We live in Washington DC suburb (NW). Our house was built in 1973,
two-story, about 3400 sq ft. It has two systems, one for each floor. The
furnace downstairs broke at the end of last winter, so we are going to replace
the whole system downstairs. Thank you for folks here who helped me reach
that decision.

I have had two contractors come and give estimates, but neither was willing
to do a manual J, including the second one who spent two hours at our house
looking at everything. They both proposed 90K BTU two-stage variable
speed furnace (80% and 95% efficient, respectively) and 3-ton AC. I will
post separately about the detailed proposals once I get another two estimates
early next week.

My concern is that the proposed systems are oversized. Our current furnace
is a Bryant 397GAW036100 from the early 1980s (100K BTU input, maybe only
60-65% efficient?), and the AC is a 3-ton Fedders from the 1970s. We set the thermostat at 79 in the summer and
68 in the winter. I timed the downstairs AC one afternoon when it was 89
outside. The AC would turn on for 6-7
minutes and off about 15 minutes. Below
are the screen shots of my Man J calculation.
I have to guess at a lot of things (e.g. insulation), but tried to be
conservative. I got a total of 105K BTU
heating and about 6-ton cooling for the whole house. Since this is for downstairs and considering
our lifestyle, I feel a 2-ton single-stage AC will be the most
cost-effective. A 3-ton two-stage AC
will work also but cost more $$$$. For
the furnace, I feel 60-70K BTU at 95% efficiency will be sufficient.

Since I will have to convince the contractors to downsize their proposals,
I'd really appreciate any advice from the experts here.

Jeff