I calculate a need for 36,000 BTU furnace for the downstairs. What happens if I use one with double the BTU?

Question

separate furnace and ducts for upstairs.

Answer 1

stop  asking silly questions and go to the professionals,.  go to a dealer that sells the equipment and they will advise you what the best  way to approach this situation. they have all the knowledge and equipment you need. their interest is selling you the right stuff you need.

Answer 2

You just are not as efficient,  the new furnaces have 2 stage fans and will run longer for more even heat, a bigger unit will cycle shorter and not heat as evenly.

Answer 3

Short run cycles.

Rarely operates at its rated steady state efficiency.

Possible short life for the heat exchanger.

More than likely too small of duct work for the furnace.

Not a very consistent room temperature.

I guess your getting a good deal on it.

Then it wont be so bad if you have to replace it in a year or two.