If he didnt show you the load calc I bet he didn`t do one. I don`t think
you can do it without writing it out, and you should demand a copy. If
80000 worked at your coldest days keeping you at 70f without running
24x7 then 100000 is much more than you need or want An 80000 btu unit ,
your old unit, say its 80% efficient put out 64000 btu, but it may be
much less, 60% efficient, poorly sized ducts, dirty heat exchanger, AC
coil, burner not running right etc etc. so it could be 48000 or less
output for example, mine was 50%. An 80000 btu infinity at 94.1% actual
[not 96] is 49000 on low fire and 75000 high fire btu, so depending on
what your unit really outputs and an honest load calc in writing, an
80000 infinity could be way oversized since it is more efficient. A
100000 infinity is 94.1% efficient not 96, outputs 94000 high 61000 low
fire btu. Oversize and you won`t be as comfortable if you consider it
goes to high fire after a few minutes in standard settings, it will heat
you faster unevenly. I went from 100000 btu to 47-74000, finally I got a
written load calc it was 50000 btu, I heat a bit uneven and am oversized
as I never need my second stage even at -20f with 30 mph winds. My
second stage is good for when I come home from set back temps. But I
will go smaller on my next unit soon. You need to learn more on when the
second stage is used, 1st stage should be lower for milder days with
second needed only when it can`t keep up, unfortunatly I think Carrier
comes to second after like 5 minutes, but im not sure. Oversizing is
common.