How well does your 120000 heat at the coldest point in winter and how is
its recovery if you set it back. Remember your old furnace may be 50-80
% efficient so a new 94.5% efficient 100000 btu furnace may put out
allot more heat than your 120000 of 80 %. I dought your old is 80 %
but if it is thats 96000 btu output. A new 100000 94.5% is outputting
94500 btu. Your old unit is probbly much less than 80 % in output
efficiency so you are lilely fine.