ZIP codes starting with 229 are in Virginia (that's WAY lower Michigan, eh?
)
Michigan ZIP codes all start with 48 or 49. I'm going to go out on a limb and guess that you're either in Britton (Lenawee County) or Ecorse (Wayne County), either of which would have a
99th percentile temperature bin in the low to mid single digits, say 3-5F.
Your windows could have U-factors anywhere from U0.28 to U0.50 which is quite a range, but it doesn't matter what it is if you calculate from fuel use measurements. To run a Manual-J you would need to know what the real U-factors are, measure up all of the window, wall, door, attic areas and apply the correct U-factors for each, measure the air leakage (or guess, which isn't an exact science), and measure the insulation depths everywhere, then subtract out 24/7 electrical loads, body heat, etc to come up with a number. With too many guesses the numbers just get fuzzier, less accurate, and it takes time, which is why they would have to charge real money for it. With a fuel use calculation you're using the existing furnace and it's efficiency as a calibrated measuring instrument, and you don't need to know ANY of that other stuff.
A Manual-J is necessary to figure out the ducting to make sure there's the correct proportion of heat in every room, but the ducts already exist, you really only interested in the magnitude of the whole house load, and it's
nowhere near 100,000 BTU/hr, it's probably half that or less.
In the time that I've responded to your posts I could have run the calculations for you multiple times if you just spilled the beans on the fuel use, full ZIP code, and meter reading dates.
Having calculated the load
before meeting with the contractors gives you a big leg up on the situation. About 8-9 out of 10 contractors will propose something that's more than 2x oversized, then try to snow you that it doesn't matter, because it's a 2-stage furnace. The low fire output of most 2-stagers is about 60% of the high fire output, so if it's 2x oversized at high fire it's still 1.2x oversized at low fire. Ideally you'd be no more than 1.4x oversized at high fire, which means it would cover the load more than 80% of the time at the more efficient low fire range, and would only step up to high-fire after a fairly long run cycle.