Design Master HVAC uses the built-in AutoCAD hidden-line functionality to display hidden lines. The hidden lines will not automatically appear while you are working on your project.
To view them, type HIDE at the command line. This will display the hidden lines. However, it will disable the real-time zoom and pan features. To turn them back on, type REGEN. This command will remove the hidden lines and enable the real-time zoom and pan.