All fans can be run horizontally or vertically.
The problem is with sleeve bearings and positioning. But you have to understand what a sleeve bearing is.
Sleeve bearings resemble a paper towel cardboard inner roll....the cardboard tube left after the towels are used. The shaft the fan is mounted on slides inside the sleeve bearing with a few drops of oil applied. Simple, right?
There are a couple of weaknesses though. First weakness is the shaft tends to pick up grit, dust and slowly move that junk down into the sleeve, making a grit paste that grinds on the sleeve's surface making it wear quickly and generate horrible noise when running. Another weakness is when mounted horizontally, the oil tends to migrate down the shaft and bearing, eventually leaving the shaft and sleeve almost dry and unprotected and not lubricated.
Ball bearing systems, all the hydro-type bearing systems, etc., all address these weaknesses and tend to work much better, last longer and be indifferent to mounting positions. The down side to these bearing systems is they tend to be slightly louder when running than sleeve bearing systems.....at least initially.
