I would greatly apprecaite some feedback on the correct furnace size for my installation? House in Seattle is 1994, 3205 square feet, 3 story, including a walk out basement, R19 wall insulation, R26 in ceiling, dual pane windows, and the basement is always cooler than the rest of the house.

I have been presented with two furnace options, both of which are 96%variable speed 2 stage furnaces: first option is a 60k BTUH furnace @ 58k hi / 37.7k low output, or second option is an 80k BTUH furnace @ 78k hi/50k low output.

My BTUH total heat loss is 48k, using 72 degree indoor temperature and 23 degree outdoor. Does the 60k unit leave me with enough extra BTUH in reserve? My concern is, what if I let my indoor temperature at night drop to 64 degrees on a real cold night, will the 60k furnace have enough BTUH in reserve to heat my house back up fast enough to a cozy 72 in the morning?

Do I go with the 60k or 80k unit?