Choose the Select Connected Elements command from the Design > MEP Modeling menu (or the shortcut from the MEP Toolbar).
Move your cursor to any connection point on any placed MEP object. All connected MEP elements from that point are pre-selected.
Click to select these connected MEP elements.
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Select Connected MEP Elements (Command)
1.Use Edit > Select Connected MEP Elements (or the shortcut from the MEP Engineering Toolbar).
2.Move your cursor to any connection point on any placed MEP object. All connected MEP elements from that point are pre-selected.
3.Click to select these connected MEP elements.
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How Walls are Connected in Archicad
This article gives a detailed explanation on each factor that affects the way walls are connected to each other.