Ventilating Fan Questions

13 years ago

I am looking for a fan for my GH. I have calculated that something in the 3000-4000 CFM range will work, and I am limited size-wise to one that fits in a 20-1/4" opening.

There are a few out there that would seem to fit, and one company in particular that has two fans that would work, a variable speed one with a 1/3hp motor and a 3 speed one with a 1/10hp motor. Both 20" fans with louvered shutters.

The 3sp one is the one I am favoring (lower cost, more efficient use of power), however I am having some trouble believing that a 1/10hp fan can really push 3000-4000cfm of air. Especially when their 1/3hp fan will only push 5000cfm with 3x the power.

I am curious if the folks can comment. Do those CFM numbers seem real? Can a 1/10hp motor really move that much air?

TIA

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Comment (1)

fruitnut Z7 4500ft SW TX
13 years ago
Steve:

My greenhouse was designed by a professional engineer with three 36 inch fans. I think two had 1/2 hp other 3/4 hp. But like you I looked at the charts and saw that 1/4 hp moved 2/3 the air of 3/4 hp. So I changed them all to 1/4 hp rated at about 8,000 cfm. Two works 98% of the time. Only run three under worst conditions. The third lowers temp 1-2F more than two. My greenhouse is 32x54x16ft.

So my fans are moving 32,000 cfm per hp. That's about like yours at 3,000 per 0.1 hp. But it's hard to tell what the actual cfm is running.

If that fan isn't enough then installing a wet wall is an option over more air movement. In our dry climate evaporative cooling is the ticket. Mine is probably cooler with one fan and the wet wall than three fans without.
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