A 6-inch booster fan with special connectors for flex duct is also available. Another type of booster fan fits into the boot and replaces the existing register in 4- by 10-inch and 4- by 12-inch boots ...

Duct Booster® register booster fans ... cross-flow blower that is ultra-quiet (1.4 sones) and uses only 20 W of electricity. Fans are available in two sizes to fit into standard 10- and 12-inch-wide ...