The difference between a UDSB and a USDB-D is that a UDSB-D unit can be connected to ductwork therefore it has a centrifugal fan attached to facilitate airflow. Please see the UDSBD-4E installation manual (https://www.reznor.co.uk/wp-content/uploads/2023/12/Reznor-UDSBD-4E-Installation-Manual-EN-2019-02-PN-06-00116-UDSBD-R2.pdf) for more information.
What is the difference between a UDSB & a UDSB-D ?
Information on Reznor's range of warm air heaters