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Old 07-03-16, 11:32 AM   #1
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Default Real World Ceiling Fan CFM

Has anybody done testing on delivered CFM of ceiling fans? Energy star has a test room where the blades are 40 5/8" from the ceiling, but this rarely happens in living rooms. Perhaps manufacturers should list actual CFM output at various mounting heights instead of the laboratory 40 5/8" that almost never exists in the real world.

How is CFM affected when installed at the default 12" from the ceiling? When used with taller ceilings, does installing a 12", 18", or even a 24" downrod significantly affect delivered CFM?

How does blade diameter come into play? Are smaller fans "more tolerant" of being mounted closer to the ceiling? Is a 42" fan a better choice than a 60" for a typical 8' ceiling due to limited space above the blades?

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Old 07-04-16, 03:26 PM   #2
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That's a good question. The only thing I found while searching for that info was a reference to the optimal height being 8-9+ feet, probably to minimize the space between the fan and the person being cooled, and that there was some maximum distance at which fans were within their rated efficiency.

My guess is that most ceiling fans are relatively inefficient, and focusing on blade design probably gives better bang for the buck.

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Old 07-04-16, 04:46 PM   #3
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That sounds like a meter of space, which is a lot. But I am not sure what CFM is in relation to a ceiling fan, and how would it be measured? The fan blades churn the air, and moving air is what we want.

I think the size and shape of the room would affect the air flow as much as anything. How close are we sitting to the fan?

I go by the blade design - the closer to a wind turbine blade, the quieter it will be relative to the amount of air flow it generates.
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Old 07-07-16, 06:03 AM   #4
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The size of the room affects a lot for any equipment installed in that particular room. Whether it is a ceiling fan, lights, heater or air conditioners, all these appliances working gets affected by the room size.
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Old 07-22-16, 07:23 AM   #5
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As long as the fan is installed with the stock hardware, it will deliver at least 90 percent of its maximum capacity. If mounted on a peak or on a slanted (vaulted) ceiling, and the blades closely approach the ceiling, ground effect turbulence comes into play and airflow suffers. The same thing happens if the fan is hung too close to a wall.

This can be demonstrated easily with a common box fan. With the fan on some sort of pedestal, it can be set up next to a wall. Measuring airspeed with an anemometer, the value will not drop until there is sufficient restriction to choke the fan. The actual distance depends on fan speed, but even on high, the distance is around 8 inches with a typical box fan. The relationship is somewhat exponential.

The government has taken the liberty of forcing manufacturers to publish airflow and cfm per watt ratings on retail units on the outside of the package. As always, some standardized fixture is required to perform the test. I imagine the reason a meter of distance is specified is so a big arse 8 foot industrial fan can be tested on the same fixture as a little 30 inch fan. For what it's worth, the cfm per watt is also an exponential relationship. Going up a size from the energy star rating will net 50 percent better efficiency or more.

Most residential ceiling fans are not highly directional, so if they are mounted too high from the floor, they cannot throw the air far enough to completely mix the air in the room. This is where the rod and chain extensions are used. Most all are designed for a maximum of 9 feet of throw to minimize blade noise. So if a unit is mounted on a tall vaulted ceiling, a long enough extension should be used to get the air thrown all the way to the floor. If you don't care if your knees are comfortable, the fan can hang 10 or 11 feet above the floor. It will still do its job just fine, but your knees aren't going to feel any wind.

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