You can see my disconnect switches creates a line gap on the HVAC equipment they attaches to. Is there something I can change to make it transparent?
Or can I do wireframe selectively? Just the HVAC on wireframe view?
The source of this behavior is under Mechanical Settings » Hidden Line. You can either disable "Draw MEP Hidden Lines" or set the gap sizes to 0.
The above is a Project-wide effect, though. You can eliminate this per View by setting the Visual Style to Wireframe, but you cannot assign a Visual Style to a specific Category. You could also change the View's Discipline to Coordination to prevent Revit from creating these gaps.
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