is rooted in basic physics. It’s purely a consequence of their larger size. It has nothing to do with (e.g.) the grilles on the box fan or the efficiency of various styles of electric motors. It is simply that to achieve some given rate of air movement (cubic feet/minute), it takes far less energy to move a large volume, slowly, than to move a small volume, quickly.
Not only are ceiling fans more efficient than box fans, they always have been, and always will be. It’s not the motor, or the housing, or the grille, or any of that. It’s just physics.
Edit: This also explains why bathroom fans are so slow at clearing the air. If you wanted a bathroom fan that could move as much air as a box fan, it would require a 500 watt motor (Post #1859).
Background
I just had occasion to look up the performance statistics for my ancient Windward III ceiling fans by Hampton Bay. I chose these almost 20 years ago because they were designed to be efficient, with (e.g.) aerodynamic blades rather than flat blades. Energy use and air movement are as follows:
Low, 12.5 watts, 2425 CFM
Med, 26 watts, 3817 CFM
High, 53 watts, 6102 CFM
The startling thing is that these elderly ceiling fans are vastly more efficient at moving air than the best modern box fan.
For example, the most efficient 20″ box fan currently on the Home Depot website is this Hampton Bay model . On high, it moves just under 2300 CFM on high, and uses 54 watts.
Comparing that to the low setting for the ceiling fan, my ceiling fan (on low) is more than four times as efficient as the most efficient box fan at Home Depot (on high). The ceiling fan moves (2425/12.5 =) 194 CFM per watt, the box fan moves (2300/54 =) 43 CFM per watt. That’s when both fans are moving roughly the same total amount of air per minute.
Just to be clear, there’s nothing hugely special about my old fans. Based on this listing, typical modern ceiling fans move even more air per watt than my ancient ceiling fans do.
Why? Why are ceiling fans so much more efficient than box fans?
I thought electric motors were all pretty efficient. So it’s not likely to be the motor. What’s the reason for this big systematic difference in efficiency?
A big, slow-moving fan blade is inherently more efficient.
Below, I guess that’s why a big brand-name in ceiling fans is Casablanca. I’m guessing 45 RPM at most, for the ceiling fans in the Blue Parrot. Near as I can tell, there are no ceiling fans in Rick’s Café Américain.
1: Even a little back pressure greatly reduces fan efficiency.
My first hint comes from engineeringtoolbox.com, which is my go-to website for any sort of basic engineering calculation. There, the main graphic under fan efficiency shows that even a little back pressure is a bad thing:
OK, maybe not the clearest graph I’ve ever referenced. Let me get to the bottom line. If you are trying to maintain a moderate air flow, a one pound-per-square-inch (PSI) back pressure on your fan leads to a roughly 8-fold increase in energy consumption.
Is that plausible? Well, a 20″ box fan moves a column of air that’s about 300 square inches in cross section. To move that, against a 1 PSI back pressure, would require overcoming about 300 pounds of force.
In fact, “static” pressure on most household HVAC fans is only a fraction of a PSI. This reference says that about 0.07 PSI is typical.
2: A 20″ box fan needs to move air 7x faster to achieve the same CFM as a 54″ ceiling fan.
Just math. The cross sectional area of the 54″ fan blades is about seven times the area swept by a 20″ fan blade. Therefore, to move the same CFM through that area, the box fan has to push the air seven times faster.
3: Moving air 7X faster requires about 49x as much air pressure.
Why does this matter? Putting all other design issues aside, assuming I’m reading the engineering tables right, air pressure required to move air goes up with the square of air velocity. It takes about 49x as much air pressure, to move air along at 7x the velocity. Assuming laminar (non-turbulent) flow.
Put aside all the design differences. for example, the front and back grilles on the box fan. This square law means that, to move the same amount of air, the little box fan is pushing 49 times as hard, per square inch, over an area that is 1/7th the area of the ceiling fan.
Ergo …
4: All other things equal …
Again, ignoring any design differences, the small box fan should require about seven times as much energy as the ceiling fan, to move the same amount of air.
So, kind-a-, sort-a, for identically-designed fans, the 20″ fan is going to require about seven times as much power as the 54″ fan, to move the same amount of air.
The fact that the modern efficient box fan only requires about 4.5 times as much energy as my ancient ceiling fan is arguably due to the age difference. I would hope that electric motors and such have gotten better in the meantime. And if I look back to this reference, they seem to show 280 CFM per watt for the average modern ceiling fan. Which would be about six times as efficient as the box fan. That’s close enough to the theoretical value of 7x, for me.
Upshot.
Box fans are vastly less efficient than ceiling fans.
It’s not the grilles. It’s not the size of the electric motor. It’s not the shape of the fan blades. Or the box that encloses it.
It’s the speed of the air that has to flow through it. Or, alternatively, the speed of the fan blades pushing the air.
Big, slow moving blades, pushing a slow stream of air through a large cross-sectional area, are inherently more efficient than smaller, faster moving blades, pushing a faster stream of air through a smaller cross-section.
In fact, this is such a basic aspect of airflow that you can see this effect in the specifications for the newer version of this fan. As you crank up the air speed, you crank down the efficiency.
The inefficiency of box fans is not due to some specific aspect of their design. It’s basic physics. Ceiling fans are and will always be more efficient than box fans. To move a given volume of air, the large size and slow speed of the ceiling fan out-perform the small size and rapid speed of the box fan.
