Let me understand this, and please tell me if I'm wrong. You have a house with an addition on the back. When the addition was built, the contractor removed 16 feet of load bearing exterior wall, and replaced it with a (must be very substantial) beam which spans 16 feet and supports what, a second floor? You don't say.
Basically, and this may not apply to your situation, but if a wood beam is being obnoxious, sometimes you can replace it with a steel beam that is smaller but stronger. In any case, it is not really a concept for a do it yourselfer, no matter how "experienced" you may be. I would have a structural engineer look at it, or a couple of contractors to look at it and give you some ideas.
Don't have anything "fancy" like steel beams or anything removed from that beam until a structural engineer stamps his approval first... This is your house and possibly your lives we are talking about here.