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Old 07-22-16, 07:23 AM   #5
jeff5may
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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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