Virtually any type of small fan can be used. The only caveate is that you must be sure to keep any water off of them. UL listed greenhouse fans are protected from water damage, shorting out, and electrocution but these cheaper fans are not. They will work great but if you choose to use them you take on the risk of fire or electrocution. I personally use the cheap box fans in my greenhouse but I hang them in areas where they are not exposed to water.
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