When a unit draws in the outside air through one duct and blows it out through a different one, pressure is lost. Centrifugal fans are best suited for such circumstances, as they enable large pressure increases by design. They are also extremely compact, taking up very little space.

When installed outdoors, noise emission needs to be kept as low as possible, especially at night. Axial fans capable of running at low speed are then the products of choice. The increasingly popular monoblock units also require solutions that are compatible with flammable refrigerants.