A coworker of mine is looking to replace his old furnace, and since I'm going through the same process I've been trying to educate him on all the stuff I've learned from this site. In particular, I got him to convince two of his four bidders to perform heat load calculations. He got two very different results.
After talking to the contractors to try to pin it down, it looks like they treated his basement very differently. His basement is unfinished (and will stay that way), and has no supplies or returns. The furnace is in the basement, and the ducts run along the ceiling, then go up into the first floor. There is a door at the top of the basement stairs, which is generally closed but often open, and not particularly tight.
Contractor A treated the basement as part of the conditioned space. He computed heat loss through the walls, floor, and windows of the basement, and factored those into the total heat load to choose a furnace size.
Contractor B treated the basement as an unheated space. He computed heat loss between the first floor and the basement (the joists are not insulated) but didn't count the basement walls, floors, windows, etc.
The difference came out to roughly 12000 BTU, which sounds nontrivial to me. In this particular case, the higher result pushes him over the line for a 80k BTU furnace and into the 100k range.
Which method is correct? Is there a definitive answer?