When placing round connectors in families, Revit allows you drive the dimension of the connector with either the diameter or the radius, but there is a catch.
All of the round connectors must be driven the same way.
When editing an existing HVAC family to add some hydronic connections, I couldn't change from diameter to radius. A quick test revealed that the round duct connector already set to diameter was locking all round connections to diameter. I deleted the duct connector and the radius choice reappeared.
In this case "all or nothing" set to radius was not a problem, it actually cleaned up the family. It just just a quirk that good to know about.