#1) Why do people get dual fuel systems when they can simply get a dual stage furnace? Isn't the first stage of a dual stage furnace about as efficient as a heat pump during mild temperatures (in general)?
#2) If my current single stage furnace puts out 100,000 BTUs and I want a new dual stage furnace, do I add up the first stage BTU + second stage BTU = 100,000 or do I look for one where the first stage is 100,000 BTU already?
#3) It's too bad that I probably have to settle for an 80% furnace instead of a 90% furnace because I can't orphan the water heater (big chimmney), and I have windows all over the house (hard to side-vent), any of you had to settle for 80% furnaces for these reasons?
Dual stage isn't so much about effciency as it is comfort. Dual stage systems (heat pumps, AC's, or furnaces) lower the temp swing between cycles, promote better air circulation (by running a bit longer) and, in a small way can be more efficient by the longer run times.
A heat pump will ALWAYs be more effcient than a furnace. A furnace is around 93% efficient, a heat pump is usually 200% efficient (or more). Sure they don't make as much heat when the temps get colder, but even at 20 below, they'll be making some heat.
If you are sizing a furnace, you want the 2nd stage to be what the heat loss is (like the 100k btu furnace you have). 1st stage will run at a lower rating, which is exactly what you want since the furnace is sized to run almost all of the time on the coldest nastiest day you'll ever have in your area. But.. most of the time it's not that cold, so you'd need less heat.
Either way, make sure the contractor does a load calc on your house to ensure you're getting the correct size unit. If it's oversized too much it'll short cycle and get rid of some of the comfort and efficiency you should get. If it's too small, obviously it won't keep up on those extreme days.
I've put in a few 80% furnaces because they would be a pain to vent a 90%. Nothing wrong with that at all, but in my opion, that's even more reason to get a heat pump. Or at least look into the dual fuel setup, so that 80% (less efficent) furnace won't have to run as much, and you'll save money. Even with an 80% furnace, you can get a dual stage furnace.
Be sure, if you do get a dual stage furnace you get a dual stage stat aswell.
"If you call that hard work, a koalas life would look heroic."