Each section of ductwork in Design Master HVAC is sized independently when using the constant pressure drop or constant velocity calculation methods. This can result in a main duct run changing size at every duct branch, as shown below.
It is possible to prevent this by changing the Sizing Method setting on the ducts.
To do this, use the Query Duct or Edit Multiple Ducts command.
Ribbon: DM HVAC->Ducts-> Query Duct / Edit Multiple Ducts
Pulldown Menu: DM HVAC->Duct Centerlines->Query Duct / Edit Multiple Ducts
Select the duct that you want to be the same size as the one previous to it. To create the image shown above, we selected the three ducts shown in red below.