Locate the manufacturer wiring diagram for the motor that powers the whole house fan. This information will be in the manufacturer's documentation that came with the fan, or the manufacturer will have to be contacted for the the information.
Look at the wiring diagram and see if the motor has a startup winding. If a startup winding is present, the motor can be reversed. If a startup winding is not present, the motor cannot be reversed.
Locate the two end wires of the startup winding. Disconnect each of the two wires and reverse their connections. For example: if wire "A" of the startup winding is connected to terminal "A" and wire "B" is connected to terminal "B," then the motor can be reversed by connecting wire "A" to terminal "B" and wire "B" to terminal "A."