Have a load calculation done, that's the only way to know what the required size is.

Be cognizant of the fact that furnaces have two "size" measurements - BTU input and BTU output. The size is most often described by the larger number, the input.

If you have a 75KBTU 80% furnace, the output is 75K x 80% = 60K

If you have a 75K BTU 97% furnace, the output is 75 x .97= 72,700

A 60K BTU 97% furnace would provide 60Kx 97% = 58,000, the same as an 80% 75K furnace but using less gas to do so. But it would cost more to buy and install.

It's the output versus load requirement calculation comparison that matters. Make sure that's done. If a contractor tells you "I'm sure this is the right size based on your square footage", call another one. That's old school and not followed by the better contractors. Also, with an older house, make sure ducts are tested for leakage and repaired as possible. That's money well spent. Good luck.

PS - The AC equipment "match" that matters is the coil inside the air path, not the outdoor compressor.

I would think a 60 KBTU furnace even at 80% eff would be satisfactory. The furnace should have a 3 ton rated blower. If you replace your outside AC condenser, the indoor evaporator coil should be replaced as well.

TD

Thanks you, keeping it 80% . Any advantages off a 2 stage furnace? Is it worth the extra money?

In my opinion the 2-stage furnace is worth the extra money. You don't get that blast of air every time the furnace turns on, quieter operation on the low stage, and less short cycling. It will also set up up to install a 2-stage AC condenser in the future.

Have been quoted the difference in cost? Is there a reason other than cost that you are not considering a 95% efficiency furnace?

96% is about $1500 more ,I dont see us staying in this house long enough to recuperate the investment, we have concrete chimney in great chape and I don't feel comfortable with the only place the pipes can exit., through a window egress.