This 20 in. dia. x 25 ft. L Flexible Duct is for HS4000NG/LP. The Heat Star HS4000NG/LP dual fuel direct fired indirect heaters can be installed in the space to be heated and ducted for the exhaust to go outside therefore reheating the warm air and resulting in less heat loss over time. Use this 20 in. x 25 ft. duct hose with the F151225- 20 in. duct adapter.