I recently had a home inspection in preparation for my home to be sold. The inspector cited that "The heating units have flexible gas lines entering into the burner chambers". I live in Louisiana and can not find anything about this.
Hard steel pipe connected from the gas valve inside of the unit extending out of unit (not sure of how many inches it must protrude but not many ) with a drip leg (to catch debris in gas) and then connected to the flex. No two ways about it, flex can not go into a unit.