To merge two rooms into one room

Click the Merge icon.
The Merge rooms tool settings appear.

Note: The New name text field is disabled.
Select the first room/plenum that you want to merge.
(Optional) Attempt to select a room/plenum that is not adjacent to the first one you selected.
Merge rooms will not accept the disconnected room/plenum.
Select another room/plenum that is adjacent to the first room/plenum that you selected.
The New name text field is now enabled, and a default name is provided; [room/plenum name 1] + [room/plenum name 2].
Enter a name for the new (merged) room/plenum. Reset to complete merging.
The partition between the rooms/plenums is removed. Room components (Ventilation, Gains, Lighting components) are also combined into one object in the room/plenum hierarchy.