Duct duct = selobject is Duct ? duct = selobject as Duct : duct = null;
if (duct != null)
{
LocationCurve ductcurve = duct.Location as LocationCurve;
Curve curve = ductcurve.Curve;
Line line = (Line)curve;
XYZ oldPlace = line.Origin;
double _Z = {новая отметка воздуховода} + oldPlace.Z;
XYZ newPlace = new XYZ(oldPlace.X, oldPlace.Y, _Z);
ElementTransformUtils.MoveElement(doc, duct.Id, newPlace);
}