Thank you very much for answering my question the way that you did. It was very informative and helpful.
I have one question though. I understand calculating the load in order to find what size unit or units you will need to properly heat and cool a building. I also understand, now that you have helped me, that once you have decided what unit your going to be using to make the plenum the size of the opening on top of the furnace. My question is, How tall should you make the plenum?