I currently have a 120k btu 30 year old upflow gas furnace. The contractor did a heat load calc and came up with 30k for a 2400 sqft house on the northwest coast - design temp 20F.
The house has been upgraded, R40 ceiling, double low-e argon windows and air sealed to 4.5 ACH. This seems too low to me but then my old furnace never ran more that 10-15mins/hr.
I am also concerned that the blower on the new furnace may not be powerful enough. It's 3 speeds are rated between 800-1350CFM at .1 ESP