Tuesday, May 18, 2021

Ceiling Fans


Dashed line is center-line of rotation. Ellipsis are fan blades. "Eyeball" is point of arrow coming at you and "X" is fletching of arrow heading away from you.

Mrs ERJ gave me a project before she flew to a far-off state to be an angel of mercy. She picked out a ceiling fan for me to install in our living room.

Those of you who are blessed (or cursed, it is a matter of perspective) to know an engineer, you know it is impossible for us to look at a product and not think "I could make that better.

In the case of the ceiling fan, I think it is warranted.

The airflow is underwhelming.

A simple, back-of-napkin analysis suggests that the blades of the fan are too close to the ceiling to effectively "grab" air.

There is half as much area between the tips of the blades and the ceiling as there is in the swept area of the blades...or close enough for government work.

Fluids don't like to turn sharp corners. The airflow would benefit if I could raise the ceiling by six inches or so.

Or I could lower the fan blades by six inches.

Or I could lower the tips of the fan blades by six inches.

Or I could lower the tips of the fan blades by anything, even if it is less than six inches.

The pink bits are extensions that can be added to the ends of the blades as a trial to verify the physics of raising the ceiling....or whatever. Back-of-envelope calcs suggest I might get 50% more airflow.

And Mrs ERJ thought this project would keep me out of trouble. Bwa-ha-ha-ha-ha-ha...


  1. How much air is circulated also depends on the direction of the spin. Most ceiling fans are reversible. It also would depend on the speed of the spinning blades. My fan is about 8" below the ceiling and has three speeds.

  2. Don't forget the stick-on weights to balance it. Ask me how I know.

  3. They sell longer down-rods at all of my local home improvement stores.

  4. Open up the box and see if there's an extension. If I remember correctly they're basically a pipe nipple. Which can be easily replaced with a longer unit...
    If it's a newer style it'll have a ball and socket with a distinctly triangular shape. The spherical ball and socket. Much more susceptible to NVH concerns.
    Usually there's ample wire to extend the circuit.
    The longer moment of angle might amplify any imbalance. The newer fans sometimes come with a balance kit. Meh... YMMV
    as long as you're putting up a ceiling fan, now's the time to perform any desired electrical upgrades suck as 3 way or 4 way switches, separate fan and light circuits etc.
    Have fun and remember:
    White is alright, green is serene, touch red and your dead.

  5. And you'll have done the work to ensure that the fan motor bearings can handle the 50% higher load and all the electrical components will be happy with a jump in current consumption.

  6. Some older homes have low ceilings that won't allow one to lower the fan with putting it at hairline level. Have you thought about just getting some thin plywood, making a stencil, and crafting some bigger blades? You could even do it out of ABS plastic, which is even lighter and readily available.

    1. That is an interesting idea.

      The ABS would also be ideal for incorporating the 757 winglets if that proved the way to go.

  7. shape the ceiling to make the air turn better. A small concavity will do wonders.

    1. The ceiling shape comes into play.

      The builders opted for the Ponderosa Ranch House look and have faux beams spanning the room.

      Placing the fan centrally nipped about 90 degrees of arc from the feed. That shaved about 3.5" from the six inches that segment of the curtain area.

      I am not allowed to make major changes until the boss comes home.

  8. Or you could just get a longer pole to hang it on... ;-)

  9. Interested in this experiment. The fan I installed in the kids room way back when, they always complained that it didn't move air very well, and as that room may become my office soon I'll be interested in improvements I can make to airflow.

  10. One other thought for you if you're going to control the fan with wall switches rather than the pull-cords: when I originally installed my two fans I got the slider switches for light and fan from the big box where I purchased the fans. Big mistake. Cheaply made, they were broken and unusable within a couple of years. Go to an electrical supplier that will sell to the public and spend the money for good quality switches. I got the rotary kind rather than the sliders, and I haven't been sorry in the many years since.

  11. Outfit named "Big Ass Fans" has some interesting stuff; they make a 16 ft diameter ceiling fan that uses - IIRC - 6 blades, each with a winglet, and both the winglet and main blade sections are airfoil-shaped. I've seen them installed in a few places and the "air column" is MUCH wider than 16 ft, well over double that it seems.

    I suspect the secret is shaping the blades - several of their smaller fans also have airfoil-shaped blades, which seems to do better at moving air than the usual ceiling fan flat slabs set at an angle. Flat slabs are much many more cheaper, though, bot from an engineering standpoint and especially, manufacture. Big Ass Fans of all sizes have a good reputation for design and effectiveness but there's a reason they're not sold in home centers next to the $99 ceiling fans.

    Looking at smaller fans - 12" - 16" etc. - the better ones (mostly, Lasko brand, from my observations lately) have blade shape that moves more air, more quietly, than simpler blade designs.

    I remember back in the '50s General Electric made very efficient 17" 4-speed all-steel oscillating table fans with a blade shape that Lasko seems to have copied.

    As for ceiling fans, I'd guess effort put into a blade design that still fits the mounting holes, and doesn't require more power than the original motor provides, could be a marketing opportunity. I've got a 72" name-brand brand fan in a large room with 12 ft ceilings that's disappointing, despite being 3 ft below the ceiling - the air column barely extends past the diameter of the fan, and reversing it in winter doesn't accomplish much; I wound up installing a 16" remote-control Lasko wall mount oscillating fan very high up at each end of the room to "sweep" warm air along the ceiling and drive it down the walls in winter. Works better than using the ceiling fan.

  12. Ceiling fans are a kinda useful decoration on a light fixture. If you really want to move air you gotta use multiple fans and/or ductwork.
    But they've Conner a long way since I was a kid.


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