Can I mount the whole-house fan vertically?

No. Whole-House Fans cannot be mounted vertically.

Air Vent whole-house fan cannot be mounted vertically. There are several reasons for this. First, the shutter will not open or close in a vertical position. Second, the fan blade will eventually crack over time. Third, if mounted on a vertical wall facing the outside the fan will pull the air from outside into the attic rather than attic air being vented to the outside. Unfortunately, the fan blades and motors are not reversible. If the motor is reversed, it will burn out. If the fan blades are reversed they will crack. The warranty on Air Vent whole-house fans requires installation in a horizontal position.