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.

In the Sizing Criteria section of the dialog box, set the Sizing Method to Same as Previous. Ducts that have this set will be drawn to match the size of the previous duct.