Elevation of objects in AutoCAD MEP?

Why does MEP objects change the elevation and break the layout when changing the inlet and outlet sizes on vav boxes?

For one thing, duct inserts differently than equipment as far as elevations go. Duct inserts to the center of the duct, but you can set it so that the elevation is to the bottom or top of the duct. But equipment and devices are always inserted to the bottom of the equipment.

The problem is that the content creator made the insertion point (0,0,0 BCS) at the bottom corner. So when you swap out sizes, you are actually swapping out the parts. Since parts, like AutoCAD blocks, reference the same 0,0,0, the insertion point of both parts are at the same location in your drawing. However, since the new part is larger, the distance between the insertion point and the connector is larger causing the connector location to shift breaking the layout.

Picking an insertion point for an MvPart is an art. Typically, equipment that is placed first (before duct) has its insertion point at its base or top. For parts placed in-line after duct is drawn, the insertion point is at the connector. If you want to change the VAV boxes so that the insertion point is at the connectors, navigate to the VAV dwg file located in the catalog (typically here: C:\Documents and Settings\All Users\Application Data\Autodesk\ACDMEP_2008\enu\Aecb Catalogs\MvParts US Imperial\Mechanical\VAV Units\VAV Boxes), redefine all the model blocks to have the the appropriate connector at 0,0,0. Then, make a block of one of the symbols, then purge all the otherblocks (top, bottom, etc.). Now start content builder, modify that part, and regenerate the missing blocks. You should now be all set.