How subjective are Manual J calculations? I had two contractors perform Manual J calculations are recommend different heat pump sizes for the second floor of my house. The first contractor measured the basic dimensions of the second story, size and location of the windows, and insulation quality. He said that a 3 ton unit was the perfect size and that I really couldn't go any bigger.

The second contractor measured the size of every room and window. The living room and entry way are open to the second floor so he factored that area into the calculation. Since the first contractoar didn't measure every room, I'm not sure he counted all of this space. The second contractor said that a 3.6 ton was optimal so he recommended a 3.5 ton unit.

Are differences in Manual J calculations typical?