Complex sentence: Dual duct systems provide cool air in one duct and warm air in a second duct to provide an appropriate temperature of mixed supply air for any zone--an extra duct, however, is cumbersome and expensive.
Split into simpler sentences: Dual duct systems provide cool air in one duct and warm air in a second duct to provide an appropriate temperature of mixed supply air for any zone. An extra duct, however, is cumbersome and expensive.