Help! Gas furnace size proposals are all different
I would greatly appreciate any feedback on what would seem to be the correct furnace size. I have received 3 different bids for Trane XV95 95% efficient 2 stage variable fan furnace. All three contractors proposed different size BTU units including 60, 80 and 100btu. One said he did a load calc, to which I saw him measuring the house but I didn't see him measuring any windows. The other two contractors took a good look around, but did see any measurments. All see they are confident in the btu selection. All have are recommended from the Trane website and have great Angieslist ratings. I am obviously try to figure out who know there stuff best.
Here's my stats:
Portland Oregon area (temperatures can get in the twenties and maybe teens a couple days at year)
3 story house, including 880sf daylight basement with crawl space not slab
House built in 1993 with original Carrier furnace
Double pane windows
2x6 framing (I don't insulation ratings)
Trane XB13 3.5 Ton AC installed in 2007. Has hard time cooling the top floor below 75 degrees on 92+ degree days.
Daylight basement of ~900sf runs about 8 degrees cooler than rest of house. I do have a Rannai direct vent gas wall heater in the basement which can heat it quickly, but I would prefer to heat the basement with the main furnace, or at a minimum close reduce the 8 degree gap with the main furnace, and supplement with the Rannai if necessary.
Which one seems most correct for my stats. Thanks!