Thanks, dipep41, for the BTU calculator. Had no idea how to guess the window or door area but I fiddled with it and it was very helpful.
Justin--we came to your conclusion today, when the local furnace guy (who knows our house) said we can't do a power vent. Our foundation is ridiculously thick and they can't get the vent high enough off the ground w/o putting in an unwanted bend. So we're pricing out coal/oil furnaces or wood/coal/oil b/c it turns out any new appliance that goes into a chimney in Maine since 1998 must be on a lined chimney, regardless of what it's burning. So if we replace the current oil furnace, we have to fork over to line the chimney. Argh--of course that adds a lot to the cost (at least to us, $1600+ is a lot).
We might have to do the coal stoker stove this year (less than half the cost, it looks like) and plan on a bit of oil use to keep the upstairs warm enough. Then when the oil furnace craps out (or next year, if we win the lottery), we can replace our oil furnace with a combo unit.
Sigh. I was so excited about the furnace and having a warm house throughout. Still, my hubby pointed out that the Harman DVC-500 (or the LL Pioneer LE) should be able to make it pretty toasty, at least downstairs, for a good portion of the winter--at least until it gets insanely Mainely cold